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Q & A

I do quite a lot of attempting to answer astronomy questions on the internet, so I've decided to collect as many of these as I can. This is by no means a complete list. These are mostly short, often simplified answers to complicated questions - no fancy graphics, lengthy discussions or excessive lists of links here. I've edited the questions and answers here for clarity.

Sometimes there were follow-up questions. In these cases I've edited them so that each question has a standalone answer, you don't need to read the question immediately above to get a more complete understanding. I've also added simplified versions of the questions (which are often quite detailed themselves) in bold, so readers can quickly decide if they want to read a question or not.

Also I've removed multiple punctuation marks so that the questioners look less like a bunch of crazy people. :)

Unfortunately, while the majority of the time the discussions end to everyone's satisfaction, sometimes people believe their ignorance trumps science. This can involve long, unproductive discussions, from which I've here tried to salvage what I can.

If you find an error or would like more detail / links on any particular topic, let me know in the comments !


The Solar System (and others)
Can something in space rotate the Earth ?
Could you pee around the Moon ?
When NASA moves an asteroid to a lunar orbit, will they need to give it a push to start it orbiting ?
Could the asteroid belt have formed by the destruction of a planet 10,000 years ago ?
Are the numerous moons of the outer planets evidence for an exploding planet in the asteroid belt ?
Was there an asteroid-induced tsunami in the Indian Ocean about 5,000 years ago ?
Will a "tidal wave" from space affect our Solar System ?
Could a large earthquake shift the axis of the Earth ?
Can we calculate gravity on other planets ?
What size asteroid would cause a mass extinction ?
Is there anything that could stop the Earth from going around the Sun ?
Why do we use the Roman names for the planets and not the Greek ?
Are planets born in nurseries like stars are ?
Could the Sun split in two and spit out a mini-Sun ?
Could we stop an asteroid with three weeks warning ?
Would a 400m-wide asteroid just break up harmlessly in the atmosphere ?
Why use radar for searching for asteroids ?
If stars were larger in the past, were planets bigger too ?
Does the Sun make a corkscrew motion as it moves around the centre of the Galaxy ?
Do all the meteorites that hit the Earth make it heavier and slow down its rotation, and maybe also push it out of orbit ?
If you dropped a match into Neptune, wouldn't it catch fire because of all the methane ?
If the Moon eventually decides to leave (because it's slowly getting further away), will we be in trouble ?
OK fine, but if the Moon did leave, could we make a new one ?
If we replaced the Sun with another star during a collision with another galaxy, would we all die ?
What's the speed of the Earth through space if the whole Solar System is moving ?
Is Planety McPlanetface real ?
Why don't you like Planety McPlanetface ?
What will happen if the earth suddenly vanishes from its orbit ?
Why do meteors always land in craters ?
How can we sometimes see the inner and outer planets at the same time ?
Which planets are the most and least spherical ?
Are the most massive planets the most spherical ?
What's the truth about Nibiru ?
Could Venus just be cooling down rather than experiencing a super-greenhouse effect ?
If Callisto and Hyperion were orbiting the Sun, how would they be defined ? Planets ?
How can we detect planets around other stars given that Solar System images have such lousy resolution ?

Are the stars all dead ?
Can there be dark matter stars ?
How can stars form when gravity is so weak ?
How does dust form ?
Will stars keep recycling gas forever ?
Can a supernova destroy a planet, and if so, at what range ?
Can debris from supernovae destroy planets in other star systems ?
How do we know the distances to the stars ?
Do stars in stellar nurseries have very different compositions ?
What do supernovae have to do with planets ?
Can neutron stars and proton stars share the same space ?
When the giant star UY Scuti explodes, will we feel the effects ?
Can we measure the Sun expanding ?
How many stars are there in our galaxy ? How many new ones are born each year ?
Do stars explode when they're young or old ?
Can "rogue stars" knock stars out of their own star systems ?
Can stars merge ?
Do rogue stars pose a threat to our Galaxy ?
Were there bigger stars in the past ?
How come I can see Polaris all year round ?
Will we ever see a naked-eye supernova ?
Are we looking for gravitational waves as advance warnings of supernovae ?
Would a supernovae at the distance of Alpha Centauri be too close for comfort ?
Would we feel the blast from a supernova at the same time we saw it ?
Could gamma ray bursters explain the Fermi paradox ?
What's the deal with the star with the alien megastructures ?
If we dropped a small wormhole into a star, would it cause a supernova ?
What if we dropped a large wormhole into a star, or into a neutron star ? Would that cause a supernova ?
If a star turned into a black hole would it suck in all its planets ?
If there was a teaspoonful of white dwarf material on Earth we'd all get sucked in, right ?
If there is no oxygen in space, then, how the sun and other stars are burning ?
Are the stars we see in the night sky just in a small part of the Milky Way ?
If the distances between the stars are so large, why do they look so close together in photographs of other galaxies ?
If a star dies , does it still have the same mass ?
If a star becomes a black hole does its mass increase ?
What's it called when you live in a binary star system and one star occults the other? Is it still an eclipse ?
What's the closest two stars can get without colliding ?
What's going on with Tabby's Star ?
Shouldn't Dyson spheres resist absorbing and re-radiating heat ?
Could the dimming of Tabby's Star be caused by evaporating comets or sunspots ?
Could solar winds prevent dark matter from entering star systems ?
Why does a star's gravity get stronger when it dies ?

Are rogue stars parts of mostly dark matter galaxies ?
In the Universe is expanding, how can galaxies evolve through mergers ?
Can radio signals travel faster than light ?
When exactly will we collide with Andromeda, given that both galaxies have finite size ?
Will the gravity of Andromeda cause our collision to occur more quickly ?
Can a supernova damage our Galaxy ?
What happens to a galaxy when its central black hole evaporates ?
Could galaxies be rotating too quickly due to electical forces rather than dark matter ?
Will we really collide with the Andromeda galaxy in the future ?
How can there be hot gas between galaxies if space is so cold ?
Why study dwarf galaxies as opposed to their larger companions ?
How come we can see other galaxies given that there are no so many stars in our own galaxy ?
What will happen when the Milky Way collides with Andromeda ?
Will the collision of the Milky Way and Andromeda cause dangerous gravitational waves ?
How big is the error when weighing a galaxy ?
Would the estimated mass of a galaxy change using General Relativity instead of Newtonian gravity ?
How is the mass measured for a group or cluster of galaxies ?
What are you planning to research for the rest of the year ?
Should we get the sausages ready for when the the Milky Way collides with Andromeda ?
If there was a quasar in our own galaxy, could we see it with the naked eye ?
Will gravitational waves be significant for studying galaxy evolution ?
If the Sun moves out of the plane of the Galaxy, wouldn't it be pulled towards the centre more strongly and so spiral inwards ?
How do we know about the Sun's motion through the Galactic plane ?
Why does every galaxy have a black hole at the centre ?
Are rogue stars and planets created by galaxy collisions ?
Is there an ordered sequence to galaxy evolution ?
Could these newly-discovered ultra diffuse galaxies be ordinary galaxies at a different stage of their evolution ?
Could Unruh radiation explain the direction of galactic rotation ?
How many gas clouds are there around M33 ?
Is Keenan's giant gassy Ring really all that giant ?
What do you think of this press release about dust ?
What do you think of this silly meme about a void in space ?
Why don't galaxy-sized turbulent gas clouds look like smoke ?
What do you think of the idea of spiral arms being density waves ?
If Andromeda were to go nova instead of colliding with the Milky Way, what would happen ?
What do you think about this article about Dragonfly 44 being mostly made of dark matter ?
Could Dragonfly 44 be full of Dyson spheres ?
Are you very skeptical about Dragonfly 44 ?
What do you think about Dragonfly 44 ?
What would happen if dark matter started absorbing light ?
Does dark matter cause ram pressure stripping ?
Will star formation in galaxies be rejuvenated when all the stars go out and more gas can fall in from their surroundings ?
Could some dark matter halos just be made of normal matter that isn't glowing or reflecting anything ?

What is dark matter ?
How can we prove dark matter exists ?
With the right telescope, could we see the beginning of the Universe ?
Is the Solar System expanding along with the rest of the Universe ?
Is "dark matter" the wrong name ? Should it be "modified gravity" instead ?
Why didn't the young Universe collapse ?
Was gravity always present ?
Is cosmology a load of mumbo-jumbo ?
Is the expansion of the Universe a certainty, or might this be overturned by future discoveries ?
Could it be that space is moving rather than the galaxies ?
Is space infinite ?
What's the latest progress in detecting dark matter ?
How can we measure the age of the Universe since gravity distorts time ?
How can we measure the age of the Universe since time and space are expanding ?
Can the Universe expand faster than the speed of light ?
Can we see objects when they fall into a black hole ?
If space is flat, can we keep travelling and travel into another dimension ?
When measuring the Cosmic Microwave Background, does the telescope need to point away from the Galaxy ?
Is it really possible to tear a hole in the space-time continuum, cap'n ?
Could dark matter just be normal matter that exists in a parallel universe ?
How fast are we moving away from where the Big Bang happened ?
Can we see back to the very beginning of the Universe ?
Does the inertia of the galaxies affect the expansion of the Universe ?
Does ordinary matter become repulsive at high relative velocities ?
Is there any technology we could use to see right back to the Big Bang ?
As the Universe expands will its chemistry change significantly ?
How does the size of the Universe change over time ?
Is there evidence for the expansion of the Universe besides redshifts ?
Is there any other scientific explanation for redshifts and the CMB that could bring back a static, steady state model to explain our cosmological order ?
Why are distances in space so enormous ?
What's your take on the gravitational wave detection ?
I'd like to know more detail about deciphering the age, direction and source of the gravitational waves.
Since energy and mass are equivalent, are dark matter and dark energy really just the same thing ?
Since matter can never be created or destroyed, are you the same age as the Universe ?
What do you think of the idea that Universe had no initial singularity ?
Does space have limits ?
Why do black holes bend space ?
If space was curved enough could I see the back of my own head ?
How do we know space is curved ?
Is space globally curved ?
Is the Universe shaped like a doughnut ?
Yeah but really, could Steady State be the answer after all ?
How does the expansion of space cause resdshift ? Where does the energy of each photon go ?
Are we sure the Tired Light Theory was wrong, then ?
Who can explain to me why Einstein was trying to find the Gravitational constant of the universe ?
Isn't G related to the total amount of gravity in the Universe ?
Is gravity the weakest force ?
But if we DID have an FTL drive, could we explore the unobservable Universe ?
How do you know that dark matter is diffuse ?
How small was the deuterium fraction created during Big Bang Nucelosynthesis ?
Could the Big Bang be a white hole ?
How can we see light from the Big Bang, shouldn't it have overshot us ?
Does the total amount of energy in the Universe really add up to zero ?
How can the Universe be expanding if it's infinite ?
Could frame dragging explain dark matter ?
Do models of dark matter predict it should be self-interacting ?
If dark matter is self-interacting then shouldn't it behave like a normal gas ?
What's your take on Verlinde's new theory of gravity ?
Does the Bullet Cluster rule out self-interacting dark matter ?

Space Exploration
Why are space probes so slow ?
Are there any ideas for faster spaceships ?
Can we use the expansion of the Universe to travel faster ?
Could a big volcanic eruption launch a small moon into space ?
Could a volcanic eruption launch small particles into space ?
Should we send children into space to see if it affects them differently ?
Shouldn't the Voyager spacecraft be accelerating due to the nearest star system ?
When will faster-than-light travel become a reality ?
Are astronauts time travellers ?
Could we find new, dangerous elements on distant planets ?
Do space probes still use radioactive materials as a power source ?
How fast would you have to go to get to Mars in 15 minutes ?
When will Voyager I reach the heliopause, and will we lose communications with it when that happens ?
What if we moved to a planet with more helium ?
How can rockets work in space when there's no air to push against ?
How tall must a building be for someone on top to be weightless ?
Have NASA supressed better technology than rockets ?
What determines escape velocity ? Is it just speed and angle, and is there a better way to get high enough to see the curve of the Earth ?
What if aliens have a different name for planet Earth ?
If I started firing off relativistic projectiles into the Universe and random, should I worry about hitting something ?
Would a relativistic projectile be stopped, fragmented and/or destroyed by interstellar/intergalactic drag ?
Would nuking a dangerous asteroid at the last minute just make things worse ?
Why does the Cygnus spacecraft need to do a de-orbit burn ?
Is there any research into wormholes and negative energy going on ?
Can you spin a ball to make artificial gravity ?
Why do Space X love boats ?
Should astronauts really be running marathons on the International Space Station ?
Why is Martin Rees calling for an end to human space missions ?
Could we build a space battle cruiser if we really, really needed to ?
Is the Orion drive as stupid as it looks ?
Could the ISS rescue astronauts if they fell off ?
How can we improve communication with distant space probes ?
How fast could an Orion drive get us to Mars ?

What's the chance that aliens exist ?
Can we travel through time by eating the right plants ?
How can we detect gravity ?
Is it worth worrying about stupid memes ?
What is time ?
What's the difference between gravitational waves and tidal forces ?
Does an infinite Universe mean that impossible things will happen somewhere ?
What will the first aliens we meet look like ?
How do photons last for billions of years ?
Space is jolly cold. Don't photons run out of energy travelling through it ?
Would two charged black holes repel each other ?
Is there any known way of reversing time ?
If the speed of light isn't constant, why do we measure distance in light years ?
Can you ever escape the graviational field of an object ?
Do gravitational waves keep going for ever or do they eventually stop ?
What's the latest in the search for gravitational waves ?
If two gravitational waves collided, would they cancel each other out or create a new wave ?
Have there been any signals from aliens yet ?
Are you sure it's not aliens ?
Why are we assuming aliens are using technologies we understand to communicate, instead of something more advanced ?
Do gravitational waves cause winds in space ?
If gravitational waves are so weak, does that mean they couldn't have destroyed the Klingon moon Praxis ?
Why do different planets move at different speeds ?
Was Giodano Bruno a scientist ?
Are the physical constants really constants ?
What's the opposite of a black hole ?
What would happen if a black hole collided with a white hole ?
What causes inertia ?
If a black hole and a white hole have the same gravity, surely that makes them the same dang thing ?
Where does matter falling into a black hole go ?
How can we prove there's life or other civilizations on other planets ?
What is the escape velocity of Jar Jar Abrahams and is there a way tp reduce it to zeo ?
How do you know black holes exist ?
How long will it take for all the heavy elements to decay ?
Is it possible stop microscopic black holes from shrinking ? I am totally not plotting to destroy the world.
Can something become a black hole by going really fast ?
Does this nebula look like a brain ? This hour-long documentary is worth watching !
Why didn't you watch this 25 minute video on why gravity doesn't exist and the Earth is flat ?
Could there be black holes made of anti-matter ?
Apart form looking for aliens, what will China's new 500m telescope do ?
What do you think of this UFO report ?
What do you think of this 80 minute documentrary about UFOs ?
Do black holes rip you apart because they're made of antimatter ?
If we can see back in time, can we also see forwards in time ?
Would it take infinite time to cross the galaxy at the speed of light since time stops ?
When black holes merge, do they form a wormhole ?
Do black holes convert energy into mass ?
Can you do science without the scientific method ?
Should the Arecibo radar be subject to ethics review boards to prevent us accidentally signalling our presence to aliens ?
Will the new FAST radio telescope be better at signalling/detecting aliens ?
How about these space facts, eh ?
If nothing can escape from a black hole, why do they have jets ?
Does centripetal gravity affect different masses differently ?
Have we reached the end of physics ?
If aliens visited Earth, what would be the hardest universal human behaviour to explain ?
Are there any known lifeforms which can survive in space ?
Are black holes gateways to other dimensions ?
Could there be a white hole in the centre of our Galaxy ?
Why do you want physics to be broken ?
How do we know black holes are spinning ?
Do all astronomical imaging systems have the same contrast range ?
How could we definitely detect dark matter with an infinite budget and/or resources ?
Can mobile phones take images as good as giant telescopes ?
Is there sound in space ?



Can something in space rotate the Earth ?
Q : If earthquakes are strong enough to alter the rotation of the planet, is it not reasonable to infer that a spacequake could alter not only the rotation of the Earth,  but it could also move the Earth, if only a few feet. What I'm getting at is earthquakes can cause great big faults in the ground. Well, is it not the same with spacequakes ? Could they not disrupt the gravity well our planet sit within by moving it thereby changing the degree of axis our planet rotates on?

Currently, the axis is at around 33 degrees. Yet, what is to say that the axis could be changed to say 35 or even 40 degrees, with a large enough spacequake. Say, something as big as a star going nova. It is not in the realm of possibilities?

A : I confess I'd never heard the term "spacequake" before, so I had to do some research. A spacequake is a variation in the Earth's magnetic field. And they can be terrifying.

However, I don't see any way a magnetic event could affect the gravity well of the Earth in any way. Gravity is caused by mass, and mass alone. To alter the axial tilt of the Earth, you'd need to hit it with something massive or at least have a close flyby.The tilt of Uranus is interesting in that regard.

Could you pee around the Moon ?
Q : Because of gravity on the moon, does this mean your urine can even go further!? Maybe a circle around the moon ?
[This was obviously a joke thread, but I decided to take it seriously anyway, because it's funny]

A : Let's do this properly. Typical speed of urine, according to the internet, is around 15 ml per second. For water that's a mass of about 0.015 kg, or a volume of 0.000015 cubic metres. Assuming that the hole is a circle 2 mm across, and the urine is ejected in a continuous cylindrical stream, that's a cylinder of length 1.2 metres (so a speed of 1.2 m/s). That seems a little high to me, but sod it I can't be bothered to check because this is a stupid exercise anyway.

Assuming that's correct, the orbital velocity is given by v = sqrt(GM / r) where G is the gravitational constant, M is the mass of the Moon and r is the radius of the Moon. Which turns out to be 1700 m/s. So no, you cannot pee around the Moon.

... HOWEVER you could on a sufficiently small asteroid. Asteroid densities vary considerably, but can be close to that of ordinary water, so let's go for that. We need an orbital velocity v to be equal to 1.2 m/s. We know the density of the asteroid is 1000 kg/m^3. Assume the asteroid is a sphere, then its density will be M / (4/3 * pi*r^3) where M is the mass of the asteroid and r its radius. Re-arranging the equation for the orbital velocity, it can be shown that v = sqrt(G*density*4/3*pi*r^2) which we can re-arrange to find r...

The answer is that you probably could pee all away around an asteroid about 2 km across.

When NASA moves an asteroid to a lunar orbit, will they need to give it a push to put it in orbit ?
Q : Hearing the discussion about placing an asteroid in orbit around the moon got me thinking. Would the asteroid have to be pushed to get the orbit started? I imagine if you simply placed it near the moon, it wouldn't just begin to rotate given that an object at rest remains at rest unless acted upon by an outside force. However, one simple push should get it going after which it would continue on its orbit. Am I correct in this thought process? 

A : Essentially yes, with a but. If you did somehow simply place it near the Moon, it would just fall to the Moon. But in reality everything is already in motion. What NASA's proposed mission will involve is moving an asteroid from its current orbit around the Sun to an orbit around the Moon - so they will give it a push to move it from one orbit to another, rather than making it be in orbit at all (if that makes sense).

Could the asteroid belt be a very recent feature of the Solar System, formed by the destruction of a planet due to a collision with supernova debris ?
Q : Could a supernova explosion have ejected fragments which caused utter chaos in the Solar System about 10,000 years ago, including destroying a planet between Mars and Jupiter which then formed the asteroid belt ?
[This question was part of a larger discussion including archeoastronomy and mass extinctions. I decided to concentrate on the purely astronomical aspects of the questions - i.e. the bits I have some hope of answering !]

A : The energy needed to not just blast apart a planet but also send its fragments hurtling off into deep space (out of the gravity well of the Sun) is significantly greater than just smashing the planet. At 2.7 AU (the distance to the middle of the asteroid belt) the escape velocity of the Sun is about 26 km/s. For an Earth-sized planet, this means the energy required is about 10 times greater than "merely" smashing the planet to bits. (as a ballpark, this corresponds to raising the temperature of an Earth-sized mass of granite by about half a million degrees, but you only need to raise it by a few thousand K to vaporise it - so it's not clear to me whether such an event could result in large numbers of mountain-sized fragments)

I don't see any way of accelerating a planet in the Solar System to collide with another at that kind of velocity, or to get a smaller object up to a much higher speed to carry the same energy. A planet ejected from another star system, possibly by a supernova, might be able to do it - but this is such a fantastically unlikely event that it will never, ever happen (see

But let's assume it did. In that case the composition of the asteroids should resemble that of a planet, i.e. distinct differences between asteroids that were originally part of the crust, mantle and core. Asteroids within those categories ought to have very similar compositions to one another. Each category should be found uniformly throughout the belt since there's no way a collisional origin could cause asteroids with different compositions to be distributed differently from one another. This is not the case. Asteroid composition varies significantly (e.g. meteorite studies : and does vary within the belt, as a function of distance from the Sun :

I'd also guess (and it is just a guess) that the distribution of asteroids produced by a recent collision should look completely different to the distribution of asteroids formed in situ billions of years ago. Unless the planetary geologists are all massively incompetent, someone would have noticed this. :P

Ceres and other large asteroids are believed to have differentiated interiors and high volatile contents. Volatiles would not have survived the collision, nor would there be time for the asteroids to form differentiated interiors (or probably even form at all).

Observations of the rotation rates of asteroids are consistent with the Yarkovsky / YORP effects acting over billions of years (e.g.

Radiometric dating of meteorites has found that they were last molten billions of year ago.

In a word, no.

Would an exploding planet explain why there are so many moons around the outer planets ?
Q : Could the origin of the asteroid belt and the many, many moons of the outer planets be explained by an exploding planet ?

A : Well, the asteroid belt wasn't formed like this (see But for the sake of it, let's investigate the issue from the perspective of the moons of the gas giants.

It's widely accepted that many of the small rocky moons of the gas giants are captured asteroids. There are very clear differences between their giant moons (especially the Galilean moons of Jupiter) and their smaller moons. The giants tend to absolutely dominate the mass, with the smaller moons having only a few percent or less of the total moon mass. The orbits of the moons tend to vary. The innermost moons (which includes the giants) tend to all orbit within a very narrow plane, while the outer moons are something more like a swarm of bees. Exactly what you'd expect if some formed in situ and others were captured later.

The moons which are all orbiting in a plane very likely formed at the same time as their parent planet did. The gas and dust which formed the planet would have collapsed into a disc because of its rotation, though the details of how planets and moons formed are not at all well-understood. Even discounting the more obviously captured-asteroid moons, the gas giants still have more moons than the terrestrial planets. But they're also a lot larger, so they must have formed from more material - so there would have been more material around to form more moons as well (even if we don't understand the process very well).

I am not at all sure that an exploding planet is going to produce large chunks of debris, since the energy required to blast it apart is far higher than that needed to vaporise it. But let's assume it does anyway.

If so, we can work out the mass of the postulated planet by considering the masses of the moons and their distance from the planet - assuming that a few fragments didn't have as much velocity so were able to be captured (more on that in a minute). That gives us a mass flux which we can multiply over the area of a sphere centred on the asteroid belt (2.7 AU from the Sun). I did a very crude, quick calculation of this. I have to admit, I was surprised by the result. Ignoring the giant moons (and the small moons in the same plane), the estimated mass of the planet is about 2% the mass of the Earth for the Jovian moons, 7% for the moons of Saturn, 2% for Uranus and 12% for Neptune.

Given the enormous errors in this process, the masses of the gas giant moons are not incompatible (not the same thing as "evidence that this happened") with their origin being a planet in the asteroid belt. Of course, if you pick and choose which moons to accept and reject for the calculation, the results will vary dramatically. And remember that this is assuming a spherical explosion - if you restrict it to something more compatible with the 1 AU thickness of the asteroid belt, those mass estimates will fall considerably.

If you want to explain the asteroid belt (as well as the outermost moons, which are moving much too slowly to have been ejected as solar escape velocity), there must have been a wide spread in the velocities of the debris. But that implies there should also be moons that are on highly elliptical orbits, but there aren't. Most of the orbits are rather circular. I also note that there's a rather sudden cut-off in the outer moons. If the debris had a continuous distribution of velocities, there should also be moons even further from the planet where they could be captured at lower velocities. Saturn displays a similar sort of cut-off in the distribution of its moons (at a very similar distance, 24 million km compared with 30 million for Jupiter), as does Uranus (21 million km) while Neptune has moons at up to 50 million km away.

This really requires a specialist in orbital dynamics to answer properly. My naive impression is that the rather circular obrits on a limited range of inclination angles aren't compatible with a spherical explosion origin.

Was there a giant tsunami in the Indian Ocean about 5,000 years ago, caused by an asteroid impact ?
Q : Could a giant impact explain Noah's Flood ?
[I chose to answer this since I know the questioner is not a Creationist and not set on proving faith-based beliefs. I would probably have ignored it if I thought it would lead to a pointless science vs religion debate.]

A : This idea comes from a press release a few years ago by a group of geologists claiming to have explained Noah's Flood. The "Holocene Impact Working Group" ( is a group of geologists trying to prove that major asteroid impacts are more frequent than everyone else thinks they are. Readers should beware that this one is well outside my area of expertise !

Let's break this into two questions : 1) Was there a tsunami ?; 2) Was there an impact ? I read four main documents to answer this one : 1) Press release about a crater candidate and the tsunami (; 2) Book chapter by Gusiakov et al. 2010 about the same (; 3) Peer-reviewed article in the journal "Geology" by Bourgeois & Weiss 2009, describing a simulation of an impact at the proposed location (; 4) A response by some disgruntled geologists who are annoyed by the claims of an ancient tsunami / impact, published in GSA Today, Pinter & Ishman 2008 ( Here I'm giving a shortened version of my analysis, you can read my full discussion here :

1) Was there a tsunami in the Indian Ocean about 5,000 years ago ?

Gusiakov 2010 discuss many different features produced by tsunamis : boulders, numerous different rock formations, and the chevron-shaped dunes. However, so far as I can tell they only cite the dunes as evidence for the Indian Ocean tsunami. They make several different claims why these chevrons indicate a tsunami, but they rarely consider any alternative explanations (e.g. the internal structure of chevron-shaped dune features, the lack of sediment in a river, the orientation of river sediments, the wide range of rock fragments in the chevrons, the presence of oceanic microfossils in the chevrons, the orientation of the chevrons with respect to the coastline).

The orientation of the chevrons is categorically dismissed by the simulation of Bourgeois & Weiss 2009, which Gusiakov 2010 appear to be completely ignoring. The presence of oceanic microfossils can be explained simply by the wind according to Pinter & Ishman 2008, who note other examples where this is the case. Annoyingly, though Gusiakov 2010 spend a long time describing all the signatures of tsunamis, they completely fail to mention any of them in this case. For example, it appears to be that there aren't any massive boulders transported high up the shoreline as there were with other tsunami events. What's really worrying is that they rarely even consider other explanations besides a tsunami.

So far as I can tell, they have no evidence of the age of the chevrons and therefore no evidence of when this hypothesised tsunami may have happened - besides ancient myths.

2) Did an asteroid impact the Indian Ocean about 5,000 years ago ?

Leaving aside the evidence for a tsunami, which I find very unconvincing, the evidence for an impact rests on a crater candidate. Even Gusiakov 2010 describe this as "subtle", and - if I may venture so far - it could also be called "wishful thinking". It's half an arc in some very complex terrain - you'd need some really good supporting evidence that an impact happened. By their own admission, the team don't have this. They have samples of rock from near the crater, but : "We have learned from our work on other oceanic impacts that the minimum weight of sample we need to confidently identify the presence (or absence) of impact ejecta is between 10 and 20 grams. The cores we have been working with are old and heavily sampled. Thus, in many cases we could only obtain a 2 to 5 gram sample."

It seems to me that this is taking the flimsiest of flimsy evidence and using it to infer an extraordinary event that is in flat contradiction to the rest of geology and astrophysics. Even without knowing geology, reading Gusiakov 2010 did not feel like reading a scientific paper. Most of the time they don't consider other explanations so they don't try to falsify their own conclusions. Quite honestly it didn't even feel like science. It's a house of cards, literally built on sand.

Will a giant tidal wave from space kill us all ??!?
Q : I either heard or read, recently, where there was a "tidal wave in space", that shook up a few galaxies. Wouldn't that affect our solar system in the next few hundred years ?

A : This was about the following press release :

Here the collision of two galaxy clusters has triggered a shockwave in their gas which may cause increased star formation. The term "tidal wave" is well-chosen to attract attention (that's what press releases are for, after all !) but causes confusion because it's such an ambiguous term with many different meanings :
None of them really make much sense when applied to galaxy clusters. I think they're using it in the far more loose sense that's usually used in disaster movies - any large wave, even if it wasn't caused by tides. It's not a term normally used in astronomy.
it won't affect us in any way. It's 2 billion light years away and the effects are due to shocks within the gas in the galaxy cluster. Even in a galaxy in the cluster, individual star systems wouldn't really notice anything - not on human timescales, anyway. The effect of the shock would be extremely weak by human standards - but on the very low density gas, and over millions of years, it can trigger an increase in star formation.

There's pretty much nothing that could possibly happen in another galaxy that would have any effect on us. The distances are just too large.

Could a really large earthquake shift the axis of the planet ?
Q : What if over time, the axis of the world went from 33 degrees to say 0 degrees. Meaning that the world would shift onto its side and rotate that way. That would mean different climates all around. Water might shift, etc. Is there any kind of evidence that that could happen? OR did it happen already?

A : Earthquakes can and do shift the axis, but by a tiny amount. In order to shift it by an amount large enough to cause changes in the climate, you'd probably need enough energy to shatter the planet - at which point you wouldn't be worrying about climate variations.

Can we calculate what gravity is like on other planets ?
Q : Can we calculate gravity variables on other planet is that just speculation ?

A : The only variables are the gravitational constant, G, measured in laboratory experiments on Earth, and the mass of the planet. That can be measured from the motion of any objects orbiting it, or its affects on nearby objects.

What size asteroid would cause a mass extinction ?
Q : What size asteroid or comet would cause an ELE (Extinction Level Event). Are we talking the state of Hawaii, the State of Rhode Island or the State of Texas ?

A : A large mountain-sized rock will do it. It did it for the dinosaurs at any rate.
The energy of impact is more dependent on the speed than the mass, which for most asteroids is "bloody fast" (technical term). If I were you, I'd avoid following this twitter feed if you're of a nervous disposition.
If you're not of a nervous disposition and wondering what the energy of impact means in real terms, have a look here :

Is there anything that could stop the Earth from going around the Sun ?
Q : Is there such an event that would cause a planet to stop revolving around it's star (ours being the Sun). If so, does that signal an end to a civilization or does it have anything to do with the magnetic shifts that go on over the course of several hundred thousand yrs ?

A : Not many. The most likely is that in a few billion years the Sun will expand to a red giant. If that doesn't completely incinerate the planet, it will shed a lot of mass from its outer layers and form a so-called planetary nebula. That might throw us out of orbit, if it doesn't incinerate us.

A collision or close encounter with a very massive object could do it. That definitely isn't going to happen anytime soon. Maybe over long time scale the solar system is unstable, but again we're talking billions of years.

A close encounter with a black hole from outside our solar system could do it, but there are so few black holes that the chances of that ever happening are close to nil.

It would definitely be very bad for civilization. Probably fatal. Nothing whatsoever to do with the magnetic shifts though, which are due to internal processes of the Earth.

Why do we use the Roman names for the planets instead of the earlier Greek ?
Q : For the naming of the planets in the solar system, why did they elect the names of the Roman gods ? Greek gods are more prevalent in modern culture than the Romans are. Actually, I believe Jupiter's moon Io is derived from Greek origin. The woman who transformed into a cow. It doesn't coincide with the Roman name Jupiter.

A : It wasn't an election, it was a military victory. It was the Romans who conquered Europe and spread their names for the planets, not the Greeks. So their names were established across Europe first. Moons were discovered much more recently.

Are planets born in planetary nurseries, like stars are born in stellar nurseries ?
Q : I'm not sure how to ask this, but I'll give it a shot, hoping I'm understood. When planets are being "born", what determines what goes inside the core ? I have heard of "nurseries", but wouldn't they all be similar in the core, because they are around the same environment ? Or do things interact differently in a nursery than say a singular planet being born ? I read that and it doesn't make much sense, to me... It's ok if no one else gets it.

A : I've heard of stellar nurseries but not planetary nurseries. Planet formation is not all that well understood, but generally they're thought to form around stars. The closer to the star, the hotter the material, so things like methane and ices aren't going to hang around long. Further out colder materials can survive without evaporation. So, there will be different compositions of different planets depending on where they form.

But then you have weird exoplanets like "hot Jupiters" which are gas giants orbiting very close to their star, which throws a spanner in the works.

I didn't read it all but wiki's article looks nice.

Could the Sun ever split in two, like, spitting out a baby Sun ?
Q : If Jupiter can pop out "moons" like they are Chicklets, can the Sun also create a "mini Sun"??? OR what I'm getting at is can any star create another star or does that only happen in binary systems ? The reason I ask is I was hypothesizing that when the Sun is "nearing it's end", it might have enough energy to "have a baby"...Yeah I'm tired, and probably not making any sense... sorry.

A : "If Jupiter can pop out "moons" like they are Chicklets..."

It can't.

When the Sun dies, it will have used up most of its hydrogen fuel. At that point, splitting in two would be no help - it would be like chopping your car in two and hoping that would make things better... :P

... that said, it's thought that asteroids can gain enough spin that bits of them can fly off to create binary asteroids. Which would be quite a lot like spitting out moons like chicklets. But there's no known mechanism by which the Sun could spin up so fast that it would split in two.

Could we stop an asteroid in just three weeks ?
Q : The 400m-wide asteroid 2015TB145 nearly hit us ! If it's trajectory had been just slightly different, we'd have only had three weeks of warning. Would this be enough for us to do anything about it ?

A : Keep in mind that while yes, the trajectory would only have had to have been slightly different, that still doesn't mean a chance of a collision was ever very high. There are plenty of other slightly different trajectories if could have taken which would have sent it sailing safely past.

Suppose that the asteroid might have travelled anywhere within that 480,000 km radius of the Earth. Since the Earth itself has a radius of 6,371 km, the area of the Earth / area of the circle within which the asteroid might have passed = 0.00017. So, the chance that it would have hit is something like 0.017%, assuming it went anywhere at random within that 480,000 km.

That's a very simple approximation, mind you.

As to whether we could do anything about it, well really the only option at the moment is a nuke (probably always will be with that little warning - if you have more time, you could use a gentler, more controlled method). Could we go from zero to scrambling a nuclear-armed rocket able to hit a 400m target tens of thousands of kilometres away in three weeks ? I doubt it.

Would a 400m asteroid just break up in the atmosphere ?
Q : I mean, it's just not very big, right ?

A : What ? It's ENORMOUS ! I doubt very much that anything 400m across would break apart in the atmosphere. At 35 km/s it would travel through the entire atmosphere in less than 4 seconds - there's just not enough time for the heat on the surface to travel 200m into the interior. True, the atmosphere will slow it down - but not enough.

Even if it did break up, you really wouldn't want it to. All that energy has to go somewhere. Assuming a density equal to water (pretty decent assumption given known asteroid densities) and a spherical geometry, at 35 km/s that means it has an energy of nearly 5,000 megatons of TNT. Energy scales in direct proportion to mass, but in proportional to velocity squared - it's speed that matters, not mass. So even though it might not be a perfect sphere and the density may be different, this won't affect the calculation much. We're certainly talking about thousands of megatons of TNT.

5000 megatons = 200,000 Hiroshima bombs. It's about 20 times less than the dinosaur killer, but it would give everyone a really bad day. Hence, although asteroid impacts are rare, searching for dangerous asteroids is a worthwhile pastime.

For more scary numbers see +Winchell Chung's epic boom table :

Why would you use radar for hunting asteroids ?
Q : Well, why ?

A : Radar isn't used for finding asteroids, it's used for measuring their properties. Radar has a very narrow beam, so it can only survey a small part of the sky, so it's not a good way to look for new objects. Optical and infra-red telescopes have a much wider field of view, so they can look for moving objects over a very large area of the sky.

What radar is good for is measuring the properties of the asteroids (because it has very high resolution) and also their distance. With the distance the orbit can be constrained much more accurately, so the asteroid will be easier to track in the future.

If stars were bigger in the past, were planets also bigger ?
Q : You were wrote that it is quite possible for planets closer to the origin of the Big Bang, to be larger. Would that mean that there might be larger "Earth-like" planets with similar atmospheres closer to the beginning or am I not understanding something, yet again ?

A : For proper context see this :

It's thought (but not certain, because no-one has seen one yet) that stars in the early Universe were larger because there were less heavy elements around.

Heavy elements (meaning anything other than hydrogen and helium) are effective at cooling. Cooling is important for star formation. As gas clouds compress, they heat up. This heat pushes back against gravity, making it more difficult for them to collapse. If you throw some heavy elements into the mix, the gas cools more easily, so it has less heat holding it up against gravity, so it collapses more easily.

If you don't have heavy elements, the gas doesn't cool so much. This means you need a lot more gas for it to overcome the heat and collapse, so the stars are bigger. But, you also need those heavy elements to form planets. So you'll have bigger stars, but fewer (and probably smaller) planets. At least, that's definitely true for rocky, Earth-like planets. Not sure about gas giants though.

Does the Sun make a corkscrew motion as it moves around the centre of the Galaxy, like in this video ?
Q : That video being this one :

A : No it does not. As I wrote in this post :

Then there's his second video. This one is more objectively just plain wrong. He shows the Sun tracing out a corkscrew pattern as it orbits the galaxy, which makes no sense. The Sun simply goes around the center of the galaxy (and up and down a little bit) - nothing else. It's not orbiting anything else at the same time. For it to trace a helix is just nonsense. He seems to have an almost unique case of helix madness.

However you should read this follow-up post :

Do all the meteorites that hit the Earth make it heavier and slow down its rotation, and maybe also push it out of orbit ?
Q : I read that daily, our "Earth is bombarded with more than 100 tons of dust and sand-sized particles." ( If so, doesn't the mass on the Earth and consequently, its weight, increase leading to reduction in the speed of rotation of the Earth around itself and the Sun ?

A : The mass of the Earth is about 6x10^21 tonnes (6 followed by 21 zeros). So to increase the mass of the Earth by 1% would take about 6x10^19 days, or 1.6x10^17 years, which is much, much longer than the age of the Universe. Although the increase in mass does change the rotation rate and orbit, the increase is so small that it's negligible. Probably not even measurable.

To put it in slightly more comprehensible numbers, imagine that the Earth has been receiving the same mass in meteorites every day in its 4.5 billion year history. That comes to a total of 1.6x10^14 tonnes. If the ocean is around 5 km deep, then that mass is the same as an area of sea around 200 km on a side. So quite a bit, but still utterly tiny compared to the whole ocean, let alone the entire planet.

The mass of the Sun is about 300,000 times greater than the Earth. This is so much larger that the mass of the Earth basically doesn't matter. Even if its mass doubled, or increased a hundredfold, it would remain in exactly the same orbit.

If you dropped a match into Neptune, wouldn't it catch fire because of all the methane ?
Q : Neptune is basically a big cow fart in space, right ?

A : No, because you also need oxygen for combustion. It's the same reason Jupiter doesn't explode even though it has a lot of hydrogen and has lightning, which is much hotter than a match.

If the Moon eventually decides to leave (because it's slowly getting further away), will we be in trouble ?
Q : How would an "explosion" on the Moon affect life; as we know it; on Earth. I don't think the Moon is large enough to shove the Earth out of the "habitable zone", is it ? Yeah, the tides are going to be affected, but that's about it, right ?

A : Yes, removing the Moon probably isn't enough to change the orbit of the Earth around the Sun all that drastically. But tides are important, and I suspect removing them would have huge consequences. Life has evolved with tides as a constant background. Removing the tides would probably alter ocean currents, which would almost certainly be bad news for the entire ecosystem of the sea. Additionally, some animals rely on moonlight to hunt and navigate. My guess would be that without the Moon we'd be looking at an ecological disaster.

Actually, although the Moon is slowly getting further away from us, it's probably not going to escape at all. Just get a bit further away, until in about 50 billion years it reaches a maximum limit. The Sun will have died in about 5 billion years anyway, so it's a moot point.

OK fine, but if the Moon did leave, could we make a new one ?
Q : Would it make sense to send up an "artificial Moon", to do the job that the real moon won't be doing ? Or is that something for future generations to talk about ?

A : Probably creating an artificial mirror to replace the light of the lost Moon (hey that sounds like a title for a novel...) would not be so difficult. In fact the Russians already tried something like this on a smaller scale, and it very nearly succeeded.

But replacing the Moon's tides would require something of the same mass of the Moon. By the time we've got to the stage of being able to move that much mass around, there'll be no need to remove the Moon at all. We'd just go and mine asteroids or something instead. No point causing all that tidal chaos unnecessarily.

So leave the Moon alone. It's getting along just fine without us meddling.

If we replaced the Sun with another star during a collision with another galaxy, would we all die ?
Q : If "Sol" is replaced with a Sun from Andromeda, what is the likelihood that the Earth might change temperatures (either up or down), or would life just cease to exist, as an ELE ?

A : In the event that another star did get close enough to rip us out of the system / replace the Sun, we would all die. Any star that gets that close is going to cause massive disruption to the orbits of all the planets. Regardless of the temperature of the new star, as our orbit varies in distance we would experience extreme variations in temperature. We're talking the oceans boiling into space or freezing solid level of cataclysm. 

What's the speed of the Earth through space if the whole Solar System is moving ?
Q : If the solar system is moving at 600,000 MPH through the galaxy, how could the Earth be moving at only 66,000 MPH? Would it not be "left behind" ? Clearly, I'm not understanding the math behind the differences between the solar system movement through space and Earth's movement around the Sun. Why the discrepancy ?

A : t's about what things are moving relative to. If I walk down the street at 4 mph, I'm moving at 4 mph relative to the surface of the Earth. But the Earth itself is moving around the Sun at 66,000 mph around the Sun. I'm held to the surface by gravity, so I'm really moving at 66,004 mph around the Sun (well a bit less, depending on which direction I'm walking).

Similarly the Earth is going round the Sun at 66,000 mph, but the entire Solar System is orbiting the centre of the Galaxy at 600,000 mph. The Earth is also orbiting the centre of the Galaxy at roughly 600,000 mph, but it's only doing 66,000 mph relative to the Sun.

Or think of a CD. They can spin at anything up to 300 mph, but they don't go anywhere. Unless you've got one in a car, of course. Then it's moving forward at however fast the car is moving. But the speed at which the CD spins doesn't depend on how fast the car is going.

A simpler example might be one car overtaking another. If the first car is going at 60 mph and the second at 80 mph, then they go past each other at only 20 mph. But both of them are moving much faster than this. It all depends on if you're measuring speed relative to the road or to the other car.

It's the same with the Earth going around the Sun. It's going around the Sun at 66,000 mph, but simultaneously moving through the Galaxy at 600,000 mph. Now, if you wanted the true, total speed of the Earth, you'd have to combine them, so you could say the Earth is moving at 666,000 mph. But you wouldn't say that the Earth is going around the Sun at 666,000 mph, any more than you'd say the faster car is really only doing 20 mph. It all depends on your point of view.

Is Planety McPlanetface real ?
Q : I mean....something just ain't right here ! How is this big a$$ planet, just now being discovered ? They've found dwarf planets & all of these other structures, but missed this big a$$ planet?! Oh only comes out every few millions of years right? Well how do they know that, if its newly discovered ?!

A : It hasn't been discovered, just predicted to exist based on the orbits of a few other objects. The whole thing is rather dubious, in my view.

Why don't you like Planety McPlanetface ?
Q : Would you care to elaborate why it is to be dismissed ? I get that it's far from the first time a "new planet is inferred!" but could you explain what makes this time's arguments flimsy ? I've heard about the Kuiper orbits all bunched up and a probe trajectory model that the planet would help, but with no idea what they are worth.

A : It just seems to me that they are making incredibly strong claims from extremely flimsy evidence : "“It's such a long history of people being basically wrong that standing up and saying we're right this time makes us almost look crazy," Dr Brown said. "Except I'm going to stand up and say we're actually right this time."

That sort of "we're definitely right" attitude may be fine if you're NGT but it's ridiculous if you're doing actual research. Coupled with Brown's self-labelling as a Pluto killer, the word, "grandstanding" leaps to mind like a kangaroo on a trampoline. The current definition of planet is bloomin' daft anyway. This smacks of glory hunting.

Then there's the evidence itself. Six objects. Six ! From which they infer a clustering probability of 0.007% by chance. Yeah.... really ? Seems to me that that's a crazy-small population of objects to draw any firm conclusions from. There could be any number of as-yet undiscovered objects out there which don't show the same clustering. The idea of a giant planet as the only explanation feels like building a house of cards.

What will happen if the earth suddenly vanishes from its orbit ?
Q : Well, what ?

A : We'll all die. As for the rest of the Solar System, probably not all that much. The Sun is much the most massive component of the Solar System, so our sudden disappearance wouldn't have that dramatic an effect. The orbits of the other planets would shift, but (without checking the exact numbers) probably not that significantly. I'd cautiously speculate that the asteroid belt might do something a bit more funky though.

Why do meteors always land in craters ?
Q : Well, why ?
[It was a joke question, but I was asked for an answer so I gave one]

A : Of course, this is obvious. Every crater is home to a crater monster, sometimes known as an exogorth. A famous example of a such a creature can be seen in the 1980 documentary, Star Wars : The Empire Strikes Back ( However as passing starships are generally rare delicacies, most exogorths make do with a staple diet of rock flambé.

How can we see the inner and outer planets at the same time ?
Q : I have a question that if earth is after Mercury and Venus then how can we see them at night and moreover earth is between them and other planets so how can we see all at same time ?

A : You can only see Mercury and Venus just before sunrise or just after sunset. Sometimes it's possible to see the other planets at those times depending on which part of their orbit they're on. This online simulator probably explains things more clearly :

Which planets are the most and least spherical ?
Q : What planet is closest to a perfect sphere and farthest?

A : Google tells me that Venus is the most spherical planet...
... while Saturn is the least.
I guess that maybe the search terms aren't as obvious as they might seem.

Are the most massive planets the most spherical ?
Q : I would think that the planet with the most mass would be closest to a perfect sphere and the planet with the least mass would be the furthest from being a perfect sphere - due to gravity.

A : That would be true if all the planets had the same composition and were rotating at the same speed. But this is not the case. Earth is much smaller and denser than Saturn, but it's closer to being a perfect sphere - partly because it's mostly solid and rotates more slowly. Saturn is mostly gaseous, much less dense than Earth, and rotates very much more quickly (around 10 hours to complete one revolution at the equator). It's so much less dense than Earth that even though its mass is nearly 100 times greater, its surface gravity is about the same. But since it's also rotating much faster and made of much less rigid material, it's less spherical despite its greater mass.

What's the truth about Nibiru ?
Q : Planet-x, nibiru. What is the truth behind the rubish

A : The truth is it's utter nonsense. Fortunately I have a file prepared on this already*. The super-short version is that it's nothing but the ravings of crazy people making wildly contradictory claims that make absolutely no sense.
* This was for a group discussion in work back when some genuine (but probably wrong) scientific claims were being made for a distant planet in the Solar System. I decided to educate my colleagues as to why People On The Internet were so excited about this. [Please remember that I retain the absolute right not to answer questions and am under no obligation to discuss conspiracy theories, if I choose to do so that's up to me, but the purpose of these AAAAAAAA posts is primarily for education, not debate]

The long version :
Original idea from a mad lady in Wisconsin who said the aliens talk to her through an implant in her brain. Claimed :
""The Hale-Bopp comet does not exist. It is a fraud, perpetrated by those who would have the teeming masses quiescent until it is too late. Hale-Bopp is nothing more than a distant star, and will draw no closer."[9] She claimed that the Hale-Bopp story was manufactured to distract people from the imminent arrival of a large planetary object, "Planet X", which would soon pass by Earth and destroy civilization. After Hale-Bopp's perihelion revealed it as one of the brightest and longest-observed comets of the last century Lieder removed the first two sentences of her initial statement from her site, though they can still be found in Google's archives."

Lieder associated object with Nibiru, a planet apparently described in ancient Sumerian texts according to some dude named Stichin. Stichin denies the connection because obviously Nibiru won't return until AD 2900, though he did think the aliens might return in a spaceship before that.

Nibiru is also predicted in the Bible and will have astrological consequences. It's sending us a message that the stock market needs reform :

Nibiru has been causing chaos for 510 TRILLION years !
"According to historians, Tiamat, a planet which had lain between Mars and Jupiter around 510,000,000 million years ago, was a victim of Planet X, as Tiamat collided with one of the moon's of Nibiru; it crashed, broke into half, as one half became the asteroid belt, and the other Phobos: Mars moon, while the other half is our home, planet earth. " [Yes, that's three halves]

"This strange planet has known to be twenty times bigger than Jupiter, with a burning moon which acts like Nibiru's personal sun. Since Nibiru goes much, much further away from our sun; this theory actually does make sense, and stands out."

"This is why the earthquake that are happening in Japan, Chile and other places, could be due to the fact that magnetic pull from Nibiru is increasing as it nears our plain. The pull from Nibiru will increase gravitational force of each planet in a rubber band effect. Most of this informaiton is not provided in Wikipedia."

Orbital period is 3,600 years. Or maybe it's 740,000. Could be it's four times bigger than the Earth, or maybe ten, or maybe 850 times bigger. Might be going to collide with the Earth, or just cause global warming by disrupting the magnetic field of the Sun. Aliens created humans by having sex with animals 300,000 years ago, or maybe it was only 25,000 years ago but who's counting. It's going to cause chaos later this year, or maybe next year, or maybe hundreds of years from now.

Could Venus just be cooling down instead of experiencing a super-greenhouse effect ?
Q : When the Venus Pioneer mission was sent to investigate the source of Venus' heat, it was initially determined that Venus is not in thermal equilibrium -- that in fact the planet is releasing 15% more heat than it is taking in, an apparent violation of Carl Sagan's Super Greenhouse Theory (arguably the primary focus of this mission was to generate evidence for the greenhouse effect). A glance at the original papers reveals that the data was corrected to reflect the greenhouse theory as an assumption. It seems that when the data did not cooperate, theorists simply assumed their hypothesis.

My question is: How do theorists rule out an alternative cause for this heat, such as a recent planetary catastrophe? Is it not possible that the planet is actually cooling down from some recent event?

See snapshots of the original papers here:

A : [Regular readers are well aware that I think this idea of re-writing the scientifc narrative is a load of old cobblers. On one hand we've got people worried that I'm being too dogmatic and others who think I'm not being dogmatic enough. Methinks that means I've got the balance just about right.]

There's no way I'm going to read 23 papers (and books !) just to answer a question. It's not even clear which quote comes from which source.

What I will say is that atmospheric physics is _hard_. For instance there's a strangely colder layer in the upper regions of Venus' atmosphere :
Like asteroid impacts, planetary atmospheres are a grey area between astronomy and geophysics. So I don't feel at all comfortable in trying to understand the details of whether Venus is in thermal equilibrium or not, or whether the instrumental measurements had significant errors and were adjusted for good reason or because of malpractice. My experience in observational astronomy suggests that errors are usually larger than people like to claim, but I know nothing of this specific instrument or research area.

Were Venus to have a negligible atmosphere like Mercury, I might try a simple calculation to work out how far in the past it would have been molten in order to get an estimate of when the catastrophe occurred. But the presence of that incredibly thick and complex atmosphere - which at the very least is going to provide a lot of insulation, runaway greenhouse or no, strongly suggests that this wouldn't give a meaningful result. Someone with better knowledge of radiation transport could probably model it.

So could we rule out cooling in some other way ? Hard to say, you should talk to a geologist or a meteorologist or a planetary scientists, or preferably all three. Depends what sort of catastrophe, I guess. The surface of Venus doesn't look much like Earth, or Mars, or Mercury - but then it's more massive than Mercury, much hotter than Earth or Mars, and has by far the thickest atmosphere. So probably it's not too surprising that it also has very different topography. The problem would be to differentiate between this naturally different morphology due to the different properties of the planet and the difference caused by whatever catastrophe is proposed. As far as I know, planetary models aren't sophisticated enough for us to predict things with this level of detail yet.

If Callisto and Hyperion were orbiting the Sun would they be considered to be planets ?
Q : Well, if Callisto (undifferentiated) and Hyperion (big and fluffy) were orbiting the sun, how would they be classified ?

A : [This question was in response to my proposed definition of "planet" to clear up the Pluto issue once and for all : My definition is quite simple - anything that's massive enough to be round (we can quantify exactly how round later) would be considered a planet while anything else wouldn't. But within that definition there would be many, many subcategories. For instance the current IAU list of "dwarf planets" wouldn't change, what are currently considered to be "planets" would become "giant planets", thus putting them on an equal linguistic footing. Further refinement could be used - for example, gas planets, rocky planets, icy planets, etc.]
If we take the definition I've proposed, Callisto would be considered a dwarf planet (or possibly a dwarf ice planet) since it's big and round. Hyperion isn't big or round so it would be considered an asteroid.

How can we detect planets around other stars when photos of planets in our own Solar System have such low resolution ?
Q : I got the impression that our science claims its ability to find earth-like planets from the distant of lightyears, yet failed to produce ultrahigh resolution pictures of moon, mars etc. At least to be openly shown to public. Earth-like, mars-like, venus-like, they have big differences in supporting life! Can you really tell how far those claims go for earth-like planets? i mean this kind of pict. Or does anyone have them in better resolution?

A : That Mars picture is certainly very low resolution but that looks like poor image compression by someone on the internet. Google searching, "Mars surface" or "Moon surface" will get you many, many vastly better images. But landers are extremely difficult : so far there have been successful landers on the Moon, Mars, Titan, a couple of asteroids and a comet, and that's all.

As for planets around other stars, we won't be getting high resolution images of them anytime soon. We can barely resolve even the nearest large stars, let alone planets which are hundreds of times smaller. Even detecting Earth-sized planets is very challenging, never mind imaging them. But we can make some determinations about what conditions must be like there if we know their orbit from their parent star, which will tell us roughly what the surface temperature is. If we could measure their spectra (i.e. colours), which is feasible in the next decade or so, we could work out the chemistry of their atmospheres. Pretty pictures, however, are a very long way off.


Are the stars all dead ?
Q : When looking up to the stars it's intriguing to know that the light you see has taken millions of years to reach the Earth. As you are effectively looking back in time (because the light has taken so long to reach the Earth) how do we know what exists in space at this very moment? Is there another way of viewing objects in space that gives a more recent representation than waiting for the light to get here?

Probably a stupid question but it crossed my mind that there could literally be nothing in space anymore but yet what we see is what used to be there...millions of years ago.

A : It hasn't taken all that long to reach us. More like a few thousand years or less. Only 4 years in the case of Alpha Centauri.

The most distant thing you can see with the naked eye is (pretty much) the Andromeda galaxy, from which light has been travelling for about 2 million years. But since it would take about 200 million years for the galaxy to rotate even once, there's just not enough time for there to have been any significant changes.

By measuring the velocities of stars and galaxies, in principle we could work out where everything is right now. This is difficult to do accurately, and there wouldn't be a lot of point to it. Since nothing can travel faster than light, we'd have no way to verify our findings other than waiting a long, long time.

Stars live for many billions of years, and the Universe is about 13 billion years old. New stars are being born all the time, so there's not much chance that all the lights have gone out yet. :)

Does dark matter burn ?
Q : Is it theoretically possible for there to be dark matter "stars" ?

A : Although dark matter is not thought to be able to undergo fusion like normal matter does, some models of it postulate that dark matter particles may sometimes annihilate each other when they collide. In that case, it might just be possible to get something like a dark matter star :
They're a pretty speculative concept at the moment at there's no evidence (to my knowledge) that they actually exist.

How can stars form ?
Q : If gravity is such a feeble force, how is it able to explain the current theories of stellar dust / gas coalescing together from tiny bits with almost non-existent gravity into bigger and bigger clunks ? Something is missing here and maybe gravity is not such an omnidirectional force as believed ?

A : Think bigger - it's not so much about the attraction of individual particles to one another, so much as the collapse of massive clouds. Once you form a cloud the mass of a star (this can happen through various instabilities, e.g. Jeans instability : it will collapse in a time dependent on the mass and size of the cloud. The mass of that cloud being many times the mass of a star, the collapse can easily happen on timescales of millions of years. As it does so, density increases, and the collapse gets faster.

Another way to think about it is that right now, the entire mass of the Earth is pulling you down. Similarly at the edge of a gas cloud, the entire mass of the cloud is acting to cause it to collapse. 

The cloud as a whole may have the mass of a star, but this is very, very different from saying that the center of a cloud has the mass of a star. Collapse of the cloud occurs because the total mass of the cloud is very large.

How does dust form ?
Q : Where's this dust coming from anyway? I thought that after 13 billion (or 40 billion) years there wasn't any left... there is still plenty of it so why is it not all gone yet and what regurgitates it somehow ?
[This question was part of a larger, unproductive discussion]

A : This not so well understood. Fusion inside stars can create heavier elements, which can then be ejected by winds and supernovae explosions. Exactly how much dust can be produced in this way is very controversial. However, what I think you're also referring to is not just the dust, but also the gas, which is a little bit better understood.

The reason there is still plenty of gas around is that star formation is more complicated than a simple collapse of a gas/dust cloud. The gas also has temperature (pressure) which resists the collapse. So first you need to cool the gas for it to start to collapse (exactly how this happens is complicated to say the least). It can also be supported by its rotation and (possibly) magnetic fields. Then, other stars disturb the gas by winds and supernovae (which also help replenish the interstellar gas - not all gas which forms a star is forever lost). So you don't have all gas collapsing at once - only a few, small regions where conditions are just right for the gas to collapse and stars to form.

See also star formation wars.

The lifecycle of stars
Q : Is there a sort of endless lifecycle or are we "blaming" again to be in a Universe just the right age that still have gas hanging around ?

A : Since we know of the existence of white dwarfs, neutron stars and black holes, the recycling isn't perfect - some material is lost. Eventually we will run out of gas (wiki sez this will take a few trillion years). Maybe it's interesting we live so close to the start of this very long era when the Universe has gas... that's almost a philosophical question. I don't know.

I am an evil supervillain, and I'm considering using a supernovae to destroy the Earth. Can you help ?
Q :If I wanted to destroy a planet, would a supernova do the job ? Can I use one to take out planets in neighbouring star systems ?

A : Certainly. Knowing the energy output of a supernova and the energy required to obliterate Earth (both from Atomic Rocket's boom table :, it's easy to calculate the maximum distance at which a planet would receive enough energy to be blown to bits moving at such extreme velocities that they could never recombine (i.e. escape velocity).

Depending on the energy of the supernova, this distance can be as low as 2.5 AU or as high as 250 AU (1 AU = 150 million kilometres, the average distance of the Earth from the Sun). But technically this is only the energy needed to move the entire Earth to its own escape velocity. Of course such an immense, sudden blast is going to do a lot more than give the whole thing a gentle shunt. The energy is equivalent to detonating a bomb made of TNT that's ten times more massive than the Earth.

The temperature of the mantle is typically well above the rock's melting temperature, so when it's exposed to space by the blast (releasing the enormous pressure keeping it as a semi-solid) it will melt of its own accord. A lot of it will even vaporise since it's even above its boiling temperature (iron boils at 2,800 C; Earth's core is at 5,400 C). So I'd speculate that what you'd get it a lot of gravel-sized grains rather than rather than mountain-sized fragments. Difficult to be certain though.

Supernovae are devastating for their own planetary systems, but not nearly powerful enough to actually blast planets apart in other star systems. At a distance of one light year, the energy received by an Earth-sized planet would be billions of times too low to actually destroy it. It might well still sterilise the planet from its radiation, but it wouldn't actually smash it into pieces.

Yeah but seriously, there's this planet I don't like and I don't want it to be there any more. I can use a supernova, right ?
Q : Can debris from supernovae destroy planets in other star systems ?

A : Sorry, but it's extremely unlikely. A large supernova will probably disintegrate just about every planet in its system (see above question). A less powerful one will merely destroy planets within a few AU. The fundamental problem is that you have to give a planet-sized body enough energy to destroy another planet-sized body without somehow destroying that same planet-sized body... tricky.

However, there's one other effect from a supernova that just might be able to do the job. If enough mass is ejected by the explosion, and it's not too powerful, distant rocky planets (say ~100 AU from the star) might just survive relatively intact as massive bodies. But now most of the mass of their parent star will have gone. They may now find themselves at the escape velocity from the stellar remnant, so effectively they get hurled off into interstellar space at several km/s (it's not likely to be much higher than this since orbital velocities are lower the further you are from the star - get closer and the velocities will be higher but you also get closer to the obliteration zone).

Assuming that things are setup just right - supernova just powerful enough, and planets just far enough away - that's potentially sent a swarm of hundreds of rocky bodies careering through the galaxy. Sounds deadly ? Only in the ludicrously unlikely event that they ever hit anything. My favourite analogy for the size of space is to shrink the Sun to the size of an aspirin - at that scale the nearest star is around 300 km away. Planets would be the size of specks of dust. And if you scatter a a few hundred dust grains, the chance that they will hit another dust grain 300 km away is basically nil. A probability of about 1E-22, by my reckoning.

How do we know how far away the stars are ?
Q : How can we know for certain that a star is as far away as we think it is ? I looked it up on Google and it said that for stars over 400 light years away we had to rely on how bright the star is. Is this a precise way to measure the distance from here to a star ?

A : It's true we can only directly measure distances for stars within a few hundred or few thousand light years, using the parallax method ( Beyond that, we have to use other methods which are checked using stars for which we can get direct measurements. There's a variety of different methods used to reach further and further distances, the distance ladder : The further away the object is, the greater the measurement error.

The Gaia mission will greatly increase the number of stars for which we can get accurate distance measurements. As well as giving a more accurate picture of the Galaxy, it will allow us to get more accurate estimates of objects at greater distances :

Are stars born in stellar nurseries all very different to each other ?
Q : Why do you think stars in the same nursery have different make-up ? The nursery is very large, a light year across or more, there could be different constituents in that large of a space. I think our star grew up with Barnard, Proxima Centauri, and 4 other siblings, 7 all told.

A : I don't think they do have dramatically different initial compositions. Most interstellar gas (about 70%) is hydrogen.

What certainly will have an effect on their later composition is their mass. Low mass stars slowly and steadily fuse hydrogen into helium for trillions of years. High mass stars have a much higher rate of fusion, quickly exhausting their supply of hydrogen. Then they begin fusing helium into carbon and then carbon into heavier elements.

We probably didn't grow up in the same cluster as our current neighbours. The Sun is about 5 billion years old, so it's orbited the galaxy probably twenty times. Stars have random motions as well so most likely our current neighbours were formed somewhere else.

Do supernovae form planets ?
Q : It's my understanding that depending on mass, several things can happen at the end of a star's life, one being supernova, one being implosion/creation of a dense black hole. Do supernovas not stable star nurseries called nebulae ? Or is that a spin off of the star life cycle, that one of the 2 can occur, star nursery or planetary formation ?

A : Stars are born inside nebulae. When they die a lot of their mass can be returned to the nebula and form more stars.

Really low-mass stars can live for trillions of years. Eventually they swell into red giants, but then collapse again. Not much of their mass ever gets back out.

Larger stars live for a few billion years but when they reach the red giant phase they may also shed their outer layers into a so-called planetary nebula. It's a historical name, it doesn't actually have anything to do with planets. A lot more mass gets re-inserted into the nebula from these stars.

Really massive stars (say > 10x the mass of the Sun) explode as supernova after a few million years. Being incredibly hot, they can also spend a long time shedding much of their outer layers. The surviving remnant of the explosion can form a neutron star or black hole depending on how massive it is. Most of the star ends up back in the nebula either from the winds from the star or from its supernova explosion.

So yes, stars can and do form nebulae, and nebulae can and do form stars. Planet formation, it's thought, happens alongside star formation. A small part of the nebula, being a bit denser than the rest, collapses. As it does so it forms a disc, since random motions within the nebula will have given it some spin. Most of the mass ends up in the centre as a star, but some parts of the disc can fragment and form planets in orbit around the star.

At least, that's the super-simplified version. Huge amounts of details are not well understood.

Can proton stars and neutron stars share the same space ?
Q : Can a proton star and a neutron star share the same space, or does there have to be some distance between the two ?

A : Neutron stars form because the star gets so dense than the protons and electrons can combine (neutrons being very slightly heavier than protons). You can't combine a neutron with anything to make a proton, so proton stars can't form. Anyway if it did happen it would have a massive positive charge and blast itself to bits.

When the massive UY Scuti explodes, will we feel the effects on Earth ? How bad will it be ?
Q : When UY Scuti goes nova, because light waves, shock waves/gravity waves, and sound waves all act differently... how soon after would those in our Solar system have knowledge of that, not by the Hubble or it's replacement, but physically ? Because UY Scuti is so distant, does that mean our descendants won't feel anything for centuries ? Or would not the light, gravity wave and sound waves travel that distance ?

A : The energy released from a type 2 supernovae (as UY Scuti will be) is massive : But the distance is also massive. If the explosion is roughly spherical, the energy received per square meter on Earth comes to around 3000 J (3E44 / r*pi*8.98E19^2). That's more than I was expecting, but still isn't very much - enough to heat up 1 kg of water by about 0.7 C (although not really because the energy release takes a few days/weeks, so the actual power, the rate of energy received, will be much lower). We won't even receive this much energy though, because the intervening gas and dust will absorb a lot of it. My guess is that like the supernova that created the Crab nebula (6,500 light years away - a little closer than the 9,500 light years of UY Scuti), it would be visible as bright star but nothing more than that :

If the Sun is expanding into a red giant, can we observe this process happening ?
Q : Have there been any type of measurements of the Sun taken to see how much it has grown over the centuries ? Also, since Mercury is so close to it, would our first sign of it growing and expanding by taking Mercury out. Sorry, I'm not sure how to word that last sentence and make sense.

A : Offhand I'd say the expansion process is so slow that we won't have been able to measure it over the course of a few centuries. Quick check : let's say it take about 1 billion years to double the Sun's diameter, that means it's growing by about 1 metre per year. But, the expansion is likely to be strongly non-linear, so probably it will be even slower until eventually it expands much more rapidly.
Mercury is a few tens of millions of kilometres from the Sun, so we would certainly be able to measure the Sun growing well before it starts to engulf it.

How often are new stars born ?
Q : How many stars are born in our galaxy every year, and how many are there in total ?

A : The number "1 new star every 18 days" is often quoted, though I'm not sure where it comes from. It's equivalent to a star formation rate of about 20 stars per year, which is extremely high. Other estimates go as low as about 1 star per year; some say 2-3, the highest I can find is 7.

A caveat is that star formation rate is typically estimated by mass of stars formed, not number of stars formed, and some stars are less massive than the Sun. But most of the links seem to suggest that this isn't that important.

As for the total number, probably 100-400 billion.

Do stars explode when they're young or old ? What sets them off ?
Q : Is it just age that makes stars explode, or is there more to it than that ?

A : What happens to a star depends pretty much only on its mass. If a star is greater than a certain mass (>~5x the mass of the Sun), it will eventually explode after a few million (or perhaps a few hundred million at most) years. There are no massive stars of a different "type" that avoid this, except maybe for a few which just collapse directly into a black hole.

Also note that novae and supernovae are different phenomena. Supernovae are exploding stars that are unable to continue fusion. Novae are eruptions on the surfaces of white dwarfs (which are long-dead stars that no longer shine by fusion) which are accreting material from a nearby companion star.

So it depends on your definition of age. You could very well say that a star is "old" based on how far through its life cycle it is - that's a perfectly reasonable definition. On the other hand it's also reasonable to say that only young stars explode, in the sense that supernovae occur very soon compared to the death of low-mass stars, which live many times longer than massive ones.

What would happen if a star from one system went into another star system ?
Q : Would it be out of the realm of possibility for a rogue star to either join with or kick out an existing star, in a solar system ? I tried looking online for the answer to that question and all I got was Star Wars related links. So, I figured I'd come here and ask the question. I know stars have been kicked out of their orbits, but not sure how that happens.

A : A star passing through another solar system could certainly cause chaos. Stellar mergers are not impossible, but they are very rare because the distances between the stars are so massive.

Stars can scatter each other out of their original systems though. The gravity of two stars moving past each other can change the orbits of both stars around the center of the galaxy (just as the trajectories of space probes change due to the gravity of planets in the solar system).

Another way this can happen is if in a binary star system, one explodes - the other star no longer orbits anything, so it gets flung off into space.

"Rogue stars" are known to exist which are not part of any galaxies. These are thought to form when galaxies collide and parts of them get torn off into intergalactic space.

If two stars really like each other, can they merge ?
Q : If two stars interact and get close enough, can they merge to become a bigger star ?

A : Yes, but it would depend on the exact geometry of the encounter. Mergers are believed to happen inside star clusters, and are thought to be important in the formation of massive stars (at least some people think so) :

That can happen in the case where the stars orbit each other and lose energy through tidal forces. It's not as simple as having the stars get close enough to each other, e.g. Phobos is moving closer to Mars whereas our Moon is moving further away from the Earth. The point is that it's a slow process which can take millions of years, so the stars gradually merge rather than slamming into each other.

Direct head-on collisions are very rare because the distances between the stars is so much greater than the size of the stars. If it did happen, it's safe to say it would be a spectacular event. Because of the gravity of the Sun, any star which collided with it would be moving at at least 617 km/s by the time it hit. That would release as much energy as the Sun produces in ten million years.


Are rogue stars dangerous ?
Q : A rogue star, primarily known as an intergalactic star, is a star that has escaped the gravitational pull of its home galaxy and is moving independently in or towards the intergalactic void. More loosely, any star in an unusual location or state of motion may be termed a rogue star. In other words........ IT WILL KILL OUR GALAXY IF IT WANDERS IN OUR GALAXY!!!!!!!!!!!!
[Yes, this quote is verbatim]

A : A rogue star is absolutely no more dangerous than any other star. The mass of a star is about one hundred billion times less than the mass of a galaxy. There are precisely ZERO ways in which a star, rogue or otherwise, can destroy a galaxy.

How one goes from "unusual location" to, "threatens the entire galaxy" is quite beyond me.

Were there larger stars in the past ?
Q : What is the likelihood that there is a star larger than UY Scuti closer to the beginning of the universe ?

A : Extremely high. In the early Universe there was nothing but hydrogen and helium, whereas today there are also a great deal of heavier elements. These can be important in gas cooling, which is what you need to get gas to collapse and form a star.

Since there were almost no heavy elements in the early Universe, cooling was not as efficient. To overcome the pressure from the heat of the gas (pushing it outwards) would have required much higher gas masses. Whereas UY Scuti is maybe 50 times more massive than the Sun, early stars may have been many hundreds or even thousands of times as massive as the Sun.

How come I can see Polaris (the North Star) all year round ?
Q : Do I have a celestial stalker or something ? I mean, jeez, take a hint already !

A : The size of the Earth's orbit around the Sun is extremely small compared to the distance to even the nearest star, and the Earth doesn't change its orientation as it orbits. So the north pole of the Earth is always pointed towards Polaris.

Are we likely to see a supernova with the naked eye any time soon ?
Q : I want the Earth to have a second Sun, damnit !

A : It's not impossible, but don't get your hopes up.

Will gravitational waves provide advance warning of supernovae ?
Q : Is that why NASA has sent out the gravitational wave "buoys", So that Earth could "at least have a chance", at preparation ?

A : Gravitational waves should travel at the speed of light (if the theory is correct) so they won't give any advance warning. But they may be useful in finding supernovae that are hidden by all the gas and dust that blocks our view of much of the Galaxy.

But mainly the point of looking for gravitational waves right now is to win a Nobel prize. :) They're on of the few (possibly the only) remaining major predictions of General Relativity that has yet to be observed. People have dedicated their lives to finding them, so the detections would be an end in itself. A non-detection would be even more exciting, because then we'd know that relativity has a serious flaw.

Would a supernova at the distance of Alpha Centauri be dangerous ?
Q : It's possible that I may have gone a little too far with my desire to see a second Sun.

A : Interesting question. Fortunately none of the stars in the Alpha Centauri system are big enough to go supernova. If they did, we definitely wouldn't get blasted to bits, so that's good news :

However, by my very quick and rough calculation, the energy we'd receive from the explosion would be equivalent to covering the Earth with one 100W light bulb in every square metre, and plausibly 100x that. Factor in that much of this radiation will be dangerous UV and gamma rays, and it's safe to say that things would be bad.

There's also a very nice explanation here :

Would we feel the blast from a supernova at the same time as we saw the explosion ?
Q : Now photons travel the speed of light (given) so some energies match that? Can particles travel a percentage of the speed of light ? Or would a sun going nova reach us en mass about the same time as its distance 20 light years away (20 years)? At 40 light years would the mass of the explosion take much longer and have less mass because of its expansion ?

A : Some particles will be accelerated to nearly the speed of light, but only a small fraction, and we're talking subatomic particles rather than large meteorites. Supernovae expansion velocities are generally measured at at most a few thousand kilometres per second (10% the speed of light), and usually less, so it would take decades or centuries after we see the explosion for the blast wave to hit us.

Indeed the mass that hits us will be much less than the mass of the whole supernova, because the explosions are very roughly spherical. Even at 4 light years, the fraction of mass that hits us would be far, far less than a trillionth of a percent.

Could gamma ray bursters explain the Fermi paradox ?
Q : The Fermi paradox, for unfamiliar readers, is that "if they existed they would be here", or, "where is everybody ?". That is, intelligent life has arisen at least once in the Galaxy (us), so it should have done so many times in the past as well. There's been more than enough time to colonise the whole galaxy by now, so where is everybody ?

Fr a complete overview see :

A : On GRBs, I don't know so much about them but I'm not sure we really know enough about them to say what influence they might have had on the Galactic habitable zone (

My feeling is that the Fermi paradox is still a paradox. Although there are large stretches of geological time where change was slow and gradual, there also seem to have been a few sudden, important events. Multi-celluar life didn't really get going until the Cambrian Explosion, possibly triggered by the end of Snowball Earth. The extinction of the dinosaurs was due at least in large part to an impact event. The development of the human brain seems to have happened in a few million years or less. So, it seems to me that there should be a spread of development of lifeforms, from some planets which are still hosting unicelluar life to others which could have civilizations much more advanced than ours.

Given that, and that even at sub-light speeds (which could be done with today's technologies if we really, really wanted to - a massive Orion ship or a solar sail comes to mind) the Galaxy could be colonized in just a few million or tens of millions of years, the lack of a Galactic Empire seems surprising.

None of the proposed explanations seem convincing to me. Life just isn't common ? But it seems to have started on Earth very quickly. Intelligent life is rare ? Perhaps, but once it started on Earth it took off very quickly indeed. Difficult to reach other stars ? Yes, but surely not that difficult. A Great Filter (e.g. a huge GRB) that prevents life from evolving beyond a certain point ? Difficult to believe a galactic-spanning civilization wouldn't be able to do something about it (even if that just meant hiding).

What's the deal with the star with the alien megastructures ?
Q : Come on, it's aliens, isn't it ?

A : [I wasn't actually asked this directly, but I'v written quite a lot of responses to this so I thought I'd collate them]

A : Normally my advice is to wait a year (literally a year) to decide if a press release story is really interesting or not. It takes a long time to do a proper, independent analysis, and often stories depend on quite complicated reasoning.

Not in this case however. Here we have something where the observational data show that something seriously weird is afoot, and no amount of analysis is going to change that. Out of literally millions of stars observed over the years, this one is unique.

"Tabby's Star" is weird because its brightness is changing in not one but TWO ways that have never been seen before. First though, it's a main sequence star - we know this from very precise measurmenets of its colour. This means it's in the most boring part of a star's life where all it does is sit there fusing hydrogen into helium. OK, it might have the occasional flare and starspots, but that's about it. Major changes should take millions of years to become detectable. For comparison, the Sun (which is only a little bit smaller and cooler then Tabby's Star) varies in brightness by around 0.1% over the course of an 11-year cycle.

Tabby's Star is doing two incredibly weird things. The first is that its brightness occasionally drops dramatically - by as much as 20% in a few days. No other (main sequence) star does this. The second is that its overall brightness has been steadily decreasing over the last century - also by around 20% in total. Unfortunately there aren't any records further back than this. Assuming this rate to be constant, it would have been (just about) naked-eye visible around 5,000 years ago, but to be unmissable you'd have to go back 7,000 years. So there aren't any ancient star catalogues which could help us.

Thus far, no-one has a good explanation of either of these behaviours. Even the alien megastructures hypothesis isn't without problems, because the heat from the star ought to make them radiate in the infra-red, but this isn't seen. Comets could cause the rapid dips, but it requires an impluasibly large number of giant comets - and an outright insane number for the century-long dimming (it's not obvious to me why the number of comets should be increasing with time either), plus there's again no IR excess. No-one seems to hae suggested that it's just not a main sequence star, presumably because the data is good enough to rule this out.

For once, the over-used "scientists baffled" headline may be right on the money. If a mundane explanation isn't found - give it year - then my guess is we're looking at 5-10 years before we can say with confidence what the heck is going on here. What that will turn out to be I have no idea.

If we dropped small wormhole into a star, would it cause a supernova ?
Q : What happens if you drop a black hole/wormhole into a star? How does it vary depending on the star (mass, stage of life/remnant), and the hole size (stellar, primordial, varied intermediary sizes) and type (black hole, wormhole with empty space on the other size) ?

There is one relatively famous SF example where they drop a (6.7m diameter, disc-shaped) wormhole in a (possibly Sun-like) star (with a planet in decaying orbit around a black hole on the other side) to blow it up it up in a matter of hours (and those are the good guys).

A : I am familiar with the example of which you speak. It always struck me as an odd way to creating a supernova given that stars which are less massive don't explode. Maybe it's about disturbing the balance between pressure and gravity by rapidly removing a lot of mass.

It's easy to estimate how much mass can be removed, but it depends strongly on where in the star the Stargate is since temperature varies massively from the interior (~15 million K) to the "surface" (~5000 K). That means the particle speeds will vary from something like 6.5 km/s to 350 km/s.

The average density of the Sun is only slightly higher than that of water, so the following approximation will give us a handle on how fast the Stargate will drop :

A terminal velocity of a few m/s means we're looking at several years for the Stargate to reach the core. Or maybe it never will, since the density of the Sun at the core is many times that of iron or even (probably) naquada, which isn't that heavy.

Anyway, near the surface let's assume the average density for the Sun - 1410 kg/m^3. The area of the wormhole is pi*(6.7/2)^2 = 35 sq m. Velocity of the plasma into the wormhole is 6.5 km/s. So in 1 second we get a volume of material equivalent to 35*6500 = 2.3E5 cubic metres = 3.2E8 kg. In 12 hours that's 1.4E13 kg. Alas, the mass of the Sun is 2E30 kg, so we've only decreased the mass by around 7E-16 %. This will achieve precisely diddly-squat.

If we allow the Stargate to have some technomagical propulsion so that it reaches the centre of the Sun, the total mass lost in 12 hours will be about 3E-16 %. Still useless. [Oops - I forgot to account for the greater density in the core, but since it's only 100x greater that's still nowhere near enough to be significant !]

You may be wondering about the black hole's gravity sucking in matter faster, as clearly shown in the original "oops, we've connected to a black hole" episode. Well, even at its most extreme that clearly never reached more than a few gs, which means the infall rate won't be significantly higher.

So the method in the show just doesn't work at all. As to whether removing a really large amount of mass would cause a supernova, I'm not sure. Maybe if you removed all of the outer material around the core, it would expand to reach a new equilibrium. I don't think it would explode though. To do that you'd need to increase the pressure drastically, and I can't see any way to do that by removing mass.

Stars being eaten by black holes have been observed. They don't explode, but they do do interesting stuff :

What if we dropped a large wormhole into a star, or into a neutron star ? Would that cause a supernova ?
Q : I want to cause a supernova using a wormhole. Can you help ?

A : Yes I can. if you threw a neutron star or white dwarf into a giant star, you could potentially trigger a supernova or at least a nova. Novae happen when material accumulates on the surface of a white dwarf to the point where there's a massive fusion explosion. I don't think this is possible in the case of a wormhole, where material is being drained.

Still, if you did remove enough of the star you could destroy a planetary system, albeit in a more stately manner. If you remove the entire fusing core of the Sun (which extends to about 0.25 solar radii) you'll have removed enough mass to send all the planets into highly elliptical orbits. Not quite enough to send them hurtling into intragalactic space, but close. Which is not terribly threatening to Apophis [Evil alien in Stargate whose ship is destroyed when our plucky heroes drop a womrhole into a star] though.

But then it hit me that the best way to destroy a star with a wormhole is obvious : don't link a star and a black hole, link two stars. Massive stars for preference. But if you really really want to make sure, link to a neutron star.

If you had a massive Stargate that could swallow the neutron star whole and spit it into the target star, you'll get a TZW object. That might be short-lived on astronomical timescales, but probably won't instantly explode. Probably the increased density of material around the neutron star causes an increased temperature, preventing material from quickly infalling.

However, if you have a regular sized gate (assuming it can survive), then things get much more spectacular. Pulled by the neutron star's tremendous gravity, the gate will slice through it at nearly the speed of light. As it does so it will spew neutron-degenerate matter into the target star. Without the gravity of the neutron star to hold it together, this will be bad. Like, seriously mega-hyper-total bad.

If a star turned into a black hole would it suck in all its planets ?
Q : You know how planets orbit the sun because of its intense gravity? The planets around this black hole are sucked inwards because of the intense gravity.

A : No, for two reasons. Firstly, most stars that form black holes first explode as supernovae (it's only the surviving core which collapses to a black hole) which most likely will destroy any planets which were in orbit. Secondly, the gravity of the black hole at any given distance is no stronger than that of the star.

The mass of the black hole is no larger than that of the star that created it - and usually a lot less as most of the star is blasted away in the supernova. So, if any planets did survive, they'd actually move further away from the star since the gravity at that distance is now weaker than it was before.

The gravity only reaches extremely high levels very close to the black hole. With the star, the strongest gravity will be at the surface of the star. If you were to move inside the star, the gravity actually gets weaker. Inside a sphere, it's only the mass beneath you that pulls you down gravitationally - and the deeper inside the star you are, the less mass beneath you, so the weaker the gravitational pull.

Now, with a black hole, things are different. All the mass is now contained within a much smaller region (technically an infinitely small point in standard GR). Let's say the star was 1 million kilometres across. At 1 million kilometres (or more) from the black hole, you experience exactly the same gravity as if the star was still there. The difference is that now, if you move closer, you experience higher gravity, because the all the mass of the star is still beneath you. So, if you were just 1 kilometre from the center of the star you'd experience basically no gravity at all (because almost all the mass is above you, so effectively doesn't count), whereas if you were 1 kilometre from the black hole you'd be experiencing massive levels of gravity since all the mass is still beneath you and now you're effectively much closer to it.

If there was, like, a teaspoonful of white dwarf material somewhere, would it destroy the Earth ?
Q : If you had a tablespoon of a white dwarf and you put it into your coffee, it would go through the cup, and travel through all the layers of the earth until it reaches the centre. Then, the earth would probably collapse...

A : The density of a white dwarf is incredibly high, but a teaspoon of white dwarf matter would only weigh a few tonnes - not enough to cause the Earth to collapse.

If there is no oxygen in space, Then, how the sun and other stars are burning ?
Q : Yer fancy book-learnin' don't scare me nuffin !

A : Stars are not like giant lumps of coal. They shine by nuclear fusion, where light elements combine to make heavier elements. In the process, a very small amount of mass is converted into energy. But honestly, googling "how does the Sun shine" will get you dozens of good website explaining this. Here's one :

Are the stars we see in the night sky just in a small part of the Milky Way ?
Q : Is this meme about all the stars you can see being within this yellow circle accurate ?

A : Yes it is ! Well, within reason. Of course we can also see other galaxies, e.g. the Andromeda Galaxy, which is about 2 million light years away :

However, to me the word "stars" in this context suggests individual stars, not star clusters or galaxies. And it suggests stars we actually do see routinely, not by deliberately looking for a particular star at a particular distance. So I downloaded the image and measured the size of the circle, and by my reckoning it's around 3,300 light years in radius. That's pretty much on the money as far as seeing individual stars with the naked eye goes.
(If you Google this you'll find that there are maybe one or two stars outside this circle but I think this is case of unnecessary pedentary. Probably > 99% of stars are within this circle, which in my view is good enough)

On the other hand, once you allow, say, double stars and other multiples, things quickly get much, much fuzzier. If you count supernovae, then they can be seen at very much greater distances indeed. But obviously we don't normally actually see them, and the meme seems to be about what we usually see.

If the distances between the stars are so large, why do they look so close together in photographs of other galaxies ?
Q : Well, why ?

A : There are two reasons for that. First, the stars illuminate gas and dust within the galaxies, so as well as the light that comes directly from the stars, you're also seeing light reflected off nebulae within the galaxy - which are much larger than the stars.

Secondly, and more importantly, there's the effect of resolution. Your eye (or a telescope) can only perceive details down to a certain size - but features smaller than that can still be seen. The size they appear to be is determined by the point spread function, which depends on the size of the lens (among other things) :

Simpler explanation : Imagine someone holds up a shiny coin which reflects sunlight toward you. If they're standing 10 metres way, it'll look like a bright shiny coin. 100 metres away and you'll have no idea what they're holding, but you'll still see a bright point of finite size. 1000 metres away and it will look exactly the same, just dimmer. The important thing is that beyond a certain point, the coin won't look any smaller.

It's the same with stars through a telescope. If you have high enough resolution, what appear to be diffuse clouds can be seen as individual stars. Hubble has been able to do this for nearby galaxies, but beyond a certain distance even its resolution isn't high enough - everything looks fuzzy.

If a star dies , does it still have the same mass ?
Q : Well, does it ?

A : Mostly no, but it depends on how massive it was to begin with.

Really massive stars don't live very long (a few million years or less) and emit strong winds. More than half of their initial mass can be ejected into space before they finally explode as a supernova. The final remnant core could either be a neutron star or black hole, but is usually only a small fraction of the initial mass of the star.

Less massive stars like our Sun live a lot longer (a few billion years) but don't explode. Their winds are much less violent. However when they start running out of hydrogen fuel they expand into red giants. They can then shed their outer layers to produce so-called "planetary" nebulae (which are nothing to do with planets), leaving behind just a small white dwarf which again has just a small fraction of the star's initial mass.

But very low mass stars have even less violent histories and can shine for trillions of years. They never shed much of their mass or explode. So the final white dwarf is probably much closer to the star's initial mass than for larger stars. Not sure of the numbers though.

[Someone pointed out that the stars convert mass into energy, with the Sun losing 5 million tonnes per second. This is true but irrelevant.]

5 million tonnes per second is actually not significant. Over the ~5 billion years of the Sun's existence that comes to a grand total of just 0.04% of the Sun's total mass.

If a star becomes a black hole does its mass increase ? Does its size depend on its density ?
Q : If a star becomes a black hole, its mass will be some fraction of the mass of the star. But if there is surrounding material, the mass of that black hole can then increase over time. For example most stars are binaries, so the black hole might grow by accreting material from the second star. This is how stellar mass black holes are detected.

It's not really correct to say that the more the density, the bigger the black hole. Technically the density of the singularity is infinite - which is a big problem for relativity. For a non-rotating black hole the size depends only on its mass, not the density :

What's it called when you live in a binary star system and one star occults the other? Is it still an eclipse ?
Q : Apart from "brrzzzkkglblob", or whatever the alien is for "eclipse", obviously.

A : I think it would still be called an eclipse, since the systems where this is observed are called eclipsing binaries.

How close can two stars get without colliding ?
Q : I read that there is a planet which orbits two suns. I don't remember the name, but what is the closest two suns can be together without colliding? I imagine it depends on the size too..?

A : The absolute distance will depend on the sizes and masses of the stars. But there isn't really a strict limit, since stars can actually be in contact with each other yet still orbiting each other for many millions of years without merging.

What's going on with Tabby's Star ?
Q : Can somebody explain this to me ? More and more scientists are claiming that the star dropped to 20% in terms of light emission. Some are even now hypothetically stating that it is a Dyson Sphere. Is it still possible that gas or cloud deposits enclose a star ? Wouldn't that destroy rocks or anything close to it due to heat ? (sorry not an astronomer)

A : It's probably not a Dyson sphere. Anything that blocks the light should re-radiate it in the infra-red as it heats up, but no infra-red excess has been observed.

That said, none of the other proposed explanations (and there aren't many) do much better. It could just possibly be a swarm of giant comets, but there would need to be an awful lot of them and while they might explain the sudden dips, they wouldn't explain the slow dimming.

Since no other star shows behaviour quite like this, right now we just don't know. We need more data. Fortunately there are plans and funding in place to get it :

Paul Carr has compiled by far the best summary of the situation :

Shouldn't a Dyson sphere be heat resistant and therefore hard to detect ?
Q : Are the materials resistant to heat ? Wouldn't it be logical for engineers of the "dyson sphere" to construct it using materials or elements that does not harbour any heat to prevent damage or catastrophic failure ? And doesn't a Dyson Sphere supposed to absorb energy and heat not reject or radiate it?

A : Being resistant to heat (i.e. maintaining its shape and rigidity at a high temperature) is one thing, but absorbing but not radiating heat is quite another. That would be entirely new physics. No such material is known to exist - in any case, you'd probably want it to radiate heat otherwise its temperature would only increase.

Although Dyson spheres are supposed to absorb energy (in order for the inhabitants to make use of it), there's no known way they can absorb energy without re-radiating some of it. Just as, for example, solar panels absorb a lot of the Sun's energy and convert it to useful electricity, but they still get hot and radiate waste heat. Physics says this must happen. Actually, people have done deliberate searches looking for Dyson spheres based on the waste heat they should be re-radiating.

Could the dimming of Tabby's Star be caused by evaporating comets or sunspots ?
Q : If a star is surrounded by comets circling it, wouldn't there be possibilities of some of that ice melting and causing a reflection of the star...similar to the wavelengths of light reflecting off a swimming pool...uneven and off-focus light reflections ? I was thinking along the lines that the water is in the comet like a thick viscosity...light would dim if more 'viscos mass' was in direct path? Doing such could also make the 'comet/ice planet' a perfectly placed magnifying lens towards us and its dimming because the refraction is no longer towards us?
Another thought was sun-spots...but those would have to be huge, really huge?

I did all ?'s because a . makes it look like I know, but I'm just throwing ideas...I'm sure if we keep an eye on it we'll figure it out.

A : It won't melt exactly. Because the pressure is so low, it will sublimate directly from ice to water vapour.

Comets could possibly explain the sudden dips in the emission, but not the slow, steady dimming that's been observed. However, I've been wondering if something like huge cometary tails blocking the light could be responsible, rather than the comets themselves. I'm not sure. The dips seem to occur very quickly, within a matter of hours, then lasting for a few days or weeks. My guess would be that if it was comet vapour causing the dimming, the time to reach the lowest brightness and the total time of the dip should be roughly the same, since comet tails are generally pretty diffuse things.

I don't think it can be sunspots. The rotation period of the star is known to be about 1 day, so if part of the star got significantly darker, we should see the brightness varying rapidly every day as the bright/dark parts moved in and out of view. In principle a really large band-like sunspot stretching around the entire star would avoid this, but this would be a super-weird discovery, much larger than anything ever seen on the Sun.

Then again since this discovery is unique (not to mention the long-term dimming), whatever explanation is correct is going to be interesting. In this case, extraordinary evidence may require extraordinary explanations. The difficulty is that it's easy to come up with a really weird explanation for a really weird event, but very much harder to determine if it's correct. Maybe it's a combination of factors. Maybe all the speculation so far is completely wrong. I doubt we'll know for sure until we get more data.

Could stellar winds prevent dark matter from entering star systems ?
Q : Could the "Solar winds" be preventing the DM from entering (for the most part) any solar system ? If DM has very little weight, the closer it gets to a radiating object the stronger it is pushed back and clumps up.

A : There could be some dark matter in the solar system. Not very much, but a little. If dark matter is the diffuse, collisionless substance we think it is, then it fills the entire galaxy more or less uniformly.

I think that in order for dark matter to be affected by the solar wind, it would have to interact with normal matter or photons. If it absorbed light, we'd be able to see regions where the stars (or whole galaxies) looked darker due to the dark matter blocking the view. Such regions do exist, however the evidence is extremely strong that these are caused by dust. Models predict that the dust should cause not just absorption of the light but also reddening, as observed. Dust also re-emits in the infra-red, which is also observed. As far as I know all absorption regions are consistent with being due to dust rather than dark matter (e.g.

If dark matter interacted with the particles in the solar wind it would have a clumpy structure in the galaxy and this would then affect the motions of stars. It's hard to say how significant this would be, but my guess is we'd be able to see different motions of the stars in regions of different stellar densities. On the larger scales, the standard model of dark matter being totally collisionless seems to work pretty well. The most famous example is the Bullet Cluster, where two galaxy clusters have collided. The gas in the clusters gets stuck in the middle, while the galaxies and the dark matter keeps going - exactly as the model predicts.

Why does a star's gravity get stronger when it dies ?
Q : Why the gravitational pull gets so strong when the star dies and its size and mass compress, why is it not that strong when the nuclear fusion is going on inside the hole and the star is of a huge size and mass ?

A : That's because of the density of the star is much lower before it collapses. If you were able to travel through the Sun (or the Earth, or any other spherical object), the strongest gravity you'd experience would be at the surface. It turns out that as you go deeper into an object, the only gravity you feel is from the mass below you (closer to the centre). At the very centre you feel no gravity at all because all the mass is above you.

Imagine being inside a hollow shell. Strength of gravity depends on mass and distance. It turns out that the gravity from the smaller mass of the part of shell closest to you is exactly balanced by the larger mass that's further away. The maths-heavy version is here :

Since a sphere is the same as a series of shells, only the mass closer to the centre matters.

When a star stops fusing, its core collapses. Some of the star can be blasted away, but not much, so let's ignore that. Say you were in a spaceship orbiting just above the surface of the star, and you keep your ship at that exact same distance from the centre. As the star shrinks, the gravity you experience doesn't change - the mass below you is the same, and you're the same distance from the centre. So if the Sun were to shrink, we won't notice any difference - the planets would stay in their same orbits. But now you can fly your spaceship lower than you could before. The mass pulling you down is the same... but you're closer to it. So the gravity can be much, much stronger. Acceleration due to gravity is given by a = GM / r^2, where G is the gravitational constant, M is the mass below you, and r is the distance away. If you keep the mass constant but halve r, the acceleration increases by a factor of four. Make r 10% of the initial value and the acceleration becomes a hundred times stronger.


Are "rogue stars" part of dark matter galaxies ?
Q : Every now and then "rogue" wandering stars are discovered, and I am curious if they might, in fact, be luminous matter that are part of a predominantly dark matter galaxy.

A : Galaxies which are mostly dark matter have been proposed as a solution to the "missing satellites" or "dwarf galaxy" problem. Simulations of the evolution of the Universe have predicted about ten times as many dwarf galaxies as are actually observed.

Until very recently, these simulations have only included the dark matter since the calculations of the gas and stars are much more complicated. Theorists try and work out what the simulations really predict by using simple scaling relations from observations : e.g. a dark matter halo of this mass will normally contain this much gas and this many stars.

It has now become possible to include the gas and stars in the simulations directly. One preliminary, non-peer reviewed suggests that dark galaxies are still important :

Dark galaxies are viewed as intensely controversial. There is not yet a consensus as to whether a dark matter halo could accumulate a detectable amount of gas without forming any stars. My own research involves looking for dark galaxies by searching for hydrogen clouds without stellar counterparts. There are a few very interesting candidates, but none so far that are entirely convincing.

However, could these rogue stars be parts of largely dark galaxies ? Tricky. Star formation theory isn't all that well understood. The first massive stars which formed might eject the remaining gas through stars and supernovae explosions, however, it's difficult to believe that just one (regular-sized) star would survive. Additionally, rogue stars can be explained by galaxy collisions or possibly by one star in a binary system exploding :

In short, the answer is "probably not".

EDIT : Update ! Over the last year, so-called "ultra diffuse galaxies" have become an incredibly hot topic in extragalactic astronomy. Currently we know that there are large numbers of very faint galaxies and we think they're extremely dark-matter dominated. Recently some researchers have claimed that three isolated stars could indeed be part of a dark galaxy. I haven't read the paper but I'm rather skeptical based on the press release. You can't get kinematics from just three stars, so without gas measurements there's no way to know if the stars are in a dark matter halo or not (except maybe through gravitational lensing). I stand by the original answer of "probably not", but I thought this new information should be added.

If the Universe is expanding, how can galaxies evolve through merging ?
Q : Do you have an explanation for the counter-intuitive theory of galaxy evolution through merging when dark energy is (possibly) separating galaxies apart ?
How about something a bit science-fictiony like an invisible thread that from the central black hole keeps each star in the galaxy together in whatever shape it is and then travels through the Universe as a whole based on its needs to keep everything in balance ?

A : Dark energy is supposedly causing the acceleration of the expansion, but was much weaker in the past. So mergers were far more likely in the past. Moreover, the expansion is only really important on very large scales - get galaxies close enough and their gravity easily overcomes the expansion.

... at this point, the discussion should have ended, but it didn't. I include the following an an example of someone behaving as though their ignorance trumps genuine science.

Questioner : Rhys, how convenient: apply a different set of rules each time: very small, quantum; regular, gravity; very large, dark energy. it looks like we'll never find a unifying theory this way.

Me : Ummm, no, that's not what I said. I said one force dominates over another at different distances. The expansion of the Universe still happens on very small scales, it's just not enough to overcome gravity. An analogy would be that you're currently feeling a larger attraction to the Earth than the Moon - it doesn't mean the Moon's gravity is having no effect at all, only that the Earth's is much stronger. It's simply the case that one number is greater than another, that's all. This is not at all the same as the conflict between quantum mechanics and general relativity, which are fundamentally different.

The discussion continued unproductively. As far as I'm concerned my answer fully satisfies the question.

Can radio signals travel faster than light ?
Q : If a radio signal from a nearby galaxy can be gauged at 4x's the speed of light, does that potentially mean that either there is a more advanced life force out there, or does it mean that a planet or star went boom and its remnants could be headed our direction ? Would it not be a wise idea to plan for a "shower" of some sort ?

A : This is called "apparent superluminal motion". If you were to watch, say, a blob of gas in a fast-moving jet move across the sky, and assumed it was only moving across the sky, the calculation could show that it's moving faster than light. In reality, the jet may be pointed toward us. If so, the blob gets closer to us, so the light now has less distance to travel to reach us than it did originally. If the jet is pointing almost directly towards us and moving very fast (close to the speed of light) it can appear that it's moving faster than light.

Basically, the signal arrives unexpectedly early, but only because it was emitted from somewhere that's now significantly closer to us.

I don't know how exactly the inclination angle of the jet is determined, but this effect was predicted before it was discovered (1966 by Martin Rees).  It isn't really faster-than-light travel, it's a projection effect. Here's a better explanation than mine :

Do we know exactly when we'll collide with Andromeda ? Are we measuring things correctly ?
Q : If Andromeda is the closest galaxy to the Milky Way galaxy, how do they measure the distance, from center to center or from the outer edge to the outer edge ? The reason I ask is because if it is from center to center, the two galaxies would have collided long before the two centers even kiss each other... Is that a fair assessment ?

A : In terms of estimating when the collision occurs, it makes little difference. Andromeda is about 2 million light years away, while Andromeda and the Milky Way are about 100,000 light years across. The errors in determining the distance are about the same as the size of the galaxies involved. So at the moment, it really doesn't matter if the distances are center-to-center or edge-to-edge, because the uncertainties are too large.

Will the gravitational pull of Andromeda cause it to collide with us as faster-than-light speeds, or at least get faster when it gets near us ?
Q :  Even though Andromeda is 2 million miles away, currently; as far as I know; galaxies don't have a speed limit... unlike humans. So I was thinking that that 2 million light year gap could lessen depending on the energy and gravitational pull or conversely repulsion of the two galaxies. Does that make sense?

A : Well, maybe. :)

We've been able to measure the speed of Andromeda both across the sky and towards or away from us. That means we know its true 3D motion through space. Based on that, it appears to be heading straight for us, and will collide in a few billion years.

The collision will be long, drawn-out, and above all messy. Tidal forces will stretch the galaxies as they get close together - the edges of the galaxies will be more strongly attracted than their centres, due to the difference in the gravitational forces. We can model how that will happen to get some idea of what it will look like :

The only "speed limit" galaxies have is the speed of light, but neither the Milky Way nor Andromeda will ever move anywhere near that quickly relative to one another. The speed of light is an absolute limit because accelerating something to that speed requires infinite energy.

Can the debris of an exploding star damage our galaxy and/or travel faster than light ?
Q : I was thinking that external forces, like the eventual implosion of Betelgeuse would cause waves of movements of other galaxies. Meaning that, the way I am interpreting things, once Betelgeuse explodes (even if it already did), it would cause inertial waves of other galaxies (due to its immense size, is how I was thinking), speed would then increase over the speed of light.

A : Nothing can accelerate faster than the speed of light because that requires infinite energy.

Betelgeuse, or any other star which explodes, is just one of ~100 billion in our galaxy. It's only a few times the mass of our Sun - its "immense size" is, to be blunt, completely pathetic compared to the Galaxy. Supernovae are powerful, but they're not that powerful... the explosion will re-distribute most of the star's mass throughout the galaxy, but it won't actually change the overall mass of the galaxy at all.

I am not sure what you mean by "inertial waves of other galaxies". The mass of the galaxies is not affected by supernovae (OK, maybe a very little, negligible loss due to gas that's ejected by the explosions); if the explosion is perfectly symmetric then a supernovae doesn't even emit gravitational waves. The gas within the Milky Way will absorb the blast wave from any supernova - it won't propagate beyond our galaxy.

The interstellar medium - the space between the stars - is not empty. It's extremely thin - actually it's closer to empty than the most perfect vacuum ever created on Earth, but it's not empty. If we could see the hydrogen gas that fills the Galaxy, the night sky would look far more spectacular than what we actually see (shameless plug

When a star explodes, its matter is flung outwards, sweeping up all this interstellar gas, and so the blast wave gets heavier. Conservation of momentum means that it slows down and eventually stops. I don't remember the exact number off the top of my head, but the total mass of gas in our galaxy is similar to the total mass of stars. So, the matter from one exploding star is very, very much smaller than the mass of gas it would have to sweep out of the way to leave the galaxy.

The momentum of the explosion also cannot change the motion of our galaxy. If it's spherical, then the total momentum is zero, because everything is moving in opposite directions. If, somehow, the explosion was not symmetrical, and the whole mass of the star just decided to rush off in one direction, then the Galaxy would have a small momentum change. But remember the mass of one star is still far less than a billionth the mass of the whole galaxy - a supernova just isn't enough to really affect the galaxy.

What happens to a galaxy after its central supermassive black hole evaporates ?
Q : If black hole shrinks and vanishes then what occupies the space of black hole and how when there is no strong gravitional force ? And galaxy having one black hole in center continues its way or is dispersed ? If nothing occupies the space after black hole evaporates and its gravity becomes weaker then what provides centripetal force to galaxy ?

A : Even supermassive black holes are much less massive than their host galaxies, so nothing much will happen to the galaxy when the black hole evaporates. Mind you, their evaporation time is so long that all the stars would have died long before this.

Supermassive black holes have masses typically around a few million solar masses, whereas galaxies have more like a hundred billion solar masses in stars alone (plus a similar amount in gas and maybe around ten times more in dark matter). This is the source of the gravity holding them together, not the black hole. All together, the supermassive black hole makes up less than 1% of the mass of its host galaxy.

The spin (centripetal force) of a galaxy arises from very early in its formation as clouds of gas move past each other. This won't be affected by the loss of a black hole, which is just not large enough to have much effect on the galaxy (except for stars which are very close to it).

Could galaxies be rotating too quickly due to electical forces rather than dark matter ?
Q : Why can't an electrical or static charge be involved for dark matter they way they do on the space station with a statically charged knitting needle ?

A : Several reasons. Since the Universe seems to be electrically neutral overall, it would be hard to get any object to have a strong charge : it would tend to attract opposite charges and become neutral again.

Most of the gas in galaxies seems to be neutral atomic hydrogen, so it's not obvious how an electrical field could give it a strong motion, or where that field would be generated so far from the center of the galaxies.

Gravitational lensing measurements of clusters of galaxies indicate there is missing mass, which agrees with the strong motions of the galaxies. While this doesn't necessarily rule out modified gravity, I think it does rule out electrical forces. AFAIK the electrical force cannot deflect light.

The Bullet Cluster (and others like it) provide particularly strong evidence. Here the gas (hot, ionised gas, not neutral this time) and stars have become separated. Gravitational lensing measurements indicate that the dark matter has followed the stars and become separated from the gas. No lensing effects are seen from the gas. So, even if an electrical effect could deflect light (which AFAIK is not possible), it would be strange that it is only associated with the stars.

Will the Milky Way collide with the Andromeda galaxy in the future ?
Q : Is there real evidence that Milkyway and Andromeda will collide in the future ?

A : Measuring its velocity along our line of sight is easy. Measuring its velocity across the sky is much more difficult, but the results say it's basically negligible. It's pretty much headed straight for us.

How can there be hot gas in deep space if space is so cold ?
Q : Gas cooling in space ? I was under the impression that space was a very cold place until one was close enough to a star then in the stars radiant field things could be heated ? How in the vastness of space can gas be hot, warm or cooling ?

A : The short answer is that the density of gas in space is very, very low - about 30,000,000,000,000,000,000 times less than air at sea level, and about 100,000 times thinner than the most perfect vacuum ever created in a laboratory. This low-density gas doesn't need much energy to heat it up. It's so thin that it tends to stay hot because collisions between particles are very rare (roughly, every few million years or so IIRC).

Because the density is so slow, if you were floating unprotected you'd freeze to death in this hot gas. The amount of energy you'd lose by radiation would be far larger than the amount you'd receive from the gas.

It's not true of all gas - when its density becomes higher, collisions increase so it loses energy much more quickly. Then it can become very much colder, sometimes only a few degrees above absolute zero.

Why study dwarf galaxies ?
Q : What is it about small dwarf satellite galaxies that makes them worth studying ?

A : In this context (, it's all about the numbers. There are ~30 known dwarf satellites of the Milky Way, but more isolated galaxies have only about one companion each. Models of galaxy formation typically predict many hundreds of dwarf companions.

One possibility is that some of the dwarfs never accrete enough gas to form stars. Maybe star formation is triggered in the Milky Way satellites by the other galaxies in the Local Group (Andromeda and Triangulum). These isolated galaxies don't have any giant neighbours, so maybe their dwarf companions haven't been detected by optical searches because their gas has just been sitting there all this time doing bugger all.

It turns out that this is not the case - the isolated galaxies not only lack optically bright companions, but they have hardly any gas-rich, optically dark companions too. So it seems that galaxy formation really works differently in different environments, although as to exactly what the differences are and why they occur is anyone's guess.

How can we see other galaxies with all those pesky stars in the way ?
Q : I mean, there are like 100 billion stars in the Milky Way. How can we possibly see outside our own galaxy ?

A : It's not quite so difficult because our galaxy is relatively thin. The disc is about 100,000 light years wide but only 1,000 light years thick. That means it has roughly the same proportions as a CD.

It is true that in certain directions it's impossible to see through all the stars and gas. For that, other techniques are needed. But in most directions the density of stars is not so awful.

What will happen when our galaxy and Andromeda collide ?
Q : What will happen when Andromeda and the Milky Way collide ? Will there be more stellar mergers, and what will happen to the Sun ?

A : It's believed that the two galaxies will merge to form an elliptical galaxy - a giant ball of stars. My guess would be that the merger rate would increase simply because the density of stars will increase, but given that this rate is incredibly low to begin with this isn't something to worry about.

It's impossible to predict exactly what will happen to the Sun - we don't know the orbit of the Sun or the other stars precisely enough to make predictions that far into the future (about two billion years). It could end up inside the new elliptical galaxy, or get thrown off into intergalactic space to become a rogue star.

Will the collision of the Milky Way and Andromeda increase the amount of dangerous gravitational waves ?
Q : With the Milky Way and Andromeda Galaxies colliding, would that mean an increase in tidal waves in space? I figured I'd ask, just to make sure I was understanding the whole picture. Also, would that possible increase of tidal waves in space, would that also correlate to an increase in tidal waves on a water packed planet, like Earth ? Should we be moving away from the oceans, just in case ?

A : The terms "gravity waves", "gravitational waves", "tidal waves" and "tidal forces" all refer to very different things, but have annoyingly similar names. See In short, gravity waves and tidal waves are, in space, not a thing.

What you do get in space are gravitational waves. These are so incredibly weak they can't even be detected without some of the most sensitive equipment ever devised. No amount of collisions with other galaxies will ever increase these to a level where they would ever have any effect on us whatsoever.

But what you also get are tidal forces, e.g. the gravity on one side of (say) the Earth is different than on the other. These are far stronger than gravitational waves, and depend on the mass and distance of the object causing the tides. In the case of the Earth this is due to the Moon and also the Sun to a lesser extent, because although it's bigger than the Moon it's also much further away (the planets also have a very small effect).

Very roughly, the number of stars in Andromeda is about the same as in the Milky Way. So during the collision the number of stars in the merging galaxies will have approximately doubled. This will increase the tidal forces experienced by any star or planet, since there are now more massive objects around, but not to a degree where anyone would notice. The reason is that the distance between the stars is so vast that the chances of any star passing close to another is still extremely small.

In short, what the collision will do is bring in a lot more massive stars, but these won't cause any serious tidal effects for us because they'll still be too far away.

How big is the error when trying to estimate the mass of a galaxy ?
Q : It's fairly easy to see how the mass of a galaxy is estimated as Newton's Gravitational Force being equal to the Centripetal Force. Easily enough found'98/mass1.html but I have a few further questions. Such as, what is the general error involved in the estimate ?

A : A surprisingly complicated question.

We can estimate the rotational velocity pretty well, to within a few km/s or better from raw measurements. The complication here is correctly for the inclination angle of a galaxy - if we see it face on, we can't measure the rotation angle at all since we can only measure line of sight velocities. If it's perfectly edge on we can measure the rotation width directly and all is well. Most galaxies are somewhere in between so we have to try and correct for that, making assumptions about how thick the disc is based on statistical studies of large numbers of galaxies. Still, this correction is usually small, though for galaxies at small inclination angles (<30 degrees) we would probably chuck out the estimate because the correction starts to be really difficult to do accurately.

A bigger difficulty arises because calculating the radius of a galaxy depends on its distance, which is much, much harder to estimate. This is a huge topic which I won't go into here, but estimates can easily vary by a factor of two.

The worst complication of all comes from the dark matter. Since we don't really know how far into the dark matter halo the visible matter extends, all our estimates are lower limits.

Would it make much difference if one used Einstein's equations for gravity instead of Newton's when estimating the mass of a galaxy ?
Q : The usual formula uses Newton's Law of Gravity instead of General Relativity, obviously for simplification. Have estimates been done using GR? What would be/is the difference in estimates ?

A : If I recall correctly the difference becomes negligible for the low accelerations involved from the gravitational field of a galaxy. So if anyone has tried this they result they'd get wouldn't be much different from the Newtonian approximation.

How is the mass measured for a group or cluster of galaxies ?
Q : Is it as simple as adding up galactic mass estimates (seems doubtful to me) or are other methods used ?

A : Indeed, you can't simply add up the masses of the individual galaxies because of that pesky dark matter. In groups and especially clusters, galaxies are moving faster than expected based on the masses of the galaxies (just as individual galaxies are rotating too quickly based on the masses of their visible matter). You have to use the position and speed of the galaxies from the approximate cluster centre, just as when using the rotation of an individual galaxy to estimate its mass. Since you can only do this for a few galaxies, and since clusters are chaotic places, this means the error is a lot larger than for individual galaxies. Cluster (unlike groups) of galaxies also contain hot X-ray emitting gas, which can also be used to estimate the dark matter content - one can estimate the mass of the gas based on its brightness, and then estimate how much mass there would need to be in the cluster to prevent it evaporating.

Another technique is gravitational lensing. This is free of the problems of accurately estimating the inclination angles and rotation velocities, but has its own problems which I am not at all expert in. From what I remember, errors from this technique are also of order a factor of a few.

What are you planning to research for the rest of the year ?
Q : Something cool, I hope.

A : Right now I'm working on some optically dark hydrogen clouds I found in the Virgo cluster for my PhD. They're relatively isolated from the known galaxies in that region, they have rotation widths as large as giant galaxies, and they don't show any sign of being part of extended streams. Details here :

For the last few months I've been working on two projects to try and understand these things better. The first has been to construct a catalogue of similar features. It turns out that these particular clouds have some of the highest velocity widths of any other known starless clouds. Similar features are often attributed to being just hand-waving "tidal debris", though there are only two papers showing that this is possible. One of these, frankly, just isn't very good. The other models a very specific system which has quite different properties to these particular clouds.

Hence the second, bigger project to try and simulate their formation. We know gas is being lost in the Virgo cluster galaxies, but long gas streams are very rare despite their being predicted by simulations. So, maybe the streams get torn apart by galaxies interacting with them in the cluster. I drop a gas stream into a simulated Virgo cluster and watch the ensuing chaos. Thus far, the results are pretty definitive that this process cannot reproduce the observed clouds. This part of the project is almost done and the paper should be submitted soon. The major task for the rest of the year will be to add in the hot gas of the cluster itself, which may change the results dramatically.

The reason these little gas clouds may be significant is that if they're not tidal debris, the other obvious explanation is that they're galaxies which have never formed stars. That has major implications for the missing satellite problem, particularly as these are rather large. Galaxies this large really shouldn't be able to avoid forming stars.

Should we get the sausages ready for when the Milky Way collides with Andromeda ?
Q : That is, I've heard it's going to be a spectacular event and the galaxies will merge to become an elliptical, and all the gas will get burned up. So should we get a barbecue ready ?

A : That is the commonly-held view, but I'm not so sure.

It will definitely be a spectacular event. Stars are going to be flung about all over the place, that much is certain. The common wisdom is that there will also be a massive surge of star formation which will quickly use up all the gas and leave the galaxy as an emaciated wreck - a monstrous "red and dead" elliptical galaxy.

Elliptical galaxies are deceptively simple things. They look like big red featureless balls of stars, and simulations show that they could be formed by the mergers of spiral galaxies. The problem is that sometimes they contain a lot of gas but no dust, or dust but no gas, sometimes both, and sometimes neither (especially in galaxy clusters). Could this just be explained by mergers ?

I have my doubts - for one thing most spirals possess both gas and dust, so the presence of one or the other in ellipticals is not so easily explained. Also, in complete contrast to what's expected intuitively, gas during mergers does not appear to be apidly consumed by star formation. The gas content might even increase slightly, which only goes to show that one should be very careful indeed about drawing "obvious" conclusions. The Universe isn't a simple place and doesn't always do what you think it should.

Moreover, spirals are dominated by dark matter but ellipticals appear to contain very little. How can this be if they're formed by the merging of spiral galaxies ? Personally I think the current view of galaxy evolution is woefully incomplete at best.

If the black hole at the centre of the Milky Way was a quasar, could we see it with the naked eye ?
Q : So I am watching "How The Universe Works" and they are talking about Quasars at the moment. Suppose our Milky Way's SMBH would still be a Quasar, what would that look like from Earth ? Would we see be able to see the north and south beam, or would that still be too far away ? They did say Quasars outshine their whole galaxy, so I am assuming it would look like an extremely bright star in the sky.

And in such a Galaxy, noting the awesome power the Quasar puts out, would life on a planet like ours still be possible ? Or would the fact it is outshining it's whole galaxy be a problem in other forms, for planets such as ours ? That is, in the form of radiation/particles perhaps, not being thrown our by the SMBH north and south poles but in perhaps other directions ?

A : It's actually not so difficult to estimate exactly how bright a quasar would be at the distance of Sag A*.

3C 273 is the brightest quasar in the sky with an apparent magnitude of 12.8, or an absolute magnitude of -26.1 at optical wavelengths (v band, presumably). This is a measure of how much energy the thing is putting out, accounting for its distance (apparent magnitude is how bright it actually appears in the sky).

Given the actual distance to 3C 273 (2.4 billion light years or 750 megaparsecs) and its absolute magnitude we can then work out how bright it would appear if it was as close as Sag A* (26,000 light years or 8,000 pc) using the distance modulus equation :

That gives an apparent magnitude of -11.5. Not as bright as the full Moon, but brighter than anything else in the sky. Maybe as bright as a cresent Moon, but I'm not going to work out which phase precisely.

... but, there's a lot of gas and dust in between us and the centre of the Galaxy. That will dim things dramatically. According to NED (*&extend=no&hconst=73&omegam=0.27&omegav=0.73&corr_z=1&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES) this will be by over 200 magnitudes ! It probably won't really be as much as that since that calculation includes dust on the other side of the galaxy as well, but it will definitely be enough to make the quasar invisible.

Therefore, the effects of the quasar would only become important on a long timescale - after hundreds of millions of years, all that gas and dust would have been consumed/destroyed, allowing us to see it. Another bright light in the night sky would affect animal hunting behaviour, migrations, breeding, etc. I'm not too sure if the nastier X-ray radiation would pose a more sinister threat or not, but I doubt it.

As for the visibility of the jets, they can be surprisingly bright at optical wavelengths - you can even see them with a large amateur telescope ( What I can't find is the surface brightness of the jets, which would determine if we could see them with the naked eye. I suspect not.

Will gravitational waves be significant for studying galaxy evolution ?
Q : I mean, you must be super excited now that all your problems are gonna be solved, right ?

A : Probably not. The only sources which can be detected at the moment are black holes and neutron stars, which are just too small to have any effect on entire galaxies. Even though the energy transmitted in gravitational waves can be enormous, their effects are minuscule unless you're very close to the source.

The merger detected by LIGO saw 3 solar masses converted into gravitational wave energy : since the energy itself basically doesn't affect anything, the only real effect will be from the mass loss. That's tiny compared to a whole galaxy. So in terms of studying galaxy evolution, gravitational wave measurements probably won't be useful anytime soon. Maybe when we get to detecting waves from supermassive black holes, or from the very early Universe when galaxies were first assembling, they'll reveal new information we couldn't have gained otherwise. But that's years away.

Make no mistake : gravitational waves are exciting and will likely open whole new fields of study - but let's not go nuts. They aren't going to help answer every problem in astronomy.

If the Sun moves out of the plane of the Galaxy, wouldn't it be pulled towards the centre more strongly and so spiral inwards ?
Q : We know the Sun is moving out of the plane of the Galaxy but how can it be on a stable orbit if that's the case ?

A : We are attracted towards the centre but we're also rotating around it. The further away from the center we get, the weaker the gravitational attraction but our rotation velocity remains constant - so if anything we would spiral outwards. But as we get further away, there's now more mass pulling us back in - hence the galaxy is stable.

How do we know about the Sun's motion through the Galactic plane ?
Q : Does it involve strange rituals including chanting and a lizard ?

A : As I understand it (and I'm no expert in this), by measuring the motions of nearby stars relative to us. By subtracting the systematic (average) velocity by which they all appear to be moving out of the plane we can work out the Sun's true motion out of the plane. But this isn't a simple problem and I don't want to give an impression of knowing more about it than I really do !

Why does every galaxy have a black hole at the centre ?
Q : Well, why ?

A : They don't. Above a certain mass (I forget the number) almost every galaxy has a supermassive black hole, but below this they're much rarer. No-one understands yet why this is the case.

Are rogue stars and planets created by galaxy collisions ?
Q : What examples of recent history, can show how planets might react when a galaxy the size of Andromeda collides with a galaxy the size of the Milky way. Is this how rogue planets and stars are "born"... meaning after a collision ?

A : Rogue stars I'd say definitely yes. It's not the only way stars can get ejected from their galaxies (they can also get too close to a black hole or another star - unless they head straight for it they can get flung out at tremendous speed), but it's probably the main mechanism at work.

Rogue planets I'd say (cautiously) probably not. Very roughly, the number of stars in Andromeda and the Milky Way is about equal. That means that on average the collision will roughly double the density of stars, which isn't enough to have much of an effect on individual star systems.

To rip a planet out of a star system you'd have to have a very strong difference in gravity over the scale of the system. While the collision of the galaxies will generate forces which are very strong over large distances - so whole star systems can be chucked out - it most likely won't generate significantly different forces on the much smaller scale of individual star systems. So probably the rogue stars which are formed will carry their planets along with them into the deep void of intergalactic space. Any entities living on said worlds will have a nice view for a few billion years but end up feeling incredibly lonely.

Is there an equivalent of a Hertsprung-Russel diagram for galaxies, showing how they evolve ?
Q : More generally, as we know with stellar classification evolution is obviously strongly taken into account with the inclusion of the "main sequence", is there a similar scale for Galaxies? Put it more plainly; is there are a "main sequence" in galactic classification for those galaxies that are in the more "steady" phase of their evolution OR is galactic evolution not really much comparable to stellar at all ?

A : It's complicated. There are the well-known red sequence and blue cloud, which is the equivalent of a galaxy HR diagram :
But if and how galaxies move from one to the other is not known. Stellar evolution appears to be dominated entirely by mass. For galaxies it also depends on mergers (which are at least common - many people would say even the dominate evolutionary mechanism), tidal encounters (which depends on the density of nearby galaxies) as well as ram-pressure gas stripping and other gas-only effects (which depends on the surrounding gas outside the galaxy), and possibly the initial mass function (how many massive stars are formed that can explode/have strong stellar winds which can ionize/blow out the gas completely, which is in turn dependent on the chemical composition of the gas (more metals = more cooling).

And that's the simplified version. Some people think galaxies can move more-or-less freely from the blue cloud to red sequence (by running out of gas for star formation) and even back again (by merging with a gas-rich galaxy or re-accreting material from the intergalactic medium). Others think that movement between the two sequences is a rare and not significant process. The bottom line is that there's no widely-accepted "typical" process of galaxy evolution.

Could these newly-discovered ultra diffuse galaxies be ordinary galaxies at a different stage of their evolution ?
Q : [Asked in relation to this post :]
One element, I assume being considered, but that I'd like to know more about, is consideration regarding the possibility that those galaxies are in a non "main sequence" stage of their evolution, such as in the process of formation, merging, or tidal disruption caused by other galaxies or dark matter influences.

A : For these UDGs, one idea is that they formed all of there stars early on in a relatively short burst, blowing out all of their gas. That would explain why they're so faint - they just never had the chance to form many stars. The difficulty with this is that they're so dark matter dominated it's not yet clear if the stars/supernovae would have had enough energy to remove their gas. It also wouldn't explain why the number of detected galaxies is so much lower than cosmological models.

But it seems even less likely that these are unstable, transient objects changing from one type of galaxy to another. They have very simple, smooth, spheroidal structures and their globular cluster distribution appears to be quite symmetrical. If they were experiencing significant disruption, they should be messier. An exotic possibility suggested for similar objects in the Local Group was that these aren't stable galaxies at all, but are just "tidal debris" that's evaporating - we happen to see it at the stage where it's still barely detectable. Given the new discoveries of large numbers of similar objects in cluster, that now looks to be very unlikely.

The short answer is that we don't know.

Could Unruh radiation explain the direction of galactic rotation ?
Q : [Unruh radiation was recently proposed as an explanation to the current darling of the pseudoscience world, the EmDrive. The claim is that this device can produce thrust without expelling any material or direct physical contact with anything - in Clarke parlance it is a "space drive" or "star drive". Conventional physics says this would violate conservation of momentum, a principle so well-established that breaking it is basically saying that "a wizard did it.
A sort of perverse hope recently appeared with a paper claiming that another well-known effect called Unruh radiation could explain what was going on without violating conservation of momentum after all. Unfortunately it also claimed that this could also explain the acceleration of the Universe without dark energy and the rotation of galaxies without dark matter. That's a hell of a claim based on barely measurable effects. The scent of "too good to be true" is wafting through the air like the fetid aroma of a decaying whale carcass.]

"...the idea behind it can also explain galaxy rotation ..."

Hmmm. Would that make the rotation direction (CW/CCW) predictable in galaxy formation ? Or is CW/CCW just an "above or below view" issue ? How could you know ?

A : As you say, it's an above or below issue. There's no standard against which to measure the rotation direction. Actually some galaxies have components which rotate in opposite directions :

As far as I can tell, this wouldn't necessarily cause any problems for this idea (or indeed most other ideas of gravity/inertia). It only predicts the speed of the rotation, not the direction. It's not obvious to me if this model has any fatal flaws from a galactic perspective, I have to think about it some more.
Fortunately I did not have to think about it because John Baez already explained in some detail that although Unruh radiation is a thing, it is weaker than Piers Morgan's claims to journalistic integrity. It's about as good an expalanation as saying, "because sausages".

How many gas clouds are there around M33 ?
Q : I've been reading your new paper about the HI clouds you and your colleagues observed around M33 ( I wonder if you can clarify one small point for me. In the results section it states that you found 32 clouds of which 11 are newly observed, but later on it states that you found 22 discreet clouds which matches well with the (25) predictions of simulations. So how many clouds did you find ?

A : It's not quite as simple as that. Previous observations discovered a population of clouds around M33, nice and straightforward. But our deeper observations revealed that the disc of M33 is more extended and some of the "clouds" are actually just extensions of the disc - hence we "found" 32 "clouds", but some of these aren't really clouds at all. Since the simulations predict the number of dark matter subhalos (i.e. galaxies) we discount these disc-extensions when comparing the number of clouds with simulations. So we discovered all the previous clouds and more besides, but it turns out that the number of true clouds is only 22. See also :
And the original paper :

Is Keenan's giant gassy Ring really all that giant ?
Q : The article ( says "If the cloud [a.k.a. Keenan's Ring, a giant gas cloud near the M33 galaxy] is at the distance of M33, it is larger in size than the galaxy." This seems to indicate that the determination of distance is weak. But the rest seems to assume that the cloud is at the distance of M33... ?

A : Indeed, there's no way to directly measure the distance to the Ring. It's very unlikely to be much further away than M33, otherwise it would be both enormous and with no plausible origin. It's also very unlikely to be within our own Galaxy since the redshift is all wrong. If it's as close as M33, it's still as large as M33 itself (very few other starless hydrogen clouds this large are known) and it's difficult to see how it could have originated from M33, but at least it would be a possible source of the gas. If it's closer, it could be part of the much larger Magellanic Stream, but it's quite far away from the rest of the stream. In any case, there's no good reason why it should be ring-shaped at all. It's a strange one.

What do you think of this press release about dust ?
Q : Have you seen the civilian press reporting on this paper ? I've always been interested in star and planet formation. How significantly does this change star formation models ?

A : First off, I thought the outreach version must have suffered a garbled translation. But then I read the original ( and it's scarcely any better. They have NOT solved one of astrophysics great conundrums. They haven't "solved" anything, for starters, and I'd hardly call it a great conundrum anyway. What they've done is formulate a statistical model to see how likely it is galaxies detected in Herschel, which has a pretty low resolution, are actually composed of several galaxies. They find that this could mean that galaxies are forming stars about 30% less rapidly than previously thought. Is it important ? Sure, if you're studying those sorts of distant dusty starbust galaxies. But I'm left with a lingering confusion as to why they did a press release. 30% is not exactly a revolution : a galaxy which was thought to be forming 1,000 stars per year is now "only" forming ~700 stars per year. That's even assuming their model is correct, and that this stage it's very much in the "only a model" stage.

As for how this might change star formation models in general, it's hard to say. Starburst galaxies are pretty extreme systems, and despite the claims in the paper, I have a tough time accepting that a 30% drop really makes them much less extreme. Plausibly this makes no difference at all at the level of individual star formation - it could just mean that conditions throughout the galaxy are a bit less conducive to star formation. Really the correct approach is to wait and see how other experts in the field (and that's not me) respond to it.

What do you think about this silly meme about a void in space ?
Q : What do you think of this meme ? Is this the same region that was used for the Hubble Deep Field ?

A : This is not a void in space. Voids in space as described in the caption do exist, but do not appear this empty because you can still see the galaxies behind them. The image is in fact of a Bok globule, a (much smaller !) cloud of gas and dust which absorb radiation, hence appears dark.

This particular Bok globule is also nowhere near the Hubble Deep Field, nor was the HDF selected to target any kind of galactic void - just a region where there was nothing especially interesting.

Why don't turbulent extragalactic gas clouds look like smoke ?
Q :
I'm in slightly over my head over here, but I have to ask: why is it 'spiky' ?
If we considered it as two gasses of different temperatures and densities, then wouldn't they curl up like smoke-balls and like ?

A : Well, that's complicated. The structure will depend on the velocity gradient of the turbulence - if the velocities vary strongly over small distances, you'll get a different structure than if the gradient is shallower. Plus all the gas has significant self-gravity : it wants to collapse. Simultaneously it's being heated by by the surrounding hotter gas, which makes it expand. Unlike fire though, there's no source of energy to maintain the turbulence, so its continuously being resisted - only the initial energy allows it to keep moving. Also unlike fire-borne smoke, the dense gas here starts off stationary with respect to the intracluster gas - if we put a wind in, things might change significantly. Finally, the structures seen depend to some extent on the resolution of the simulations, which here are rather low to keep things fast.

The short answer is that there are really too many variables to intuitively predict the shape of the gas.

I expect Rhys Taylor knows about this press release on spiral arm formation.
Q : [I assume that this was really asking, "what do you think about this ?" rather than a generic declaration to the world that I'm probably aware of it, so...] What do you think of the idea that spiral arms in galaxies are density waves, as in this press release ?

A : He does, but he's not terribly impressed by the press release. I have to mention that the very first galaxy simulation showed spiral arms as density waves back in 1941 (not in the '60's as the article claims)... but what's more interesting is that since computers weren't a thing then, the guy used light bulbs instead (one of those seriously crazy experiments that deserves to be mentioned at every opportunity).

Modern simulations routinely show spiral arms as density waves (e.g. shameless plug so it's a bit odd to call this a discovery. Also a bit strange to mention pitch angle because I'm pretty sure variation in pitch angle was already well-known. There was even a paper showing a near correlation between pitch angle and mass of the central supermassive black hole (

Maybe the paper itself is better, I'm confused by the press release but it wouldn't be the first time the original article got garbled in translation.

Later, I read the paper completely if not very carefully. Seems like a decent bit of research to me, but it's about a rather obscure (though important) detail of the accepted theory of spiral arm formation. Specifically they compare the position of the spiral arms in different wavelengths. The theory being that the density waves should move at different speeds at different distances from the galactic centre, and the resulting stars which form should then appear at different positions relative to the density wave (in the gas) itself. They look at different wavelengths which trace the gas, young stars and dust and find that it agrees very well with the density wave theory.

Which is all well and good but I am left wondering why they did a press release on this. They barely mention the alternative theory and anyway the density wave model is already widely-accepted. Essentially, the validate a model that everyone already believes is true. Well, I guess one has to satisfy the paymasters somehow...

If Andromeda were to go nova instead of colliding with the Milky Way, what would happen ?
Q : If Andromeda were to go nova, instead of colliding with the Milky Way galaxy, how would such an event affect the Milky Way galaxy, since they are almost touching on the edges ?

A : Andromeda won't go nova - a nova is an eruption on the surface of a white dwarf star. Like a much less powerful supernova, it's something that can only happen to an individual star.

Novae are common in our own galaxy, estimated at a rate of about 40 per year. They don't have any direct effect on us - again, like a supernova, they'd have to be really close to do that. There's really no chance of that happening - 40 eruptions per year is not so many in a galaxy of ~400 billion stars. So a nova in Andromeda would have essentially no effect on us at all.

But on a much longer timescale (millions of years or more), novae in our own Galaxy probably have at least some role in heating the interstellar medium and injecting heavier elements, which is important in star formation. Imagine being in a room with a small fire burning, slowly heating it up and filling the air with soot. Now imagine that every so often someone chucks in a grenade for good measure. The interstellar medium is a bit like that, full of constant winds being blown out by hot stars but also punctuated by very violent novae and supernovae explosions - there are shocks, bubbles, filaments, and lots of fun stuff we don't really understand yet.

As for this very extended gas around Andromeda and the Milky Way (see, my guess would be that at least some of it comes from supernovae and novae explosions blasting material out of the galaxies, with the rest being leftover material that hasn't yet fallen into the galaxy's discs. This gas is very thin but also very hot, which means its particles are moving very quickly in random directions. So if there was an explosion in/near this gas, it would send a shockwave through it but it would be quickly smoothed out because of the gas' high temperature.

I found a lot of press releases about this extended gas claiming that the Milky Way and Andromeda are going to collide sooner than expected. This is only sort-of true. It's true in that they have these much larger clouds of gas which may already be interacting. But this gas is very, very thin, much thinner than the gas found in the discs (by a quick glance at the original paper I estimate it to be around a million times thinner than the normal gas in galaxies). So it's not likely to have much of an effect on the collision, and certainly the stellar discs still won't collide for several billion years.

What do you think of this article on Dragonfly 44 being mostly made of dark matter ?
Q : is it row sombrero galaxy ?

A : What ? I don't know what that means, but here are my thoughts on the article.

Why they are using the Sombrero Galaxy in the image I have no idea. Worse, they label it as an "ultra diffuse galaxy", which it most definitely is not ! Also :

Van Dokkum and his team later realized that there was something very odd about Dragonfly 44: a galaxy that big couldn't possibly hold itself together with so few stars. There wouldn't be enough gravity, and the stars would drift apart. They suspected that dark matter was responsible for holding the galaxy together, and this particular galaxy seemed like it contained a ton of it, so they set out to determine exactly how much.

That is badly misleading. There's nothing odd about galaxies which require dark matter to hold them together, that is normal. It's the very reason why dark matter was proposed to exist !

There are two unusual things about galaxies like this :
1) They're incredibly faint for their size, 100-1000 times fainter than normal galaxies. They hint that there could be even fainter galaxies, possibly with no stars at all. That would solve the "missing satellite problem", the fact that cosmological models predict far more galaxies than we see.
2) They are (it seems) extremely massive for their brightness, so much so that they fall off the normal relations for galaxies. Standard models do in fact predict this, but until now no-one had detected any galaxies like this. The leading alternative to dark matter, modified gravity, does not predict galaxies like this AFAIK, so this may rule that model out.

On the other hand, no-one has a good idea why some dark matter halos of equal mass may form a few stars while others form a thousand times as many. That one's going to be tricky.

Could Dragonfly 44 be full of Dyson spheres ?
Q : [In relation to this article, about one of many newly-discovered "ultra diffuse" galaxies - very faint but massive objects]
I'm waiting for someone to opine that all the visible stars in that galaxy are shrouded by Dyson spheres. I'll do my ::eyeroll:: in advance.

A : Wait no longer !

Basically the Tully-Fisher relation is how bright a galaxy is compared to its rotation velocity. Like the galaxies the authors search for in that paper, this galaxy is rotating much faster than most galaxies of the same brightness... which "could" be attributed to most of the stars being surrounded by Dyson spheres. :)

(Of course Dyson spheres should cause an infra-red excess, which so far as is known is not the case.)

Are you very skeptical about Dragonfly 44 ?
Q : Didn't I read something by +Rhys Taylor being very skeptical about this ?

A : I would describe my position as "cautious" rather than very skeptical. I'll have to read the paper again but IIRC they extrapolate the large mass (much larger than directly measured) based on dark matter profiles from numerical simulations. This might be a very sensible thing to do, I'm not sure. I'm concerned because we're using rules established from pure dark matter simulations that we aren't all that sure work very well to explain normal galaxies to also explain these new things, which are a thousand times fainter. I'd be a lot happier with some independent mass estimate.

What do you think about Dragonfly 44 ?
Q : Tell us how you really feel.

A : OK, that's enough with the Dragonfly 44 questions, people. Just read my blog post with the executive summary here :

What would happen if dark matter started absorbing light ?
Q : When normal matter absorbs light, it re-radiates it another wavelength. Absorbing light heats matter up, and the wavelength it radiates at depends on its temperature. The higher the temperature, the shorter the wavelength. The Sun, stars and incandescent light bulbs all radiate at visible wavelengths because their temperature is a few thousand Kelvin. Interstellar dust tends to be rather cooler and radiates in the infra-red. So dust lanes in the galaxy appear as dark, opaque streaks in normal light but show up brightly in the infra-red.

Dark matter is different. We don't even know if it has a temperature : it seems that it only interacts with normal matter through gravity. But, if it did absorb light, it would have to make everything we see darker. Normal matter both absorbs and scatters light in such a way that the shorter blue wavelengths are more affected, so as well as darkening any stars behind the dust it also causes them to appear redder. Whether this would also happen with dark matter would depend on the detailed physics of what it actually is.

The effects this would have on galaxy evolution would also be strongly dependent on what happens to the dark matter and exactly how it absorbs the light. For example it might cause the dust and gas in galaxies to receive less heat from the stars so they would cool, causing them to collapse and so form stars more rapidly. But if it also re-radiated the energy as normal matter does, it might not have much of an effect. My guess would be this could be very important if dark matter had absorbed light in the early universe, when stars and galaxies were still forming, but probably wouldn't have such a dramatic effect today(except on very long timescales) since galaxy formation is largely over.

Is dark matter involved in ram pressure stripping ?
Q : What about this press release, eh ?

A : It's wrong.

"The answer paints a picture of these galaxies falling through their larger dark-matter halos, having their star-forming gas removed in a fast-acting process called ram-pressure stripping."

To be fair, the rest of the article correctly points out that it's not the dark matter halos which are doing the stripping at all - it's the hot gas inhabiting the halos. At high speeds the force that builds up from the pressure of the external gas exceeds the force holding the gas in the galactic disc and it gets blown away. The fact that there's a large dark matter halo as well is somewhat incidental, and definitely shouldn't be stated like this in the last sentence !

Once all the stars go out, will all the hot gas around galaxies start collapsing and form more stars ?
Q : So I guess on the very longest scales this is a cyclic process, since all that keeps the gas found outside galaxies hot is ionization by starlight, and that's all that keeps it from eventually recollapsing. So once the quenched galaxies go out, the gas will fall back into them and a new (much more stretched-out) burst of star formation might begin ?

A : That's controversial. Heating of the intracluster gas can happen as galaxies and galaxy groups are accreted and lose gravitational energy, as well as through active galactic nuclei (a nice size comparison : it's like a marble heating up the whole Earth). Hot young stars can ionize gas directly, but not on very large scales. They can also heat the gas through stellar winds and more significantly through supernovae, but these are again mainly on galactic scales rather than cluster scales.

Its temperature is indeed what prevents it from recollapsing. But if and when it does re-collapse is not well understood. Some people think that gas streams seen around some galaxies are evidence for cooling flows, but it's very difficult to prove that this gas hasn't been stripped out of the galaxies instead. Elliptical galaxies on the edge of clusters sometimes contain gas whereas in the cluster interior they're barren, which is sometimes interpreted as evidence for the gas cooling. But elliptical galaxies outside clusters often contain lots of gas anyway, so again it's hard to prove the link. In principle though, the cooling gas could indeed eventually lead to more star formation on longer timescales.

Could some dark matter halos just be made of normal matter ?
Q : Sorry for a silly question, but what is the reason to exclude the possibility of (some) dark matter halos around galaxies being just ordinary unlit and non-glowing matter that would interact with the galaxy's matter following the laws and customs of ordinary matter?

A : Complicated question. I shall divide the answer into two parts - first you must understand why we think these dark matter halos exist at all, and then I'll look at ordinary matter that remains optically dark.

First, your question implies that we already know of such dark matter halos and they need explaining. In fact we don't know of any such electromagnetically-dark halos from direct observations, but they are inferred from models. We only know of dark matter at all thanks (largely) to the rotation and motions of galaxies - plenty of evidence for it inside optically bright galaxies, but little or no evidence for pure dark matter halos.

"Dark matter" has become synonymous with non-baryonic (i.e. a new type of hitherto unobserved particle) matter that doesn't even interact with normal matter except through gravity. This is very computationally cheap to simulate since you just need a bunch of particles with gravity, with none of the pesky gas dynamics and other complicated physics associated with ordinary matter.

Numerical simulations of the Universe have therefore traditionally used only this particular non-baryonic dark matter. The number of detectable galaxies is computed based on how much ordinary matter should fall in to each halo. This, unsurprisingly, does not give the right answer - but the consistency of the method in being wrong by a factor 10 or more is so large it's hard to dismiss as being due to the method of calculation.

Hard, but not impossible. Actually more recent simulations, with computers now (just about) powerful enough to handle the ordinary and dark matter at the same time, are finding that some dark matter halos never accrete enough normal matter to form any stars. Still, the reason these "dark galaxies" exist in the simulations at all is because of the non-baryonic dark matter, so they're not the purely optically-dark clouds of normal matter you're suggesting. Such clouds would not help explain the missing galaxies problem (since it's really the dark matter that's missing), so in that sense there's no particular reason to assume they exist at all.

The second aspect to the question is whether dark but otherwise normal matter could exist at all. To that the answer is "yes with a but". As well as missing galaxies there's also the "missing baryon" problem, where models of Big Bang nucelosynthesis predict around 5x as much ordinary matter as is detected. It's thought that most of this missing normal matter ends up in the hot, very low density halos of galaxies and galaxy clusters, where it's very difficult to detect. There are some hints that this is the case (e.g. but it's very tentative at this stage.

This hot matter would not be particularly clumpy, so you wouldn't get it forming structures on galaxy scales until it cools. On the other hand, probably not all of this missing normal matter is hot ionized gas. Some of it could be very cold molecular (or perhaps atomic) hydrogen, which is hard to detect indirectly and nearly impossible to detect directly. That has the potential to form dark galaxy-like objects, and is similar to the stuff you're suggesting.

How much there could be is controversial. Big Bang nucelosynthesis allows there to be just enough missing ordinary matter to explain the dark matter inferred from galaxy rotation curves (which is an interesting coincidence), though not the much greater amounts implied by the motions of galaxies in clusters. So it's still just about at the edge of respectability to propose that significant amount of "dark matter" is just hard to detect normal matter.

But even when it's not illuminated (you can never get zero illumination mind you - all normal matter has a temperature and radiates), normal matter would still make its effects felt in other ways. Microlensing studies have more-or-less ruled out objects like black holes, planets and other small bodies. Vast amounts of cold, undetected gas could explain away the non-baryonic dark matter, but at the expense of totally smashing every aspect of star formation theory and maybe gravity as well - it doesn't seem possible that such gas could exist without collapsing and forming stars at a far higher rate than observed.

So the short answer is "no". We don't know how many dark matter halos are really out there, and we know of no form of ordinary matter that could create such a structure that's compatible with observations of dark matter where we're confident it exists.


What is dark matter ?
Q : I want to ask is there any dark matter or dark energy in the universe because nobody had ever seen this or seen colliding this with any matter in the space ? Is all the darkness in the space dark matter ?

A : No, dark matter isn't the darkness in space. That's just an absence of light. Dark matter is believed to be completely transparent. However, dark matter is thought to fill much of the space between the stars. The exact nature of dark matter is still a mystery. It is believed to be some as-yet unidentified particle which a) doesn't interact with normal matter at all except through gravity and b) is probably collisionless, meaning it doesn't even interact with itself.

Some models of dark matter suggest that dark matter particles may, occasionally, interact with each other and produce a burst of energy. If that's the case, we might be able to detect gamma rays where the density of dark matter is particularly high. No convincing evidence of this has been found so far.

Lastly, there are still alternative theories of gravity which could do away with dark matter entirely. In my opinion, these aren't very likely and the evidence for dark matter's existence is pretty strong. However, we won't be certain until we detect a particle directly. There are several experiments underway that are trying to do just that.

More details here :

Does dark matter exist ?
Q : How can we prove dark matter exists ?

A : Right now, we can't prove it exists. We can only say that all our observations are consistent with dark matter : e.g., galaxies are rotating too fast so should fly apart without dark matter; galaxies in clusters are moving so fast the clusters should fly apart without dark matter; gravitational lensing (light being distorted by massive objects) is consistent with the idea that there's matter we can't see (in amounts consistent with that predicted by the other methods).

The only way to prove its existence once and for all would be to directly detect a particle with the properties that could explain all the observations. This is very hard to do since dark matter only interacts with normal matter very rarely (except through gravity), if it ever interacts at all. Nonetheless, several experiments are underway to try and do just that, e.g.

The main alternative to dark matter is to modify our theory of gravity to fit the observations. However, finding a theory which works in every situation (e.g. from masses of planets right up to those of galaxies; from nice stable orbits to the chaos that is interacting galaxies) is proving to be very difficult. In my opinion, dark matter is a much more likely explanation.

Can we use better telescopes to see the beginning of the Universe ?
Q : Will there come a time, when we could see all the way back to the beginning of the universe with much more powerful telescopes than the current Hubble telescope. Or are we pushing the bounds of technology as we know it, now ?

A: Complicated topic, to say the least. So, bear with me. :) It's true that we can't see galaxies beyond a certain distance because of the expansion of the Universe. However, because the Universe is so large, we're also looking back in time.

This isn't such a big deal when looking at nearby galaxies like Andromeda which are "only" a few million light years away, i.e. we're seeing them as they were a few million years ago. That's just not enough time for anything really significant to have happened to them (large spiral galaxies take a few hundred million years to rotate even just once).

But we can see galaxies very much further away than that - billions of light years away, or to put it another way, galaxies as they were billions of years ago. And that is long enough to see significant changes. But more than that, we're seeing them when the Universe was significantly smaller than it is now.

Go to high enough distances and you start seeing the Universe when it was so young that stars did not exist. At some point you reach the fireball of the Big Bang itself. The most distant thing we can see is called the "surface of last scattering" - the time when the Universe cooled enough that it became transparent and light could move unhindered through it. This is thought to have been at around 300,000 years after the Big Bang.

Ordinary telescopes will never be able to see beyond that point because the entire Universe was a hot, opaque fireball until that point. Gravitational waves (which we haven't detected yet, but we do keep trying) would allow us to "see" past that point, since they're not light waves and wouldn't be affected by the high temperature.

Is our Solar System expanding along with the rest of the Universe ?
Q... well, OK, comment : Our solar system is expanding and moving away from the Sun, while at the same time the Sun is moving towards Andromeda. Too bad, we won't be around to see the "union".

A : No, the Solar System is not expanding... at least not yet :

On small scales, yes, our perspective is changing because galaxies can be  gravitationally attracted to one another, so their motions aren't what you'd expect if the expansion was perfectly smooth. However, on the very large scale, these"peculiar motions" are negligible.

Dark matter - wrong name ?
Q : Could or should we rename dark matter as augmented gravity or accentuated gravity?

A : No, because dark matter is a specific idea that there is really extra mass that we cannot detect directly. MOND (Modified Newtonian Dynamics) is the most popular theory proposed to modify gravity in such a way as to mimic the effects of dark matter.

Why didn't the young Universe collapse ?
Q :  If dark matter was prevalent in an earlier Universe what has prevented a general collapse of the entirety of matter ? Equilibrium ? Why did structures form ?

A : The early Universe was hot, so there was a pressure pushing outwards. I don't know why it was hot - I'm very much hoping that someone can give me a good explanation of that - except that if it hadn't been hot, it would indeed have collapsed, and we wouldn't be here to discuss it.

If the Universe had started off with exactly the same density of matter everywhere, then nothing much would happen. As it expands, the density would just drop (or if the mass was high enough, re-collapse). But that appears not to be the case. All matter had some random initial motions, so the collapse wasn't the same everywhere. While there problems in the details, simulations have done an excellent job of reproducing the observed structures (networks of filaments and voids) just through slight variations in the initial density and motion of the dark matter particles.

Dark matter particles are thought to be collisionless, so particles just move through each other. This means you don't get a runaway collapse of the dark matter - it re-expands a bit after the collapse. Gas, however, does collide with itself - but the other complex processes at work prevent star formation from occurring in one massive burst.

Was gravity always present ?
Q : If it all came from a very small object (in size and time) what force contrasted gravity on such a dense / compact initial "object" or was gravity not present at the beginning and only came into existence at a later stage, much like we imagine dark matter and dark energy to have had different "levels" at different times.

A : Gravity was and always will be present*. I am not quite sure what you're asking, but the basic view of the early Universe is something like this : the very hot, small early Universe expanded extremely rapidly. Gravity continuously acts to slow the expansion down - like throwing a ball into the air. For a long time it was very uncertain if the expansion speed or gravity would win, but current observations strongly favour that expansion wins. There was simply too much expansion speed for gravity to ever bring it to a halt (like throwing the ball upwards at escape velocity).

However, the early Universe wasn't quite uniform. Density variations mean there is still some localised collapse into what became the filaments of galaxies we see today.

* Except possibly in the first few nanonseconds of the Universe's existence, when all forces were unified.

It all done just don't make no darn sense !
Part of a larger, unproductive discussion. I include this here as an example of someone who thinks their ignorance trumps all of modern science.
Q : Rhys, so "expansion" is a force that was, is and will be stronger than gravity, also thanks to dark energy, and gravity only acts whenever one need to explain a more common object like a galaxy or a (extra)solar system [I had clearly explained to the inquirer that this is nonsense]. Nebulas are oddly-shaped but still produce round objects [I explained that one too]. Dark matter greatly increases gravity / attraction but not enough to counter expansion [Yes, and that]. Galactic black holes don't grow over half percent of the total mass, although it's projected that they will engulf the whole galaxy [Sigh...]. It all so totally don't make any sense :-)  Let's revisit this whole concept when we meet some ET !

A : Nope. Sorry, but astronomy is not a soft science. Thousands of people have run thousands of simulations to explain the structures of nebulae, galaxies, and the Universe as a whole. If you do not understand the principles behind these, let alone the physics and the numbers involved, you are no more entitled to pronounce judgement upon them than I am to comment on, say, neuroscience. Mathematics matters. You can't ignore it because you don't agree with the end result.

Is the expansion of the Universe really accelerating ?
Q : How firm is the data indicating that the expansion of the universe is accelerating? Is this something that with new data or better analysis might be found not to be the case 20 or 30 years from now ?

A : I'd say it's pretty solid. Maybe not 100% certain, but good enough for government work. :)

The evidence that the expansion comes from supernovae explosions - specifically, type Ia supernovae. These happen when a white dwarf gains enough mass from a companion star that it re-start fusion. The point at which this happens is thought to only depend on mass so the resulting explosion should always be of the same energy. Knowing that energy, we can work out the distance to the supernova pretty accurately.

The benefit of using supernovae is that we can measure both their redshift and distance even in distant galaxies. Because light travels at a finite speed, the further away a galaxy is, the younger (and therefore smaller) the Universe was when light left that galaxy. After accounting for this, the supernovae data indicate that the acceleration of the Universe is increasing - which is something nobody was expecting.

There was an alternative interpretation of the supernovae data : we live in a void, a region of the Universe which happens to have much less matter than the rest. Since there's less matter inside the void, there's less gravity to slow down its expansion, which would look like acceleration.

This led me to a rather interesting paper-chase. Here's an original press release from 2009 categorically stating that we don't live in such a void :
And here, on the exact same day, is another wesbite (mis)interpreting this to mean that we DO live in a void and dark energy is wrong !

More recent, independent evidence futher supports the notion that we don't live in a void. As far as we can tell, the expansion really is accelerating.

Eddie's in the space-time continuum, cap'n !
Q :  If space is the thing that is moving, why is it the Milky Way and Andromeda galaxies can not simply go under or over one another. Why is it that in the next billion or so years, the two will collide ?

If space moves, than that would mean that there are "currents" in space. Why can't a current go over another one, like air does ? There are different levels of clouds, at various levels, each moving in their own direction, separate from each other, but also together, to form the cloud formations we see.

Why can't space be the same way ? Or is it ? Where there are currents in space guiding the various solar systems and galaxies around.

A :  The standard view is that space is expanding in all directions equally. This lets us predict how galaxies should be moving. When we see differences, we assume it's because the galaxies are moving independently of the expansion of space, usually because the gravity of other galaxies is affecting them. That's a bit like a boat moving against a current.

But, the idea that space itself could be flowing in different directions is super interesting, and really it's above my pay grade. But I'll try anyway. :P

Gravity is curved space. Here's a very nice demo (first 2-3 minutes are enough, but it's worth watching in its entirety) :

So, if space was severely distorted enough to carry galaxies along like a flow, that would basically be the same as gravity, and we'd see all kinds of funky gravitational lensing effects :

I really don't know if it's theoretically possible - or even makes sense - to talk of curved space without gravity. But it's going to be pretty tough for Andromeda and the Milky Way to avoid each other. Like two sumo wrestlers rolling around on a trampoline, things are gonna get ugly...

Both galaxies have mass. so both have gravity. Lots of gravity (or, big dips in the spandex sheet if you prefer). Since it seems they're heading more or less directly toward one another, their gravity is only going to make things worse - they'll get faster and faster until they collide.

Q : A very simple question from all of you, is there any end of space or it is infinite ?​

A : SIMPLE ?!?!

Nobody knows. However, it cannot be both infinite in size and have existed forever, because the sky is dark at night. See this exceptionally wonderful 4min video :
In principle, it could be infinite in space but not in time. Which has some crazy implications - keep going long enough in any one direction, and you'll eventually encounter exact copies of yourself :
You might wonder if the Universe isn't infinite, does it have to have an edge ? Not necessarily. Space could be curved on a very large scale, so that if you fly off in one direction for long enough, you end up back where you started (like flying around the world).
Personally I like this idea because I can't imagine how a Universe becomes infinitely large in a finite amount of time, or what the edge of the Universe would be like if it was finite in size. In this view, the Universe is both finite in size (but still really frickin' large) and time and has no edge.

However, it still means that at some point in the distance past the Universe went from a state of complete non-existence to existence, so in that more woolly philosophical sense, it doesn't necessarily avoid all the infinities.

UPDATE : I give a much more complete explanation here :

Q : What's the latest progress in the recognition of dark matter ? [April 2015]

A : There have been a few hints of direct detections of dark matter, most notably from an instrument on the ISS. The latest claim was as recent as September 2014 :
However, this was a very tentative "result" and even the press release states, "We have not found the definitive proof of dark matter." A similar claim was made in April 2013 :
I can't find any follow-up to this; it's pretty much fallen by the wayside so I assume the result was never confirmed.

Even more recently, the Fermi gamma ray observatory discovered gamma rays from a dwarf galaxy. This is also a possible signature of dark matter particles :
There was also another tentative claim of detecting dark matter in the Milky Way in the same way :

The problem is that all of these are only claims that the result is compatible with dark matter. They're intriguing, but no-one is convinced by them. The search continues.

Does the Universe even have an age ?
Q : Why do people try to give an age to the universe when time is an entirely relative expanded fabrication? The age of a black hole is not the same age as the rest of the universe because the passage of time is not the same and you can't use expanded time as a valid measure of time anyway.

A : Time does vary depending on speed and the gravitational field you're in. However, these effects are very small unless you're going close to the speed of light or near (as in within a few kilometres) the event horizon of a black hole.

In the standard model, space expands uniformly. Away from any stars or galaxies, time passes at the same rate everywhere. You get some weird local deviations around black holes, but that's about it. You also get deviations wherever you have any mass at all, but so small that it's basically negligible. It doesn't really make much difference to the age of the Universe if time is passing 0.0000001% more slowly near a star than in deep space.

If you go back far enough, the Universe was so small that it was filled with dense matter, but here the gravitational field would have been more-or-less uniform everywhere. So time was still passing at about the same rate everywhere, because gravity would have been strong everywhere. This is "proper time" in relativity : time time experienced by an observer accounting for their velocity and gravitational field. Since everywhere was experiencing (pretty much) the same gravitational field, proper time didn't vary throughout the Universe. So it absolutely makes sense to define an age of the Universe.

Granted, black holes do cause problems. So does the singularity in the standard model at the Big Bang. Light is also weird, since the proper time for a light beam is always zero no matter how far it travels. To a photon, everything happens at once. And I have no idea what the philosophical implications are of that.

Does the Universe even have an age if space and time are expanding ?
[Part of a larger, unproductive discussion] Q : If space is expanding, then time must be expanding as well. So how can we even say that the Universe has an age ?

A : I am no expert in relativity, but here is my naive, non-mathematical, simplified interpretation. Short version : the cosmological principle states that space is and looks the same everywhere. So whatever passage of time we've experienced, the rest of the Universe has too.

Imagine that early in the Universe you scatter a bunch of stopclocks at random positions. Some of them will end up in completely empty space. Some will end up inside galaxies. A very, very few will be unlucky and fall into a black hole.

Let's further imagine an observer with the magical ability to travel instantaneously from one clock to another. We'll have them spend 14 billion years just watching one of the clocks which is in empty space, because we'll also imagine that they are seriously dedicated to looking at clocks and immortal. We shall also give our observer an enormously powerful telescope with which they may examine more distant clocks.

Our observer spends 14 billion years just watching this one clock in empty space, far from any galaxies. Finally tiring of this, they use their telescope to look at a clock inside the nearest galaxy. They will see that that clock does not quite read the same as their clock, since the galaxy has gravity and probably some motion relative to the observer (but not very much, since it's the nearest galaxy). But the difference will be small, no more than a few months after 14 billion years.

Then they look at a clock that's falling into a black hole. Here they will see that this clock reads quite different to their own clock, because of the black hole's gravity. But they realise that there are hardly any black holes, which only significantly influence time over a very small region. So they conclude that black holes haven't affected the age of the Universe at all, just very very small parts of it.

Things get much more interesting if they look at clocks in very distant galaxies. One obvious reason is that light takes a long time to travel across the Universe, so distant clocks viewed by our observer will report that less time has passed than our observer's clock. Of course, that doesn't mean that less time has really passed for the other galaxies, just that our observer can't see what's happening in those galaxies right now.

More interesting will be what happens if the observer watches one of those distant clocks for a little while. Because of the strong apparent motion of the galaxy with respect to the observer (near light speed), they will see the seconds tick by more slowly than on their own clock. This effect has actually been observed using supernovae.

But, what if our observer now travels instantly to one of those distant clocks ? If they are clever, they will know the Universe is expanding and so compensate for this. They will find that all of the distant clocks - except those very few which are near black holes - also read about 14 billion years. When they look back to the original clock, they will also see that time appears to be ticking away more slowly. All observers think the other clocks are ticking more slowly than their own, because of the apparent motion due to expansion of the Universe. But this is just a choice of reference frame. It is not that there has really been a difference in elapsed time for each galaxy and/or that different parts of the Universe have aged different amounts (they can't all be slower than each other, after all - that would make no sense at all !). It is different to the Twin Paradox, where twins travelling on spaceships moving at different speeds really do age differently.

The standard model is that the Universe has expanded uniformly everywhere. So all observers experience (almost, allowing for small local deviations) exactly the same amount of time. It's only that they perceive other observers to have experienced a different passage of time. That space is expanding doesn't make it wrong to estimate the age of the Universe anymore than it means that time hasn't elapsed.

Another point is that the expansion is negligible on small scales like star systems, and only important on very large scales like distant galaxies. That's what (I think) it makes more sense to think of this type of time dilation as being a perceived time dilation rather than a real one.

Can the speed of the acceleration of the Universe exceed the speed of light ?
Q : If space is expanding at an increasing rate, will it eventually grow to be faster than light ?

A : Unfortunately, it's more complicated than that. My highly simplified answer would be, "no, but you can think of it that way." In one sense, expansion is already happening faster than the speed of light, but in another, it isn't a speed at all - it's a speed per distance. Or not even that, since distance itself is changing - thinking of it as speed may be fundamentally flawed. Motion through space is not the same as motion of space. It's "travelling without moving", as they say in Dune. Perhaps it's best to think of the expansion as an apparent motion rather than a real motion.

A couple of links which provide more complete answers :

When an object falls past the event horizon of a black hole, how do we see it ?
Q : Around a black hole is an imaginary boundary called the event horizon. Now even light cant escape after crossing that boundary and if an object passes through it the object seems to be not in motion for an observer. Now because light cant come back how is it that we see an object passing the event horizon ?

A : The simple answer is that we can't. Once it reaches the event horizon, it's gone forever - we won't be able to see it any more. The much more complicated answer that deals with time dilation and whatnot is here :

If space is flat, if we keep travelling will we end up in another dimension ?
Q : If space is flat, would we be able to go straight, in any direction and escape this dimension of space and occupy another dimension ?

A : - that's the best answer I know to that.

Further discussion raised the closely-related question of whether the Universe must have an edge, given that it is flat.

All the observations show that the Universe is perfectly flat. In the Big Bang model, this means the density of the Universe had to be exactly the right density of matter to begin with. A tiny amount of too much matter would deform space so much we would be able to observe its curvature today, if it hadn't caused to already re-collapse. A tiny amount too little and it would have strongly negative curvature, and everything would have expanded so fast that stars would never have been able to form.

Personally I'm not sure this is really a problem. If a specific density is required to explain the observations today, then darn it that means it had that density. But the most popular explanation is inflation - the Universe was initially strongly curved, but expanded very rapidly. So on really, really large scales - much larger than we can observe - the Universe is indeed curved. It's like walking down the street : you don't notice the Earth is curved, but you would if you walked all the way around it.

IIRC a torus-shaped Universe would also appear to be flat since triangles would still have angles that add up to 180 degrees. So that would get you a "flat" Universe without an edge (I think), which is nice. I don't know how we'd test for this possibility. [Thanks to Ethan Siegal for confirming that this is the case. We can't test for this possibility except by travelling right around the torus - i.e. we'll never know if this is case or not. Bummer.]
Or heck, maybe the Universe does have an edge. A detailed simulation has been run exploring what that would look like :

Do we have to point telescopes away from the Galaxy towards the direction of the Big Bang to measure the Cosmic Microwave Background ?
Q : I do have a question about when a telescope is trying to measure the CMB : is the telescope pointing away from the galaxy ? Because there would be some distortion if it was pointing towards our sun, but the telescope can orbit the sun to look "behind it" But it cannot orbit the galaxy to avoid the distortion it would create to try to look through it to the other side of it, did we just get lucky that where we think the Big Bang originated from is easily seen from our place in one of the arms of the Milky Way ?

A : Planck measured the CMB across the entire sky. It's very difficult to get useful cosmological information from the "zone of avoidance" - the bit where the light from our Galaxy dominates over more distant sources. It can be done, but you have to be very careful. I assume that most of the CMB analysis is done on regions looking out of the plane of the Galaxy, where the sky is much less crowded.

The Big Bang originated everywhere, it was not an explosion occurring at one point in space. Rather it was the expansion of all of space, simultaneously. So we can see the CMB in all directions.

The advantage of being in a spiral disc galaxy is that although the sky is awful for cosmology in some directions (looking through the disc), it's much better looking out of the plane. If we were in an elliptical galaxy we'd be screwed because there'd be a lot of foreground stars in all directions (though much less gas and dust).

Can you really rip a hole in space and time ?
Q : In science fiction, you read about "ripping a hole in space-time"... Yet, is there anything realistic in the idea that space-time can be "ripped" or "torn" ?

A : I remember in Brian Greene's The Elegant Universe he concludes (after a very long and complicated discussion) that spacetime can tear according to string theory.

I suppose you could regard a singularity (the center of a black hole, where spacetime has infinite curvature... whatever that means) as a rip. Most people regard them as a problem for general relativity since infinities cause all kinds of horrendous problems for the equations. The suspicion is that the're not really infinitely dense, although AFAIK no-one has a good justification for this besides "infinity sucks".

The answer is a definite maybe :

Could dark matter just be normal matter that exists in another Universe which only interacts with ours through gravity ?
Q : I've got some reasons from the Q'ran to believe this, apparently.

A : No. Firstly, it's very unlkely to just be normal, baryonic matter. Microlensing experiments have ruled out dark matter as being ordinary planets and/or black holes (see point no.4.).

Having the dark matter be part of another universe is a very drastic solution to a problem that we can solve just by postulating the existence of a new, albeit weird, particle. Neutrinos are pretty close to what we need the dark matter particle to do, so invoking a new particle is a lot simpler than invoking a whole new universe.

Also, baryonic matter in another universe would have all the problems that baryonic matter would have in our Universe. When galaxy clusters collide, their gas gets stuck in the middle but the stars and dark matter keep going, exactly as models predict. If the dark matter was actually ordinary gas, it should still get stuck in the middle - it being in another universe wouldn't prevent that.

How fast are we moving away from the Big Bang ?
Q : I take it the matter of our galaxy is still moving away from the origin of the Big Bang. Does anyone know how fast ?

A : The Big Bang wasn't an explosion exactly, it was the rapid expansion of space. ALL of space - it occurred in all places simultaneously. In that sense it's still happening now. It didn't happen in one spot we can move away from. Our motion relative to the point of the Big Bang is zero, but because all of space is expanding, our motion to distant galaxies is enormous.

There's a complication in that because galaxies have gravity they pull each other in different directions, away from this uniform expansion. We can measure how fast we're moving away from what we'd expect if this didn't happen, and the answer is 627 km/s. To simplify slightly, this is how fast we're moving away from where the Milky Way first formed - but since the Big Bang happened everywhere, there's no getting away from it.

Can we see the beginning of the Universe, or at least close to it ?
Q : I thought scientists could already see back to the first trillionth of a second after the Big Bang ?

A : Nope. See, and also So far as I know we currently have no direct observations of anything that came before the CMB -"only" theory. But that's theory in the strict scientific sense of the word : that is, models which are consistent with many other observations of what came next.

Does the inertia of the galaxies affect the cosmological expansion ?
Q : So the expansion of the Universe seems to be slowing down (or accelerating nowadays, but the result is the same). If we take the balloon analogy, we have pebbles on the surface, but the surface is expanding at a variable rate, so the inertia of the pebbles should them bend the balloon surface inward (now outward), giving us back the nice trampoline surface analogy for gravity.

So why is this theory of gravity not working? Because if it was, someone else would have thought about it in a century. It seems to my older self that it wouldn't take all of gravity into account, nor that inertia works that way, but I'm not really sure.

A : Like all analogies, the balloon model of the expanding Universe is very good in some ways, but not perfect. When you want to make a prediction, it's better to rely on the maths.

To answer the question, that might work if you inflate the balloon very rapidly and then stop. A slightly simpler analogy (which can be easily demonstrated) would be a set of keys (or any small, heavyish object) on a towel. If you raise the towel slowly, the keys just sit there, deforming the surface. If you raise it quickly and then stop, the keys will shoot up before landing and settling back down in equilibrium.

The analogy with spacetime only works (as I understand it) in the slowly moving, maintaining equilibrium mode. You don't get any extra deformation due to inertia - it's only mass that causes deformation. If all of spacetime were to stop expanding right now, we wouldn't get any sort of negative curvature developing.

I would cautiously suggest that a better analogy for this purpose might be to imagine stretching a flat rubber sheet (keeping it flat on the ground) rather than a balloon, with a pebble glued to the very centre. In this case it doesn't matter how fast you stretch the sheet or how rapidly you vary the motion - the pebble will always remain in equilibrium.

Does ordinary matter become repulsive at high relative velocities ?
Q : Maybe this could be a way of explaining why the expansion of the Universe is accelerating.

A : Not as far as I know - mass ALWAYS acts to slow things down. How much it does this is debated. There are many alternative theories of gravity to the standard Newton/Einstein models, which have gravity decreasing in different ways. Such models have been proposed as alternatives to the standard model of dark matter. I'm not aware of any theories which say it should become repulsive, but it wouldn't surprise me if someone had tried this.

Is there any way to see back to the Big Bang ?
Q : Is it just a matter of inventing better telescopes and technology ? Could we ever get a video of it ?

A : Unfortunately, for the foreseeable future this is not possible.

When the Universe was young it was very hot and very dense, - an opaque soup of gas that no light could penetrate. This is called the surface of last scattering, or the Cosmic Microwave Background. Frustratingly, it stops us from seeing anything that happened earlier than about 300,000 years after the Big Bang. Seeing past it using electromagnetic radiation is literally impossible. There are two known exotic technologies that could see past this irritating barrier : neutrinos and gravitational waves.

Neutrinos are almost massless particles which pass through just about everything. They can be detected, but it's friggin' hard. Like, seriously friggin' hard. The IceCube telescope in Antarctica uses a cubic kilometre of ice and only detected 37 of the little buggers in three years, they're that stealthy.

Gravitational waves would also be a possibility but these haven't been detected at all yet. If I recall correctly, the resolution of gravitational wave detectors will be very low, so making images isn't really a possibility. Video is not likely in the foreseeable future. Science is a harsh mistress.

As the Universe expands and cools will we see changes in its chemistry that could be significant on a large scale ?
Q : As the universe continues to expand and cool, I wonder if there could be more global phase changes yet to come, such as molecular hydrogen condensing into ice, or even something far more fundamental which might promote contraction of the universe in (another episodic) big bounce.

A : Probably not. Molecular hydrogen is generally much warmer (up to ~1500 K though can be ~100 K) than the CMB (3 K). So making the background temperature at most 3K cooler won't have much effect (though you do get weird quantum effects near absolute zero). Also, the expansion of the Universe doesn't have much effect within galaxies. It could make a difference on larger scales but it would cause the density to decrease, making it more difficult for any further condensation to occur.

Do you have a graph showing the size of the Universe over time ?
Q : Can you put your hands on a graph that shows the relative universe size (as a function of "z") over time? I.e., how long ago was z = 1, z = 2, z = 3 and z = 6 ?

A : I was all set to re-learn the Friedman equations when I stumbled on this - which seems to be exactly what you were asking for :
A more detailed version, emphasising that we don't know how the size changed during its first moments very well because of inflation :

To see what a redshift of 1 means in real numbers, use the cosmology calculator :

Is there any evidence for the expansion of the Universe other than redshifts and Cosmic Microwave Background radiation ?
Q : Well, is there ?

A : This is a big question so I decided to give it a pretty thorough answer. The very short answer is "yes". The long answer is also "yes", but with many interesting caveats.

First, it's worth noting that when the Square Kilometre Array is constructed, it may be capable of measuring how the redshifts change over time. In principle you could watch a single galaxy and see its redshift change as the Universe expands. In practise because galaxies aren't just moving with the overall expansion, you need to measure a huge number of galaxies with very high precision over about ten years to see a meaningful change. This is probably about 30 years away, but would be as close to absolute proof of expansion as is possible to get.

Existing supporting evidence that the Universe is expanding includes the Tolman test, the time dilation of supernovae light curves, the anguluar diameter test, and a few others. For a review, see :

The two main arguments are the Tolman test and the time dilation of supernovae. The Tolman test says that if the Universe is really expanding, the surface brightness (brightness per unit area) of galaxies should vary with redshift - if galaxies have the same distribution of stars at different redshifts. The problem is that we don't expect this to be the case since galaxies interact and merge and star formation rate varies strongly over time. So it's difficult to account for this, and when people try they get different results. It's probably fair to say that so far the results are not inconsistent with expansion, and they certainly don't rule it out.

Secondly, if the Universe wasn't expanding, the brightness of a supernova should take about the same time to decrease in the distant Universe as it does nearby. That's not the case. Very distant supernovae take about twice as long to dim as nearby ones. And we know from other observations that these particular supernovae are type Ia (, which occur when a white dwarf accretes material from another star and eventualy becomes so massive that a huge fusion explosion is set off. The mass at which this happens should be fixed and not depend on any other parameters, so the peak brightness and the rate it decreases shouldn't vary.

Cosmological redshift/time dilation provides a very natural explanation of this. Just as the wavelength of light decreases from a source that's moving away from us, so should the time between events. Just as with time dilation due to motion through space, an observer closer to the supernova should see it take, say, 20 days to dim, but an observer far away should see it take longer.

It turns out that this effect has been observed and is consistent with the model of expansion based on redshift. This is pretty powerful, but not conclusive, evidence that the Universe really is expanding.

... but, it should be noted that quasars and gamma-ray bursters do NOT show the same effect ! Oh noes ! The reasons for this are still not understood. For quasars, one explanation may be that they evolve over time. We know that galaxies change over time, so quasar activity might also change as their fuel supply varies. Unlike with the critical mass for a type Ia supernova, this might be strongly related to the redshift, in a way that could exactly cancel the predicted time dilation :

As for gamma ray bursters, I am even less of an expert about these than I am about quasars. My guess would be that since the mechanism for the explosion is still not well-understood, it's much harder to say if the lack of time dilation is a serious problem given the scatter in the duration. I found a few references claiming that it's not such a big deal, but I wouldn't like to speculate too much.

Could the Steady State model be made to work ?
Q : Is there any other scientific explanation for redshifts and the CMB that could bring back a static, steady state model to explain our cosmological order ?

A : In my opinion, the Steady State model is decisively ruled out by other observations. We know that galaxies in the distant Universe are different to ones nearby, in a variety of different ways. For example, star formation rate varies strongly with redshift. The numbers on this "Madau plot" vary with different observations, but the characteristic hump-shape is pretty much an observational certainty :

Then there's the Butcher-Oemler effect. Consistent with star formation rate increasing with redshift, more distant galaxy clusters contain more galaxies with bluer colours (short-lived stars are bluer, so this is consistent with star formation rate being higher in the past).

Quasars are also much more common at higher redshifts :

There's also the fact that galaxy morphology (shape) varies with redshift. There are more peculiar galaxies and fewer spirals and ellipticals at higher redshifts : A nice visual example of this is the Spiderweb Galaxy. To my knowledge, nothing like this is known in the nearby Universe :

Taken together, in my opinion a Steady State model is now impossible. While redshift is not, strictly speaking, the same as distance, we know the correlation is good enough to say with certainty that more distant galaxies are different to local galaxies. This is in direct conflict with the Steady State idea. See also my article on why the sky is dark at night, which is damned hard to explain in an infinite eternal Universe, but straightforward if the Universe hasn't been around forever.

But I haven't actually answered the question yet. On redshifts, the most well-known alternative idea is that galaxies could have an "intrinsic" redshift. The evidence for this was that some galaxies which look like they're interacting with each other apparently have quite different redshifts. I don't know what the statistics are on this, but I'd be willing to bet that these are a small minority compared to the number which are interacting and at similar redshifts. There's also no known mechanism by which redshift could be altered in this way.

There's also the idea that maybe photons lose energy as they travel, so that redshift could indicate distance but not necessarily motion (i.e. expansion). This "tired light" idea cannot explain the supernova time dilation, and would predict that distant objects appear more blurry than they actually do. It also doesn't explain the CMB.

The CMB is explained extremely well by the Big Bang model, but cannot be made to work in a Steady State universe. One idea is that's actually just redshifted starlight, but the details show that this simply doesn't work.

Another idea is that supernovae/quasars create and eject "iron whiskers" which could transform the radiation of the youngest stars into the observed CMB. But this is highly contrived explanation. A friend of mine once did some research about this and found that they don't work; alas I'm not an expert in this so I can't comment further.

I would also add that I find a infinite/eternal Universe philosophically unsatisfying. Probability doesn't mean anything everything happens an infinite number of times, so if something extremely unlikely is observed, one can always say, "that's because of infinity" rather than searching for a real explanation. It's tantamount to magic.

Why are distances in space so enormous ?
Q : One thing that has been on my mind is why is there such great distance between planets and the Sun ? And on that same track, why is there such a great distance between the various stars and galaxies ?

A : That's either a really easy or deeply profound question depending on how you look at it. :)

The really easy version is that it's due to the strength of gravity. Given how fast everything is moving, if gravity was stronger everything would be closer together. In Newton's model of gravity there is a fundamental constant in the equation, G - make this value bigger and the force of gravity would be stronger. In Einstein's model it would mean that space is more curved by the same mass. Either way, it's all due to the strength of gravity.

The deeply profound version is : why should gravity take this particular strength and not something else ? That's not something I think anyone has a good answer for. Sometimes people invoke the anthropic principle : if it was different, we would not exist. That causes all manner of problems and doesn't actually explain anything unless you think the sole purpose of the Universe is life. Which I do not.

What's your opinion on the detection of gravitational waves ?
Q : Well, what is it ?

A : I was more impressed with the press conference than I thought I would be.

The signal seems like a very convincing detection. It doesn't look as strong as I thought it might, but it appeared in both detectors and matches the predicted signal so precisely it's hard to believe it could be anything else. Plus it was (mostly) under wraps for months. This is much better than the all too common tactic of doing science by press release and hoping no mistakes have been made.

I am less convinced of the importance of gravitational wave astronomy, however. Yes it's super super important for testing relativity, and yes, maybe it will tell us lots of interesting things about cosmology.... and yes, no doubt there will be some major surprises in store. For all these reasons it is a genuinely very exciting discovery and a historic moment. It deserves the Nobel prize it will inevitably get.

And yet... the spatial resolution is abysmal. We know a surprising amount of detail about happened, but the constraint on where it happened is so large you might as well wave your hand toward the sky at random and say, "THAT WAY !". Literally that - try it ! It isn't going to get much better anytime soon, with detectors coming online in the next few years only narrowing it down to a few square degrees. That's scarcely any more helpful if the source is a billion light years away. So if we don't even know which galaxy these events are happening in, I remain cautious as to how much we'll actually learn from these regarding how the Universe works.

I'd like to know more detail about deciphering the age, direction and source of the gravitational waves.
Q : Come on then. You said ask anything.

A : I did indeed, but you probably want to ask Jonah Miller rather than me !

As I understand it, direction comes from the time delay between the signal received at one antenna and the other. Since there are only two antennas, this doesn't really constrain the direction very much at all. It would be a lot better with 3 detectors, but still (IIRC) this will only constrain the location to within ~10 square degrees.

The distance (age) and masses of the sources are both inferred from the frequency of the signal. Through computer models of the interaction, the frequency tells you the mass. The mass determines the energy emitted, and since you can measure the strength of the signal and know the total amount of energy emitted, you can calculate the distance.

Since energy and mass are equivalent, are dark matter and dark energy really just the same thing ?
Q : Well, are they ?

A : No. To simplify, dark matter is largely thought to exist because galaxies are spinning too fast. It is thought to be an as-yet unknown form of matter, but still definitely matter in the regular sense - it has gravity.

Dark energy is an unknown force or energy field that is causing the expansion of the Universe to accelerate. It is not thought to be a form of matter, as this would have the opposite effect. No mainstream theory AFAIK postulates any connection between dark matter and dark energy. The similarity of the names doesn't mean anything.

Since matter can never be created or destroyed, are you the same age as the Universe ?
Q : Come on then. Make with the answer, clever clogs.

A : A small amount of matter is converted into energy during fusion inside stars. AFAIK the reverse process doesn't happen to any significant extent.More significantly, fusion joins atoms together into new atoms. So it is not correct to say your atoms are as old as the Universe, since many of them didn't exist until they were created inside a star.

I suppose you could say your matter is as old as the Universe, but I would also not say that you're even as old as your matter. You're not 'you' until your atoms are assembled.

What do you think of the idea that Universe had no initial singularity ?
Q : Specifically, as described in this article :

A : There's a nice summary here :

Personally, I'm not sure it helps. Either the Universe has existed forever, or for a finite time. The trouble is that if it's existed forever that just replaces one infinity (the singularity in standard cosmology) with another. Probability becomes meaningless in an infinite universe since everything happens an infinite number of times - this is the "measure problem" ( It's even weirder in this particular model - why should the Universe have got along for an infinite amount of time before suddenly deciding it wanted to explode ? Does "now" even mean anything in an infinite universe ? Seems to me that these are pretty deep philosophical issues our monkey brains are ill-equipped to answer.

Does space have limits ?
Q : Our observable universe is limited because of metric expansion of space over distances beyond gravitational bounds. Is there any evidence to suggest that the universe may have other limits beyond what we can observe? Is it reasonable to assume that there are no limits other than what we have observed, or should we accept the possibility of limits that have never been observed just because we can imagine such possibility ?

A : Tricky. It depends what you mean by "limits". If you mean, "is space infinite ?" then see this earlier answer :

I would add a few points :
First, I dislike the notion of an infinite universe because it means that everything possible happens somewhere. That means that science is basically knocked on the head - there's no point trying to explain anything because everything that can happen, does. Probability is meaningless.
Of course the Universe doesn't care what I think, so it could be infinite. But since we can't test this observationally, we've no choice but to resort to philosophy.

Second, the observable universe is a tricksy concept. It's not like on Earth where if you want to see over the horizon you just go for a long walk. Their are, according to models, parts of the Universe so distant that they will never, ever affect us. To go beyond this and report back to everyone left on Earth we'd have to travel faster than the speed of light.

Third, if you meant, "does the Universe have an edge ?" then I dislike this idea too. It's difficult to imagine what an edge to space would look like, but it would also violate the Copernican principle. Observers close to the edge would see a very different Universe to ones near the middle. Of course, the Universe could just be vastly larger than the visible Universe, so that most observers are never aware of the edge.

I prefer the notion of a torus-shaped Universe, which solves all of these annoyances. Flat, finite and unbounded, that's the way to go.

Why do black holes bend space ?
Q : I have a question, why do black holes bend space ? How is it work actually ?

A : All mass bends space. According to general relativity, gravity isn't exactly a force - it's the curvature of space. Don't think of gravity as a force or somehow separate from the bending of space. Gravity is the bending of space. Every single atom bends space a tiny little bit. A pretty nice explanation can be seen here (it really is easier to show this rather than trying to describe it) :

Black holes just bend space more than other objects because they have a lot of mass in a very small volume. Technically they bend space by an infinite amount according to relativity. Most people think this indicates that relativity isn't a complete description of what's going on, because infinities cause all kinds of horrendous mathematical problems.

If space was curved enough could I see the back of my own head ?
Q : Scientists said 'we can see the back of our head', that means, the matter is orbit the black holes, right ? That's why, we can see the matter behind the black holes, even in a picture.

A : Well I am not sure about the specific example of a black hole, but yes - if space was sufficiently curved it would be possible to see the back of your own head. We can see slightly less dramatic examples of this all the time. For example, stars near the Sun have their position slightly altered because of its gravity.

How do we know space is curved ?
Q : If space is curved, how do you know it is? You live on the curve itself. The distance you measure from the mass is the distance along the curve. And that as the mass is far, far away. That means all you can measure is a very, very small curvature, regardless of how close you get to the mass. If fact, the closer you get to a mass, the flatter the curve becomes.

Space is not curved and time is not a dimension.

A : [Oh God hear we go...]

Space is indeed curved and time is indeed a dimension, albeit not a spatial one.

There's a wealth of evidence that space is curved. You can measure this because the path of a light ray follows the curvature, making it appear as though distant objects are in a different position. For instance, the position of stars near the Sun are observed to shift in exact agreement with the predictions of relativity :

Gravitational lensing also produces distorted images of distant galaxies, again in agreement with the predictions of relativity. In some cases the effect of this is extremely clear - you can measure a very large curvature without having to be close to the mass at all :

[The OP decided to ignore what I wrote and continued to advocate their intuitive opinion over rigorously tested mathematics. Oh dear.]

Q : If you can measure the curvature, you are not in our universe.

This mystical curvature changes the radial distances from the mass but not the circumferential ones. An object travelling in a circle with its centre at the mass will have no changes in the distance it travels. This mystical curvature has no effect on the circumferential distances.

But when a photon passes by a mass, its path has its greatest curvature, which is the exact opposite of this mystical curvature.

You have just proved space is not curved.

A : That's not the case. See in particular the first link I posted. The rubber sheet analogy is a pretty good one. Here's another, slightly better diagram :
I don't really know how else to describe it. Curvature just doesn't work in the way the OP thinks it does.

[Still not enough. Come on, people, you can test how curvature works for yourself using a blanket and a heavy book. Einstein's maths may be difficult, but the concept of curvature really isn't.]
Q : Why people insist That Einstein never made a mistake on Relativity and had it published ?

A : We insist that Einstein had relativity published because, in fact, he did. No-one insists that Einstein never made a mistake, but everyone insists that the predictions of relativity have come true because... well, because they have.

Is space globally curved ?
Q : [This was in the context of the curvature of ALL of space, as opposed to bits of it in an earlier question ( In a sense, "does space curve ?" and "is space curved ?" are different questions/]

A : We can state with close to certainty that space has local curvature, as described.

Global curvature is another matter. Observations say no, it's flat :
(Caveat : the surface of a sphere is curved, but the surface of a torus is flat. We could in principle detect if the universe was curved like a sphere, but not like a torus.)

But theory says that it shouldn't be flat. If it's even slightly curved, it should either collapse before any stars have time to form or expand so rapidly that the density never gets high enough for star formation. Hence the idea of inflation : the early Universe expanded very rapidly, so that the observable Universe today is just a very small part of the whole. So globally it might be curved after all, just by such a small amount that it's not detectable.
(According to some there's no problem here at all and it's just a coordinate issue, however, this goes against absolutely everything I've been taught so I'm rather skeptical :

Apparently inflation does produce distinct signatures which could be detected, which resulted in the BICEP2 debacle last year. But no-one understands what caused inflation, and furthermore theoretical estimates of how large it should be based on known physics are ~10^120 times greater than observed - probably the most wrong answer anyone's ever come up with.

Personally I've never liked inflation much. If the Universe had to have precise conditions to start off flat, then dang it it must have had those conditions. The fact that the Universe is homogeneous is often cited as evidence that there must have been some period when regions of the Universe which are now distant must have been in contact with each other - i.e. inflation caused their separation. I've never understood why anyone would assume that the Universe just wasn't homogeneous everywhere to begin with anyway.

Is the Universe shaped like a doughnut ?
Q : If a torus-shaped Universe is flat, then could the Universe be torus-shaped and smaller than we think ? Could it actually be smaller than the observable Universe, allowing us to see a younger Milky Way whose light would have circled (torused ?) the Universe ? And if not, of course, how do we know/what did I get wrong ?

A : In principle yes, the Universe could be smaller than it appears. What appear to be very distant galaxies could actually be light emitted by a close galaxy that's traversed the Universe several times. Since the Universe is continuously expanding, we could even get the appearance of galaxies in a continuous distribution of distances, not just at multiples of the true size of the Universe.

I think in that case, though, we would inevitably see the same structures if we looked in opposite directions - the same large-scale structures of galaxies or the CMB, but mirror-flipped (though it would be more complicated than that since there would be multiple paths the light could take to reach us). As far as I know, no such repeating structures have ever been observed. That's easily avoided by having a larger-than-observable Universe (doesn't preclude it from bein a torus or other geomtery) but I don't see a way around this in a smaller-than-observable case.
But much better, watch this :

Yeah but really, could Steady State be the answer after all ?
Q : [Amalgam of responses from this thread :] Don't forget Tectonic Plate Theory would get you fired until it was undeniable. As much time that has been spent on the Big Bang Theory we should know it all....wait we just found gravitational waves. By your thinking we no longer need to put another dime into the big bang Theory.

I personally see more common sense in the steady state Theory. Whose to say that quasars aren't the baby stages of gravitational waves? There are just too many unanswered questions with the steady state Theory to ditch it because we have seen more of the Big Bang Theory. Maybe the steady Theory finishes the the Bang Theory. You don't just stop the research. I believe in the future scientists will be researching the steady Theory again. It didn't pop up for no reason. Someone just hasn't found the link. :-)

A : As for plate tectonics, I'm well aware of this. Don't go there or it will go the worse for you.

Steady State is pretty much dead since more distant galaxies are clearly different from nearby ones, quasars are more common at greater distances, the CMB, etc. _Un_steady State is, I guess, still possible. The idea that the Universe isn't really expanding is, I suppose, just about barely tenable, but not at all likely.

Honestly, Steady State is done. If you don't close some avenues of inquiry, you'll never make any progress.? Which I explain in great detail here :

To say that this line of reasoning means that we wouldn't have investigated the Big Bang model is wrong. On the contrary, that is precisely the opposite of what I'm saying ! It is exactly because the Big Bang theory works very well that we need to keep testing it. Same as with any theory that works - you have to keep testing it to find out where it stops working. That's the only way you learn. At the same time, you don't keep investigating ideas when you know they don't work.

Testing theories is not done as an intellectual chest-thumping exercise. It is done in order to learn more. This article has the right of it :
"The data so far has confirmed that our theory is really really good, which is frustrating because we know it's not!" Prof Shears says. "We know it can't explain a lot of the Universe. So instead of trying to test the truth of this theory, what we really want to do now is break it - to show where it stops reflecting reality. That's the only way we're going to make progress."

Classical Steady State theory really is as dead as the dodo. The whole point of it was that the Universe is not evolving with time. This was a valid idea in the 1950's, but we now know with 100% certainty that that isn't the case. No observations will ever disprove this. It's like saying that one day we will find the edge of the Earth if we just keep looking a bit harder.

Where there is considerably more wiggle-room is the idea that the Universe hasn't always been as we see it today, but is still infinitely old (and perhaps infinitely large as well). It's possible that the Universe is in some way cyclic, with many Big Bangs. That's a much harder idea to prove/disprove. At present there's no reason to think this is likely to be the case. But this is very different from the original Steady State theory.

How does the expansion of space cause resdshift ? Where does the energy of each photon go ?
Q : If the temperature of the universe at the epoch of recombination was in the visible light range, then has the expansion of the universe stretched the actual photons into microwaves, or are we viewing visible light photons that are being instantaneously stretched at the point of our receptors into microwaves because we're moving away from the source at nearly light speed ?

In a universe in which energy is conserved, where does red shifted energy loss of the photons go ? Is the energy loss gradual over time, or instantaneous at the point of absorption ?

Expanding gas cools because the molecules are continually doing work on a piston, or what ever, but the last scattering surface of the CMB photons was at the epoch of recombination, so they've done no work on anything. So is there any intrinsic sense in which CMB photons still carry the very same unshifted frequency that they were emitted with ? I.e., does a gamma ray know it's a gamma ray ?

A : [This question was, for some unfathomable reason, asked on a joke thread that reported on Saidq Khan's election to Mayor of London by playing on his surname with many a reference to Star Trek. This seems a bizarre place to ask me a question, so I responded in kind].


As I understand it, YOU KLINGON BASTARDS YOU'VE MURDERED MY SON ! both redshift due to motion through space and of space (cosmological redshift) OF ALL THE SOULS I HAVE ENCOUNTERED ON MY TRAVELS cause an instantaneous change of the photon HIS WAS THE MOST.... HUMAN !. Cosmological expansion is acting continuously, so the wavelength changes continuously over time. But the wavelength you perceive the photon will also depend on your motion, I WAS AND EVER SHALL BE, YOUR FRIEND so it's complicated.

About energy conservation, my hand-waving response would be I'LL CHASE HIM ROUND THE MOONS OF NIBIA that the energy density goes down, but the total volume goes up, so the total energy AND ROUND THE ANTARES MAELSTROM is conserved. And energy conservation AND ROUND PERDITION'S FLAMES is somewhat... subtle in general relativity BEFORE I GIVE HIM UP !.

I'm not entirely satisfied with HOW WE DEAL WITH DEATH IS AT LEAST AS IMPORTANT AS HOW WE DEAL WITH LIFE that answer. I suppose you could think of the photons as WE'RE TALKING ABOUT UNIVERSAL ARMAGEDDON ! doing work against the expansion of space, just as they do work escaping from a gravitational field for gravitational redshift. Space pulls the photons apart, so the lost energy goes into the expansion of space. DAMMIT JIM, WHAT'S THE MATTER WITH YOU ? but that's just my very naive interpretation. My suspicion is that THE NEEDS OF THE MANY it's the expansion of space which OUTWEIGH THE NEEDS OF THE FEW causes the loss of energy, but the total energy is still not really OR THE ONE conserved.

My relativity knowledge is not what it once was.


Are we sure the Tired Light Theory was wrong, then ?
Q : Pretty darn sure. Like any alternative to the expanding evolving universe, there are just too many things it can't explain - the fact that surface brightness of galaxies decreases with distance, time dilation of distant supernova, the spectrum of the CMB, the greater numbers of quasars at greater distances, the different morphologies of distant galaxies, etc.

Who can explain to me why Einstein was trying to find the Gravitational constant of the universe ?
Q : Well, what's it for ?

A : [The questioner seemed repeatedly confused about the difference between G, g and lambda, then eventually the conversation degenerated into a case of, "I admit I don't know this very basic fact, but I'm pretty sure I understand all these much more complicated aspects of the theory." Here's my answer anyway.].

There's the gravitational constant G, which as far as I know Einstein never did much research on, and the cosmological constant Λ (Greek letter lambda), which is what famously gave Einstein a headache. I assume that's what you're asking, though you keep referring to the gravitational constant. That's something entirely different ( As far as I know, it isn't possible to predict the value of G by theory (not relativity anyway), it has to be measured. Which Einstein didn't do, since he was a theorist.

As for lambda, that's another story. When Einstein came up with general relativity, he realised the equations implied that a static universe was impossible. Given enough time, it had to collapse to a point. This meant the Universe could not be infinitely old and static, which was the prevailing wisdom of the time. So Einstein added the cosmological constant as a way to make his equations agree with this widespread prejudice. That's the short answer to your question : because he thought the Universe was not expanding.

The other way to save his equations was to assume the Universe was expanding, and by direct implication it had once been smaller in the past - i.e. it was finite in age. That would at least prevent a rapid collapse, and if the expansion was rapid enough it would mean the Universe would never re-collapse. There was no particular reason to assume this was the case at the time, as there was little or no evidence for an expanding Universe. So adding the cosmological constant made sense, as a way for his theory to agree with what everyone thought the Universe was doing. I would be rather surprised if he never calculated a value for the constant though.

Of course, it wasn't long afterwards that the Universe was discovered to indeed be expanding - so if Einstein hadn't introduced the term, he would have made an astonishing prediction. Then again, by the end of the century it was found that the expansion of the Universe was accelerating, which also needs a cosmological constant (or something similar). So he may have be right after all, but for the wrong reasons.

Isn't G related to the total amount of gravity in the Universe ?
Q : I thought G was the sum total of all the gravity in the universe. Regardless of the fluctuations of force.

A : The way I see it, G is just a number to make the equation work correctly using our choice of units. E.g. we generally use kilograms for mass, not pounds or stones. The value of G (given some choice of measurement units) determines how strong gravity is for any given amount of mass. If it were larger, all gravity would be stronger : everything would be more attracted to everything else. It's not related to individual planets or other celestial objects.

So the force that holds the planets in orbit and keeps the moons orbiting the planets is the weakest force ?
Q : Not to mention it faster than light or the lights momentum would be able to escape its grasp considering how weak it is.

A : Yes, gravity is the weakest force, and yes, light does escape - very easily ! The only thing light cannot escape from is a black hole. There's a nice explanation of how weak gravity is here :

The Universe has expanded so much that some parts of it are now unobservable. But could we explore those parts if we had a faster-than-light drive ?
Q : Out of curiosity, wouldn't an advanced civilization capable of warp speed travel be able to explore the unobservable universe ?

A : Yes they would. There's nothing special about any particular region; if they had a hypothetical FTL drive I'm not seeing any kind of cosmological reason which would prevent them from exploring the unobservable regions. To an observer in any part of the Universe, more and more of it becomes unobservable as time passes, but their particular region looks normal. Unless, that is, you take the extreme case of the Big Rip, where matter itself is eventually ripped apart by the expansion.

How do you know that dark matter is diffuse ?
Q : Well, how ?

A : Because of the rotation curves of galaxies and the velocity dispersion of galaxies in clusters. Highly concentrated dark matter couldn't cause a flat rotation curve at large distances - they imply that the total amount of mass keeps increasing as you go further away from the galaxy.
Basically you equate F = GMm / r^2 with the equation for circular motion, F = mv^2/r

Dark matter is thought to be distributed in roughly spherical halos. In principle it could be part of the disc, but that should cause observable changes in the vertical motions of the stars. As for why dark matter makes things move fast in the first place, imagine if the Earth was ten times heavier. Anyone falling over would hit the ground much faster. Similarly, baryonic matter falling into a dark matter halo moves faster because there's more mass.

How small was the fraction of deuterium created during Big Bang Nucelosynthesis ?
Q : There's one part of cosmology which has always baffled me as a civilian: Recombination

I kinda get it, but know better than to think I fully understand what's going on. Here's where it kinda meshes with your H1 / H2 problem :

Only a finite amount of deuterium was created at BBN. But how small a fraction is it?

A : I've always thought recombination should just be called "combination", it's not as if the electrons and protons were previously combined... I'm no expert in BBN, so don't take this as more than an educated guess. But as I understand it, something close to 100% of the observed deuterium should be primordial because there's no other known way of producing it. Models of BBN predict the amount of deuterium, which depends on the baryon density of the early Universe. As long as the amount of deuterium hasn't been significantly altered by any other processes (which seems to be the case), observation of deuterium can constrain the baryon density of the early Universe.

Other constraints on the age of the Universe and the baryon density should allow you to falsify the model - if they implied the model predicted much more or less deuterium than is actually observed, there'd be a big problem. There's a complication in that chemical evolution isn't simple - even if the deuterium amount doesn't change, the amount of ordinary hydrogen can (e.g. by accretion). The deuterium abundance in the Milky Way is actually slightly less than predicted (so if there's any non-primordial deuterium is must be negligible), but, apparently, chemical processes can explain this quite well :

Could the Big Bang be a white hole ?
Q : Would it be possible that the Big Bang was/is a white hole (sorry if you hear this a lot) ?

A : For once the short flippant answer and the "more detailed" response are going to be the same : sure, why not ? In terms of what caused the Big Bang to happen I think we're very much still in the "here be dragons" stage. Something has to provide that source of energy and mass. Could be a magical deity, could be our own Universe in the future connecting to itself through a black/white hole, could be something to do with quantum, who knows ? Not me. ;)

How can we see light from the Big Bang, shouldn't it have overshot us ?
Q : If we can see light from the edge of the big bang ,and that's were we came from, why did that light not pass us ? or move with us ? How can we see back to a beginning that began us ? ...maybe this is a physics question?...?

A : There are two parts to that question. First, we can't see directly back to the Big Bang itself. Initially the Universe was nothing but a plasma of protons and electrons which scatter photons of light, so it was opaque. Eventually the Universe expanded and cooled enough to allow the protons and electrons to combine into neutral gas, which is transparent. So from that point on any photons were free to travel, preserving a snapshot of the Universe at that moment (which we see today as the Cosmic Microwave Background). This is known as the "surface of last scattering" and is generally reckoned to have happened 300-400,000 years after the Big Bang.

The second part of the question is that the Big Bang didn't happen at a fixed point, it happened everywhere : all of space is expanding. So our Galaxy has been observing photons from the CMB for billions of years, because the CMB was formed throughout the whole volume of the Universe. We've absorbed some photons but we're still receiving others from the most distant part of the Universe.

Does the energy of the Universe really add up to zero ?
Q : So, when Michio Kaku refers to it all adding up to zero energy, is there any "creative accounting" going on ?

A : As far as I'm aware it's not known for certain if the total amount of energy really equals zero. Yet even if it does, I'm not sure that's particularly helpful either. A rock falling in the gravitational field of a planet can have zero net energy : positive kinetic energy but negative gravitational potential energy. But clearly the rock and the planets are things which do exist, but the total energy of the system hasn't helped explain their existence. It's not clear to me how this would help explain the existence of the Universe either.

How can the Universe be expanding if it's infinite ?
Q : So firstly the universe is ever expanding, yet it's infinite. Those both contradict one another are they separate theories ?

A : The evidence that it's expanding is very strong, but there's not really anything to indicate that it's infinite. It's finite in time, so it could be finite in space as well.

Could frame dragging explain dark matter ?
Q : [This was in relation to a recent cat fight claiming that dark matter has problems explaining small variations in the rotation curves of galaxies. In fact, as has now been shown by many independent authors, that is simply not true at all.]

What about frame dragging for the galaxy ? Is gravity an emergent force ? Could the underlying principles that give rise to emergent gravity be responsible ?

A : I doubt frame dragging or any other relativistic effect could be responsible - someone would have noticed this from Einstein's equations by now ! The point is rather than the earlier claims were that this would be very difficult to explain through standard models, this is now not the case. Therefore this observation cannot be used to test whether modified gravity theories are any better than standard models.

Though there is now yet another alternative model of gravity claiming that "emergent gravity" is responsible after all :

Call me skeptical : people love trying to "explain" dark matter because if they succeeded they'd be hailed as the next Einstein. Yet the model has survived many alternatives and always come out on top. For sure there's a scientific revolution awaiting the unification of general relativity and quantum mechanics, but that doesn't mean dark matter doesn't exist.

Do most models of dark matter say it should interact with itself ?
Q : Don't most particle physics models of possible ways dark matter could exist also suggest that it interacts with itself and with ordinary matter via one of the three known non-gravitational forces on extremely rare occasions?

A : Yeah, there are a bunch of models of self-annihilating dark matter that could emit gamma rays. In the extreme case there are models showing that you could get star-like objects from dense concentrations of neutralinos : As I understand it (but the particle physics is outside my specialist area, which is galaxy evolution) the dark matter in these cases would only radiate when it self-annihilates, and the signal would only be strong enough to be detectable in extreme cases like the galactic centre :
I'm not sure what the current status of this idea is though. In galaxy evolution models this effect is typically ignored as it's not thought to be significant in the grand scheme of things, but it could be an important way to detect it.

If dark matter is self-interacting then shouldn't it behave just like a normal gas ?
Q : Shouldn't the dark matter particles in the models follow gas dynamics when interacting with each other?

A : It depends on the nature of the interaction. If they're each other's own anti-particle, then probably not : each particle will annihilate. If they were interacting in some other way, such as via electrical forces as normal matter does (but somehow limiting this interaction only to the dark matter), then yes.

But this (along with baryonic dark matter) is pretty convincingly ruled out by the Bullet Cluster, in which two galaxy clusters have passed through each other. The galaxies themselves have kept going and are largely unaffected; stars are too widely separated for collisions to be frequent. However, the gas fills a much larger volume : there's no way it should be able to avoid a collision. And it hasn't - it's very clearly got stuck in the middle. The dark matter in the clusters appears to have behaved very much like the collisionless stars and kept going, with gravitational lensing observations indicating that the clusters still possess huge amounts of unseen matter, with little or none of this ending up in between the clusters. So the standard model of dark matter as a collisionless particle appears to be holding true.

What's your take on Verlinde's new theory of gravity ?
Q : Well, what is it ?

A : Erik Verlinde is a respected, "definitely not a crackpot" kind of physicist who's come up with a new model of gravity. It apparently predicts things similar to the more well-known "MOND" (Modified Newtonian Gravity") theory that's been a thorn in the side of standard-model advocates for the last 30-odd years. The neat thing about it is that whereas MOND was created as a mathematical solution to avoid the need for dark matter, Verlinde's is a more "bottom-up" approach which predicts MOND from basic physical principles.

However, that's about the extent of my knowledge of the material, which is far too mathematically advanced for the likes of me. So take this response with that in mind. Even so, my first reaction to any alternative theory of gravity is more or less this :

I am not really qualified to judge Verlinde's specific idea. One thing that modified gravity theories in general suffer from is a lack of simulations, usually because it's not as simple as changing the strength of the force law. In MOND, which apparently Verlinde's idea is very similar to, acceleration depends on the distribution of matter in a completely different way to Newtonian gravity. Analytic predictions only work well in very limited scenarios and it's impossible to say what would happen in a full-on simulation in anger, i.e. a large-scale cosmological simulation or a smaller scale version with baryonic physics. These modified gravity ideas might work much better than the standard model... or they might have entirely new problems that the analytic models can't predict. We just don't know yet.

Does the bullet cluster interaction data rule out some of the extremely weakly interacting forms of dark matter I was talking about previously ?
Q : Well, does it ?

A : For context the type of dark matter referred to was the self-annihilating kind. Some models predict that dark matter particles can occasionally interact and annihilate each other, producing a gamma-ray photon we could detect. The signal would be strongest where the dark matter density is greatest, like in the centres of galaxies and galaxy clusters.

Actually I was going to say that the density of dark matter in the centres of clusters probably wouldn't be high enough to detect a signal, but I stumbled on this...
I only read the abstract, but it seems they do believe it's possible to just about detect such self-interacting dark matter in a nearby galaxy cluster. The Bullet Cluster is (from some quick Googling) 15x further away and 3x less massive, so the expected signal would be hundreds of times weaker (since the interaction is weak, the density during the collision couldn't really be more than double its usual value since it would be effectively just overlaying two normal-density clusters). So it doesn't give us any evidence either way for this kind of self-interacting dark matter.


Why are space probes so frickin' SLOW ?
Q : What is preventing humans from making a spacecraft that go almost to the speed of light ? The fastest I have seen the Voyager I and II go in free space is around 33,000 kph... That is 40 yr old technology, and the "New Horizon" spacecraft is going to be going about the same speed... Why can't we make spacecraft that slice through space like a hot knife through butter ? Zoom !!! I'm ready to go, are you ?

A : The energy requirement to reach a certain speed scales as speed squared. Want to go twice as fast ? You need four times the energy. Ten times as fast ? A hundred times the energy. And that's at low speeds - as you get closer to the speed of light, things change... reaching light speed requires infinite energy. Even speeds just below that require outrageously insane amounts of energy.

Then there's fuel - you need a lot of it. To get a hundred tonnes to escape velocity, you need nine hundred tonnes of fuel. To go ten times faster, your thousand tonne rocket will now need to be more like ten billion tonnes.

In principle you could go a lot faster if your rocket had a higher exhaust velocity. Problem is that that is very, very hard to do. See : 

Are there any plans to make faster spaceships ?
Q : Are there potentially other, innovative or 'off the wall' ideas that can possibly help lessen the amount of energy and fuel we need for really high speed space travel ?

A : Yes there are ! A solar sail uses no propellant at all, so is not subject to the so-called tyranny of the rocket equation. It has to be very large to ever reach a high velocity with a reasonable payload, though (many hundreds of square kilometers, I think).

A similar idea is to use a powerful laser to push the sail. In that case the sail doesn't have to be so large because the laser provides additional pressure to that coming from the Sun. You'd need another laser at the other end to slow down, mind you.

There are other technologies that can provide higher exhaust velocities (ion engines) but less thrust. So they can reach higher final velocities but they take a long time to accelerate. Those are already in use for deep space probes, e.g. Dawn (

Whether they scale up to larger masses, I don't know. I don't see why they wouldn't. For manned probes, you're still going to need a stonkin' great big ship because of the difficulties of providing life support for a long duration.

Then there's the Orion drive, which uses nuclear bombs to blast a ship forward. That would have both outrageous levels of thrust and final velocity.

Less crazily, there's the under-development VASIMIR engine (

Probably the most exotic solution is the Alcubierre "warp" drive, but this relies on very exotic physics and should be considered as interesting speculation more than a practical solution.

That's just a few off the top of my head. For more, see this :

Can we use the expansion of space to travel really really fast ?
Q :  Couldn't we use space's energy to our advantage ? After all, isn't space the thing that is moving the fastest ? Instead of fighting against space forces, shouldn't we somehow consider harnessing that very energy and use it to our advantage?

A : Unfortunately, space is expanding rapidly only on very, very large scales - as in between us and other galaxies. It can't be used in any useful way, since everything is moving away from everything else.

However, similar to a warp drive, we might just possibly eventually create a wormhole by folding space. In principle, there's no upper limit as to how fast you can deform space, so this could overcome even the limit of light speed. But this has similar problems to the warp drive - you need amounts of energy which are utterly impossible with today's technology, and, worse negative mass, which isn't yet even known to exist. It might not be impossible in the strictest sense, but it's going to take a complete revolution in physics and engineering to make it remotely practical.

Can a volcano launch a small moon into space ?
Q : Something that I have been thinking about was the massive explosion of Mt. St Helen in Washington state in 1980. What I have been wondering is what a 1/3 of its mass gone into the atmosphere, could some of it have escaped to form what might be another moon.

I realize that it is a far fetched idea, but I was wondering what the likelihood of another moon's influence on the planet Earth.

A : To send material into space you have to get it to around 11 km/s, otherwise it will just fall back down. Volcanic plumes can be extremely high (~50 km), but reaching that altitude can be achieved with a) much smaller velocities, more like 1 km/s - which requires 100x less energy or b) the material is hot, so it rises, like a hot air balloon, relatively slowly through the atmosphere.

Even if a volcano did eject some material into space, it's pretty much impossible that it could eject so much that it would become a gravitationally-bound structure, i.e. a Moon. The material would disperse pretty quickly just because of the random motions the explosion would have given it.

You'd also need a lot more mass than that of a mountain before there'd be any significant effects on the Earth - the mass of the Moon is many millions of times greater than that of any mountain.

However, strong tidal forces are certainly important elsewhere in the Solar System.

Q : Earth's escape velocity is 11.2 km/s. The escape velocity doesn't depend on the mass. And the thrust created by a volcanic eruption is much much greater than the thrust required to launch a rocket. So if a rocket can achieve 11.2 km/s why can't a small particle from a volcano ?

A :  Because of the atmosphere. A rocket is continually accelerating - it is continually working against atmospheric drag. For a particle ejected from a volcano, it will have to be almost instantly accelerated up to at least 11 km/s, and in fact even more than this because of drag. The smaller the particle, the larger its surface area for its mass, so the faster it needs to go to overcome drag. And if it goes too fast, heating by air compression will cause it to burn up.

Q : If they can send monkeys, dogs and older men (former Sen John Glenn) into space, why not send younger people (12-21yrs old) into space and see how space can affect them. Not long term, but for say maybe two weeks. That way they can get an idea of what weightlessness can do to developing bodies and also give the young people an idea of why space exploration is so important and get their input on how they think things should proceed.

A : If we're serious about colonising space, then long term this may be unavoidable. However, we're certainly not there yet. Given that the effects of weightlessness are known to be pretty nasty (bone loss, muscle deterioration), sending people who aren't fully grown into space - willingly or not - would be highly unethical. 

There'd be a chance they'd experience problems years down the line - but to what gain ? There's nothing preventing them from becoming adult astronauts who can make a genuine contribution to the mission, rather than being an incredibly expensive, potentially dangerous (12 year olds on a space station... what could possibly go wrong...) payload.

For now we should continue to investigate the effects of weightlessness on animals and develop treatments for its effects on adult humans before we even think about risking development of children. By the time we ready to start thinking about kids in space, it's entirely possible we'll have centrifugal ships to mimic gravity, thus making the experiments unnecessary. In which case we're risking their health for nothing.

To my mind the consent of an 80 year old astronaut in full command of their mental faculties does not remotely compare to the consent of a 12 year old. Children simply do not have as much knowledge as adults (intelligence is not the same as knowledge), therefore their decision cannot be as informed as an adults. 

Space travel is inherently risky, but to me this does not seem like a risk worth taking. If, in the future, we come up with effective means to prevent the debilitating effects of weightlessness, then maybe. But it seems more likely we'll develop rotating spaceships by that point.

Q : Could someone explain to me why; on one of NASA's website pages they say Voyager's speed is 37,000 mph. Why would it not be able to go faster, due to the gravitational pull of the next star system ? Wouldn't it not speed up, with the pull towards that star system ?

A : The analogy I like for imagining the distances to the stars is to shrink the Sun to the size of an aspirin. At that size, the nearest star is about 270 km (170 miles) away, while Voyager 1 is just 140 metres away.

Q : Seriously, I need to make an urgent trip to Alpha Centarui next week.

A : Put it like this... Newton realised that if you fired a cannonball fast enough it could go into orbit. It took 270 years to make this a reality. And that was "just" to make the engineering work - for FTL travel we don't even know if it's physically possible, and indeed pretty much everything right now suggests that it isn't.

But, here's an interesting consequence of relativity : a spaceship travelling at close enough to the speed of light can fly across the Universe in a few seconds. That is, for the crew of the ship a few seconds will pass. For everyone on Earth, billions of years will elapse. Einstein not only spoiled Newton's clockwork Universe but he replaced it with one which is frickin' weird.

Current science does not say FTL is definitely impossible, just really damn difficult at best, and likely impossible. Wormholes and warp drives might work, but they are on the very edge of our understanding of physics.
"As a very rough approximation, you would need the energy the sun produces over 100 million years to make a wormhole about the size of a grapefruit."

Do astronauts age differently ?

Q : I'm thinking that because the people on the ISS are in orbit around the Earth, that they would age a bit differently than if they were further away, going away from the Sun.

A : Yes they do, by about a tenth of a second over the course of a decade. Time can pass at different rates depending on speed and gravitational field. For the ISS it's speed which dominates since the strength of gravity is about the same up there as it is down here. Fortunately, someone else has already done the maths :

Could we find new - possibly dangerous - elements on distant planets ?
Q : On Earth, we have the "Periodic Table of Elements". What happens if we go to another world and find new and different elements that have vastly different properties than those on Earth ? Do we mine then and bring them back to Earth, not knowing the consequences or do we leave them there ?

A : We can detect different elements by studying light. If you pass ordinary "white" light through a prim you can split it into its component colours (a rainbow). Using the proper instruments it's possible to see dark lines in the spectrum caused by different elements. This is how helium was discovered.

I am not aware of there being any currently unidentified spectral lines that could be a similar discovery. AFAIK, theory predicts that it should be almost impossible to produce any heavier elements than those currently known by any natural process.

However, more exotic forms of matter are certainly possible. For example matter inside a neutron star is radically different from matter on earth. I'm not sure you could even label it as an "element" since it doesn't have any protons. You certainly wouldn't want to bring any of that stuff back to Earth.

Even more exotic - and uncertain if they exist or not - are strangelets, lumps of pure quarks. They're definitely not elements since they don't even have neutrons. And you really, really, really wouldn't want to bring any of them back to Earth.

Is NASA still using radioactive materials to power space probes ? Is there something else we could use instead ?
Q : Is NASA still using radioactive materials for it's "long range" space probes, like the New Horizons, or, is NASA switching over to a different type of engine and fuel system. Yeah, I know dilithium crystals are out, but I was thinking of some other form of propulsion...Maybe the use of strong magnets ( to form some sort of energy source...Clearly, I'm guessing.

A : AFAIK nuclear power sources are the only thing we've got that are small and light enough for deep space missions. Beyond Jupiter sunlight is too weak to make solar panels practical.

I'd be very wary of the magnetic propulsion idea in the link idea. Getting something to "vibrate" only in one direction smacks of violating the conservation of momentum to me (which is basically magic). Acceleration by a magnetic launcher is a perfectly sound idea, but you wouldn't be able to use this to boost the spacecraft's initial speed too much. You don't want to reach orbital velocities low in the atmosphere, because heating from air compression would be bad. Useful on the Moon though.

Here's another idea that's maybe a more plausible use of magnetic fields for propulsion :

How fast would you need to go to reach Mars in 15 minutes ?
Q : Well, how fast ?

A : Speed = distance / time.
Time = 15 minutes = 15*60 = 900 seconds.
At 36,000,000 miles (the closest Mars gets to Earth), that means speed = 36,000,000 / 900 = 40,000 miles per second. At 245,000,000 miles (the furthest Mars gets from Earth), speed = 245,000,000 / 900 = 272,222 miles per second, which is faster than light.

Bonus : Oh. I was thinking that once the craft escaped the gravity of the Earth, it would go faster.

Bonus answer : Actually, if you only fire the rocket during take off, it will always continue to slow down (unless it goes into orbit of something). Even at higher than escape velocities it will always get slower and slower. Its speed will decrease less and less rapidly the further it is from Earth since gravity is less, but there's always some gravity acting to slow it, and nothing else pushing it forward.

Since the speed decreases less rapidly, at escape velocity (or higher) it would take an infinite amount of time for it to reach zero speed. At just below escape velocity it could take billions of years to stop and fall back, but it will happen eventually.

In reality things are more complicated because there's also the gravity of the other planets. If a rocket gets close enough to one of them then its speed away from Earth can indeed increase. With proper timing this can be extremely useful.

When Voyager I reaches the heliopause, will we lose communications with it ?
Q : Well, will we ?

A : That will happen in about five years. I don't think reaching the heliopause will cause any communications problems itself, but unfortunately it's touch and go as to whether the power supplies will last that long.

Could we live on a planet with slightly more helium in the atmosphere ?
Q : If we found a planet close to us and decided to ship half the worlds population to that planet, for them to colonize, but it had a slightly higher concentration of helium, in its atmosphere; what would the ramifications be ?

A : Helium is one of the most stable, unreactive substances there is. Having a bit more helium wouldn't cause any problems except that the concentration of oxygen would be a bit lower, and that wouldn't be good. According to this website :
... the oxygen content of the air can't be more than a few percent lower before problems start occurring (though, as someone else pointed out, that's not necessarily the case if the overall density is higher)

How do rockets work without an atmosphere to push against ?
Q : Surely rockets cannot work in space because there's no air for them to push against, right ?

A : Rockets don't work by pushing against the air, they work by the property of conservation of momentum.

Try the following experiment. Sit on a wheelie chair with your legs off the floor. Now try pushing against the air with your arms, keeping everything else as still as you can. What happened ? Nothing. You can't push against the air with your arms because it's not very dense and just moves out of the way.

Now try sitting on the chair and throwing a bowling ball at someone you don't like. This time you'll go backwards. Obviously the bowling ball isn't any better at pushing against the air than you are. If you want the over-simplified explanation, you are pushing against the bowling ball. Rockets, in a sense, push against their own exhaust - not the atmosphere. The exhaust is generated by their own massive amounts of propellant that they have to carry and doesn't need an atmosphere.

How tall does a building need to be for someone on the top of it to be weightless ?
Q : My intuitive guess is the height of geosynchronous orbit, since the orbital period at that distance is the same as Earth's rotational period. According to Google, that's over 26 thousand miles.

A : Geosynchronous orbit is correct. At that height, it would be impossible to fall off the building because you'd already be in orbit. But you'd still in freefall, just like onboard the ISS. Gravity at that height would otherwise still be noticeable, but much weaker than at the surface (about 0.02g, so it would take around 3 seconds to fall 1m, if you weren't orbiting).

Interestingly, if you fell off the building at any point below the top, you'd fall ahead of the building and go into an elliptical orbit around the Earth... unless you were too low, in which case you'd just crash and burn, or in fact the other way around.

Technically you can never become weightless (without being in orbit) at any distance. Gravity just gets weaker and weaker the further away you go, but it never reaches zero.

Are "They" suppressing technologies much better than our primitive rockets ?
Q : Is it possible that we have made incredible progress in propulsion and space craft technology and it's just not in use by NASA and such yet? If so, why ? This audio documentary explains the probable reasons. It's a real awakening.

A : Original thread : [Readers please note while I make every effort to answer all questions, if you want me to comment on an hour long documentary or read a lengthy scientific publication, I do so when time permits and entirely at my discretion]

OK, I watched the whole thing. For the benefit of readers the main idea is that secret near light-speed and FTL transdimensional technologies have been developed many decades ago but have been suppressed by higher powers. They are not quite impossible according to current physics but they would require stupendous energies and negative mass, and we're not sure if that's a thing.

I have to be brutally honest here : it doesn't make any sense to me at all. It's just the standard conspiracy theory fare : everyone is lying, only this group of people know the truth. Lots of claims about how people have come forward with radical claims but they have to be kept anonymous for fear of their lives. Oh how terribly convenient.
Fortunately I have a standard go-to response for this :

In this particular case there are a number of things (leaving aside UFOs which is too large a topic to deal with here) which I do not find remotely credible :
-The claim that these technologies are being suppressed by those who are greedy and in power. Ummm, why ? With the resources of advanced space travel they could potentially have access to unlimited wealth and power. There's no advantage to holding them back.
-The claim that these technologies are being suppressed with ruthless efficiency on the mainstream media but apparently people on the internet are allowed to say whatever they want. It's not that difficult to take down a YouTube video.
-The idea that these technologies were discovered and made to work many decades ago. If that's so, it's extremely difficult to believe that no-one else has ever stumbled upon either the theory or the technology in all this time. If you can derive the principles of this stuff from physics that was known a century ago, it's not credible to suggest that no-one else has ever found it. Instant suppression, by any researcher in any institute anywhere on the planet ? No. Sorry, but just no. That is simply not how academia works. It certainly isn't perfect but it isn't anywhere close to that level either.
- "There's only one mind in the Universe and it's connected to everything". This is a classic case of replacing God with aliens.
- The unraised question : what are these few people who do have this technology actually doing with it ?

In short I don't think this presents any sensible ideas as to how or why such ideas have been suppressed. It presents a scenario in which there is an apparent short-cut to far superior technologies that either everyone else has missed for decades, or is ruthlessly suppressed except for a few plucky guys on the internet. Riiight.

What's the best way to see the curve of the Earth using current technology ?
Q : Can you help me understand what determines the escape velocity needed for a specific instance. Is it the speed of something, the angle and speed, of something ? Or does it have to do with propulsion ?

The reason I ask, is I have seen some GoPro camera videos, where they get to see the arc of the Earth and then fall back down to Earth. Bummer. What I was wondering is how to create something that might make it beyond the arc and keep going, to get a different prospective of the Earth and nearby surroundings. Does size have more to do with it than I am thinking ? Or is it the propulsion of the item that needs to be mathematically determined ?

A : The short answer is speed. If you reach 11 km/s in any direction then nothing else matters, you're leaving the planet whether you like it or not and you're never coming back. Unless you're heading straight down, in which case you're just leaving existence.

The long answer is that you'll need to go a bit faster than this because air resistance will slow you down (size will be important here). However, if you just want to go very high to see the curve of the Earth, you can - in principle - travel at any speed you want provided you keep supplying energy. This is difficult once you reach a certain altitude because the air is too thin to use to provide either lift or speed (jet engines propel themselves by pushing the air backwards).

When the air becomes too thin the only option is a rocket, which needs a lot of propellant. Unfortunately you can't simply slow down and take longer to reach the higher altitude. That turns out to need even more propellant, and so a much bigger, more expensive rocket. A rocket with a faster exhaust would need less fuel, but this isn't possible with today's technology.

If your budget is such that you'd opt for a GoPro camera, there probably aren't any good options. Possibly a small rocket launched from a high-altitude balloon or aircraft, to avoid as much of the atmosphere as possible.

If you have a few spare hundred million, you could try and develop something like the Skylon system :
This a hybrid jet-rocket engine. It uses the air in the atmosphere (when available) to generate as much speed as possible, then when the air becomes too thin it switches to a normal rocket mode. In principle, this should give you the best of both worlds.

What if aliens make contact and they have a different name for planet Earth ?
Q : What happens when we come upon an older civilization than what is on Earth and they have a totally different name for Earth and our Solar system and the planets ?

A : If they're advanced enough to have found us, we bloody well us their names instead.

If I started firing off relativistic projectiles into the Universe and random, should I worry about hitting something ?
Q : If a projectile was fired at, let's say, 92% of light speed in a random direction (from Sol system or not), what would be the chances for it to hit something before the heat death of the Universe ? This includes both objects from the galaxy and beyond it. Or, put another way, how recklessly irresponsible can I be with relativistic projectile guns before seriously risking ruining someone's day at some point in the deep future ?
While the calculations for the galaxy seems relatively simple (though I may still get something wrong), I'm not sure how to proceed for beyond that.

A : On whether it's likely to hit anything, the answer that is a definitive, "no". The mean free path ( given the size and density of stars in the galaxy (for a point-source projectile) is by my calculations about 165 Gpc - much larger than the size of the observable Universe ! So even if the entire Universe had the stellar density of the Milky Way, you still shouldn't worry about your projectiles hitting anything by accident. Since the real Universe is even more empty than that (by many orders of magnitude), and expanding, the chances of any individual projectile ever hitting anything are extremely close to zero (the calculation outside the Galaxy is actually easy since there's bugger all there to hit - the problem reduces to, "how many galaxies would it have to pass through to stand a reasonable chance of hitting something ?").

This is neglecting gravity but that's OK because the projectile is moving so fast it will be essentially unaffected by everything it encounters. Literally the only thing strong enough to affect it would be a black hole and it would have to pass within a few km of the event horizon.

I made a crude estimate of what the actual probability of a collision would be. Assume the stars in the Galaxy are all distributed in a thin band of length 50 kpc (approximate circumference of the Galaxy) and height 0.6 kpc (real height of the stellar disc). Assuming the stars all have the radius of the Sun, the fractional area of this band covered by stars will be about 2.38E-11. Or, taking the reciprocal, the projectile would have to pass through about 42 billion galaxies before a collision should be expected - comparable to the total number of galaxies in the Universe. This will be a severe underestimate since it doesn't account for stars blocking other stars - in other words, you could fire at the very least hundreds of billions of projectiles before you should start to worry.

Would a relativistic projectile be stopped, fragmented and/or destroyed by interstellar/intergalactic drag ?
Q : Assume a spherical mass of 1000 kg.

A : That projectile, for interests' sake, has an energy equivalent to about 43 billion tonnes of TNT, or about 2 million times the energy of the Hiroshima bomb - more than the largest earthquake ever recorded, but still about 1000 times smaller than the asteroid that killed the dinosaurs. It would be an extremely unpleasant thing to encounter, but it wouldn't be a planet-killer.

On the drag, let's start with some ballparks.

Clearly the object will slow down significantly if it sweeps up its own mass through particles in the ISM. Let's approximate the cross-sectional area of the sphere to be 1 sq m just to make the maths a bit easier. The density of the interstellar medium is typically about 1 atom per cc, or 1 million atoms per cubic metre. Most of this is hydrogen, so that's a mass of 1.67E-27*1E6 = 1.67E-21 kg/m^3. Therefore to sweep up 1000 kg of mass the sphere will have to travel 5.988E23m = about 20 Mpc.

Or we could try Newton's impact forumla which considers how far a dense projectile will travel through a thin medium :
For this case the result is about 150 Mpc, within an order of magnitude of the first estimate so that bodes well.

A more accurate treatment is provided here :

However this still seems to only use classical (not relativistic) momentum. But it also shows (importantly) that velocity will never reach zero since the momentum of each particle will decrease as the ship slows down. Super super rough approximation : it will take about 15 billion years to reach 500 km/s, the escape velocity of the Milky Way at the solar distance.

So although there are a lot of crude approximations here, I think it's safe to say that drag will not be important.

Would nuking a dangerous asteroid at the last minute just make things worse ?
Q : This wasn't actually asked as a question but I found this video on YouTube and thought it needed some comments. You can read them in full in the link, here's the even shorter version.

A : Of course, if a dangerous asteroid threatens the Earth you'd want to stop it as soon as possible with whatever method of deflection or destruction you use. But if you've got no choice - the asteroid isn't detected soon enough, or your first efforts fail, then you might have to resort to firing a nuke at it at the last minute.

The video claims that actually no, don't do that, it will only make things even worse. I disagree. If you only manage to blast the asteroid into a few large pieces, then sure, this is not going to help. But I don't agree with the claim that if you do manage to blast it into smithereens (i.e. gravel-sized grains or smaller) then you're still doomed.

I don't think it makes sense to say that since the net KE of the debris cloud is still 2E23 J, the heat received by the Earth will be 2E23 J. Even if that were true, given the mass and heat capacity of the atmosphere it would only raise the temperature by around 40 C, not 1000 C as in the original video or even 100 C as a commenter on YouTube pointed out. I think he also calculates the power received by the Earth incorrectly - he's using the area of the surface of the Earth, but I would have thought the cross-section of the Earth to the debris would have been what you'd want. If so, he's underestimating the flux by a factor of two. Never mind that detonating just a few days earlier will mean that the >95% of the debris cloud will miss us.

But more importantly, this is waaaay too simplistic a way to estimate the change in temperature. First, gravel-sized pellets are going to burn up at >10 km altitude, so it's going to take a while for that heat to penetrate downwards by conduction. Secondly the glowing meteors are going to radiate isotropically, so ~half their energy is going to escape into space. Then, only a fraction of that energy is actually going to be absorbed by the atmosphere and raise its temperature. According to :

About 30% is going to be reflected back into space, 23% absorbed and 48% absorbed by the Earth itself. So given the isotropically radiating meteor, that means only about 0.5x0.2 = 10% of the energy emitted is going to raise the temperature of the atmosphere. Which means we're looking at more like a 4 C rise in temperature. That's certainly going to cause weather chaos, but it's hardly as bad as if the asteroid actually impacted.

Anyway my calculations are also likely similarly naive and simplistic. I really just want to point out that you can't always get away with approximations - the "details" can change the results dramatically. To say nothing as to at what point you could use the nuke to deflect the asteroid instead of blowing it to bits...

For comparison, the Pinatubo volcanic eruption injected about 5 cubic km of material into the atmosphere and it caused a global cooling :

This asteroid would inject about 100x more material.

So I agree, a last-minute nuke is not what you want to do to avoid an asteroid - but not necessarily because it the asteroid would still heat up the planet. But we are talking about orders of magnitude difference here depending on if you blow up the asteroid a day in advance, a week or a month. So don't throw out the nukes juuuust yet.

Why does the Cygnus spacecraft need to do a de-orbit burn ?
Q : Can someone explain to me why Cygnus needs to do a de-orbit burn so it can burn up on reentry? Is it because if it came in too fast, it wouldn't have as much time to burn up ? Or a positioning thing, or...?

A : Since the obvious answer that it needs to get out of orbit somehow had already been raised and yet not deemed satisfactory...
It's more speed thing I was thinking. Why not just let it come in fast and hot. Why even do a deorbit burn. The thing is burning and breaking up over the ocean anyways.
... I assumed this confusion was about why the spacecraft needs to do such a careful, fuel-intensive maneuver. I'm guessing, but I bet when they say "deorbit burn" they don't really mean it's going to lose all of its orbital velocity - that would just waste fuel. Probably they just mean it loses enough velocity to send it heading downwards, it's still coming in hot.

Seems simple and clear enough to me - it's using just enough fuel to send it crashing to Earth so it will burn up, but not nearly enough to land safely. It DOES come in fast and hot, but it needs a de-orbit burn to do that.

Oh how wrong I was. Not about the de-orbit, but the state of mind of the questioner.

I'm talking about the Cygnus resupply cargo ship. If the thing is breaking up and crashing into the ocean, why would anyone care about saving fuel ?

Which other spacecraft he thought I was talking about I have no idea. Nonetheless I proceeded :

Because fuel has mass and transporting mass to space is what makes the thing so expensive. So on the way down you don't want to have any more fuel than you need - just enough to push it into the atmosphere and burn up in a selected area. Hence the deorbit burn probably uses the absolute minimum of fuel, so it's not the same as the much more carefully controlled deorbit of the space shuttle.

But apparently this answer makes me an asshole.

you obviously didn't read anything I said or have any clue what I'm talking about. I don't give two shits about the space shuttle. I'm asking about the Cygnus resupply spacecraft that burns up and crashes into the ocean once it undocks from the space station.

Can't really argue with logic like that. I can only assume he thinks the ship would instantly crash into Earth as soon as it undocks, for some reason.

Is there any research into wormholes and negative energy going on ?
Q : In a issue of BBC knowledge magazine whose topic was how to travel faster than light ... An article was there that talked about worm holes. In it was written that on putting negative energy into worm holes they can widen because they are microscopic. I knew that negative energy is a hypothetical idea but is there any research going on ?

A : Yes, there's definitely research going on into all kinds of far-out topics. Right now the concept of negative mass/energy is purely theoretical so although there are speculative concepts of what we could use it for, it's still an open question whether it's even possible for it to exist.

Research into theoretical and observational cosmology often seems like radically different things. Theoretical cosmologists deal with wormholes, multiple dimensions, and all kinds of other exotic physics. Observational cosmologists deal with measuring the temperature variations in the cosmic microwave background, the expansion rate of the Universe, and generally measuring things to greater precision. Most likely at least some of the more exotic possibilities the theoreticians are considering will turn out to be correct, but right now we don't really know all that much about some aspects of the Universe.

Can you spin a ball to make artificial gravity ?
Q : Can anyone tell me how i can spin a ball shape to make anything thats on it gets forced into it as if to replicate graverty ?

A : You can't really do it with a sphere, except very inefficiently. There would be gravity at the "equator" but not at the poles. So most of the interior surface wouldn't be useful. You'd have much better luck with a cylinder. The definitive, all-encompassing article is here :

Why do Space X love boats ?
Q : [On SpaceX's first succesful landing on a barge] someone explain to me (in words I can understand ) why they are doing this -- landing on a bobbing up and down barge (about the size of someone's back yard) in a gale-force wind blowing on the ocean.

A : There's a nice summary in this video :

Basically ground landings involve the first stage having to reverse direction to fly back to land. For launches that need a higher delta-v, this uses too much fuel. Also it's probably safer to keep the first stage over water (for everyone except the first stage, that is).

Should astronauts really be running marathons on the International Space Station ?
Q : These guys have nothing better to do ?!?

A : Actually no, there's nothing more important than exercise in space. If you don't do this, bones and muscles deteriorate which can cause very serious health problems.

In any case it was all of 3 hours out of a six month mission (which is barely longer than the amount of exercise required in space anyway), so I don't see why he shouldn't do this if he wants to.

Why is Martin Rees calling for an end to human space missions ?
Q : Just interests me as to why? What is the thought behind this?!? Or is it simply a major lack of thought and just simply sheer stupidity ?

A : He's not a stupid man at all, but he does tend to love the media spotlight. For example he's famous for stating that the human species doesn't have much chance of surviving the next century, which is pretty much guaranteed to grab attention coming from the Astronomer Royal, but doesn't have much basis in fact. Why would it ? He's an astronomer, not a politician.

Unfortunately the media have a tendency to weight scientific statements in a way that makes little or no sense. You almost never hear anything directly from climate scientists but almost everything the deniers say. Yet when Martin Rees or Stephen Hawking say things which are well outside their specialist area (Rees expertise is cosmology; he's a pretty good writer, but he's never done anything with human spaceflight) they're treated as all-knowing. Not that they shouldn't have their opinions or express them, just that the media give preference to people who are seen as experts when in fact they're really not.

What he's probably thinking is that in terms of pure cost and science, robots do better. And they do, to an extent. But in my opinion that just means we should invest more into making human spaceflight cheaper, because if the mission cost the same, just about anyone would like to have a human on board. They're a lot more versatile than robots, never mind that science isn't the be-all and end-all of space exploration : colonisation and asteroid mining aren't going to happen without humans.

Then again government-funded human space exploration hasn't delivered what everyone was expecting, but don't even get me started on that. :)

Could we build a space battle cruiser if we really, really needed to ?
Q : If we as a species had desperate need to put up some kind of space battle cruiser right now but no time to R&D many new technologies, what might we manage with a united humanity, real desperation, and almost unlimited budget allocation? Timeframe from R&D to battling humanity's enemies - three to five years. Three is safe, four risky, five is really pushing it. [Original thread with many interesting responses here :]

A : If you've got a maximum of five years, I say go hell for leather chemical. You need a solution that works and works now, and chemical's all we've got. I'm not convinced either a VASIMIR or Orion drive could be made to work inside that time frame. Possible, yes, likely, no. Good enough for sci-fi ? Absolutely. But if we actually really needed a space battleship, I say throw every man woman and child into building something comparable to the Saturn V en masse. I assume that a threat from space requiring such a response is an existential threat, therefore, we may consider measures that would normally be considered abominable. The entire global economy needs to be re-worked to facilitate building as many rockets as possible. The only non-rocket building task will be to keep the rocket builders alive and able to build rockets.

For starters, let's assume you can take the whole of the US military budget and re-distribute it to NASA, SpaceX, etc. That's enough for about 190 Saturn V's per year (, which could surely be improved by economies of scale, assembly lines and improved manufacturing techniques and materials. So you're talking about a fleet of, say, 500-1500 Saturn V's in 3 years. Plausibly several times that since if if the species is facing an existential threat, you can also use other government spending as well as the economies and industries of Europe and Asia. So, guestimating, let's say of the order 5,000 Saturn V class rockets, or a payload to LEO of 700,000 tonnes. Call it a million; you can maybe get more payload with larger numbers of smaller rockets. Depends what it is you need to actually launch. Your real limit might not be from money but simply the number of engineers and industrial facilities suitable for rocket production. The goal is to turn the planet into one gigantic rocket factory. Everything else can get screwed.

Assuming this threat to be alien, I say fight them in LEO. If they're coming from another star system, their ships will hopelessly outclass anything we have and we won't stand a chance at intercepting them at long range. Our only hope would be to literally throw everything we can at them in a massive, last-second, all-out nuclear assault. In fact forget the battleship completely - just launch nothing but nothing but missiles. No space heroics, no fancy tech, just sheer unmitigated energy delivered to the target. At point-blank range no technology will allow them to doge/destroy every missile. Don't outsmart them, overwhelm them. As well as the real armed missiles, launch tens of thousands more lightweight decoys. I don't care how good your point defence is, you aren't going to deal with 50,000 rockets heading toward you with 5 minutes to spare.

There, that should ensure I never have to suffer political office.

Is the Orion drive as stupid as it looks ?
Q : [Comment was regarding this old project.]
That's some terrible efficiency. You should only have to detonate once. Even if you were on your way after the initial explosion the entire vessel would crush from mechanical fatigue. I mean come on man the front of the ship would be ejected from the rest of the ship like a friggin pinball and the crew turned into chunky salsa. That is one fucked up design.

A : If you'll read the accompanying blog post in the video description and also this earlier one (, you'll see that I do in fact object to Orion actually being used, but not for the reasons you describe. So, first let me deal with your objections and then I'll summarise why I think Orion is a bad idea.

First, it's not terrible efficiency. Saying Orion is inefficient is like saying solar power is inefficient because it doesn't capture the entire energy output of the Sun. You have such insane amounts of energy to play with that it doesn't matter if you loose a lot.

Using a single (necessarily larger) detonation is impossible for practical reasons. First, you'd need a much bigger ship to survive the detonation because the acceleration would be so high - otherwise it would indeed be crushed. Second, you'd be relying on getting that first detonation exactly right - if your ship is pointing even slightly off-course, you won't end up going where you want to go. Multiple, smaller detonations completely avoids both of these issues.

With multiple detonations the entire vessel would NOT be crushed, that's what the shock absorbers are for. The initial acceleration of the pusher plate is indeed terrifying (hundreds or even thousands of g) but this is transferred to the rest of the ship much more gradually via the pistons. The acceleration would be no worse than a conventional rocket (a few g or less) so there's no reason the front part would somehow fly off. I can take you through the calculations to show this if you like, it can be done with nothing more than high school math.

However, one of the reasons I object is because it's not obvious whether or not the pusher plate could survive the detonation. Although various experiments indicate that it probably wouldn't be ablated by the heat of the explosion, AFAIK no research was done to see if it would buckle under the pressure. Possibly the gas bags connecting it to the pistons would be sufficient to prevent this, but it isn't really known. Buckling could lead to the plasma stagnating against the plate for much longer than it was designed for, leading to ablation. Also, in the event of the explosion being slightly off-axis, it's not clear to me what would happen to the pistons - would they be snapped off ? No-one seems to have addressed this.

Then there's the case of a failed detonation. If a bomb malfunctions and the chain reaction doesn't occur, the explosive within the bomb will still send shards of metal into the pusher plate - which could actually be more damaging that the smooth, brief fireball if it went nuclear. For a space-launched Orion this will only be a problem for the crew. For a ground launched version, it's not at all clear what happens in the even of a failure. With ~2000 bombs per ship, each with a mass of one tonne and launched at 200 mph via a gas-powered cannon, there must be some failure rate. If one detonates too close to the ship, what happens to the remaining nuclear material on board - does it trigger a single large nuclear explosion high in the atmosphere (which would not be so bad) or does it spread all that fissile material into the atmosphere ? Similarly if the ship is launched via rockets (as it my earlier video), what happens if one of them malfunctions - if it explodes, same question about the nukes, if the ship goes off-course, can it be aborted safely ? Then there are the political concerns, which are almost certainly going to be far more difficult to overcome than the engineering difficulties.

Orion is dead. It never had much of a chance anyway. But it's fun to speculate about it even so.

Could the ISS catch an astronaut if they fell off ?
Q : If an accident where to happen on the ISS and one of the astronauts tether broke, how fast would the astronaut be traveling and would the ISS be able to catch them in a "swing around" maneuver? I was just curious after watching a show and the astronaut went tumbling out of reach of the other crew outside. I didn't think the astronaut would move so quickly out of range. One of the risks, I guess.

A : Since spacewalks tend to be very slow, careful affairs, they probably wouldn't be moving at much more than a few cm/s. Maybe a few metres per second (a few mph - walking pace) at the very most. The risk of a tether breaking is very low - to my knowledge, an astronaut's tether has never broken. However an experimental satellite tether has broken, so it's not impossible.

In principle the ISS could be moved to catch an unlucky astronaut - it can move in three dimensions at about 1 m/s. But it isn't designed to be moved in a sudden emergency - it's a big, complex piece of equipment, and it's rarely moved without 24 hours notice. It's also controlled from the ground, not by the astronauts on board, so quickly moving the ISS in an emergency with the precision needed probably isn't possible.

Fortunately there isn't really any need to move the whole station. Astronauts are equipped with a small jeptack-style propulsion system called SAFER. This is designed to allow them to return to the station, or if their own system fails, for their colleague (solo spacewalks, if I understand correctly, don't happen) to rescue them. The risk of an astronaut floating off into the void isn't zero, but it's probably as low as it can be.

How can we improve communications with robotic spacecraft to help with exploration ?
Q : NASA is putting up a $1M challenge to guide a virtual robot on Mars. Hell, the R5 robot should have already been sent for deep space exploration to determine its capability of controlling equipment as a human would. Why doesn't NASA entertain this idea? All that is sent are spaceships which are controlled back on earth. The real challenge for deep space travel is communications. Somehow relay station's that boost signal strength need to be planted in space the same way we do on earth with satellites and transmission towers. Any ideas from all on deep space travel and the communication challenge? ?

A : Hell, the R5 robot should have already been sent for deep space exploration to determine its capability of controlling equipment as a human would. Why doesn't NASA entertain this idea?
But that is exactly what the competition is designed around - testing the robot before it's eventually sent on space missions. No point in sending a multi-million dollar piece of equipment on an irretrievable journey unless you're pretty sure it's going to work first !

Somehow relay station's that boost signal strength need to be planted in space the same way we do on earth with satellites and transmission towers.
As I understand it, there's no reason this couldn't be done except money. Every dedicated communications satellite launched to Mars is a satellite that could have been doing science. Since funding is limited and the number of Mars missions is uncertain, it's more cost-effective to have them use their own less powerful systems. Which is not such a big problem since a large radio telescope is capable of detecting a mobile phone on Mars. Of course it would be better to have a more powerful satellite transmitter, but it's a matter of priorities.

All that is sent are spaceships which are controlled back on earth. Not entirely. The current path is semi-autonomy. Rovers are directed towards certain areas by scientists on the ground but their onboard computers can decide the best way to get there.

How fast could an Orion-powered spacecraft get us to Mars ?
Q : I have read that this kind of propulsion could send a spacecraft to alpha centauri within 80 years, how long would it take to Mars ?

A : It depends very strongly on the the initial mass of the spacecraft, how much mass you want to send to Mars, and how much you want to get back to Earth. For Alpha Centauri in ~100 years you need a ship of several hundred million tonnes to start a colony, but if you just wanted to send a probe on a flyby you could use something a lot smaller.

For Mars, if you used a minimum energy transfer orbit a starting mass of 4,000 tonnes would get you 800 tonnes into Mars orbit and back to Earth, rising to 5,000 tonnes if the starting mass is 10,000 tonnes. That would take about 9 months. You could use a different orbit and get there faster, but the mass you'll be able to send will drop - maybe by a factor of a few. So you can either send 5,000 tonnes to Mars in a few months, or around 500 tonnes in a few weeks. It's probably better to go for the first option, because with that much mass to play with you don't really need to worry about the journey time - the astronauts can live quite comfortably.


Dude, aliens !
Q : What is the probability that another complex carbon based life form exists within our Universe?

A : Short answer : nobody knows. Longer answer :

We only know for certain of one place where life started, so it's very hard to judge how probable it is that life gets going if conditions are suitable. However, the first life on Earth is generally have thought to have appeared pretty soon after the Earth formed, pretty much as soon as it was habitable :
The last time I had a chance to talk to an expert on this (some years ago now), most of the evidence for the earliest life on Earth was considered a lot more controversial than generally appears in the popular press. If so, life might have taken a lot longer to get going, meaning it may be relatively difficult for life to begin.

The Universe is so large it seems very unlikely that there's no other life out there somewhere. However, I also like Arthur C. Clarkes's quote :
“Two possibilities exist: either we are alone in the Universe or we are not. Both are equally terrifying.”

Time travel
Q : I've read where if you eat only good, plant based foods and practice fasting and meditation, you'll see visions. Is this time travel? [Yes, this was a real question - someone actually asked this !]

A : Either that or hallucinations brought on by malnutrition.

"Sensors are picking up a massive gravitational field, Captain !"
Q : How can we detect gravity ?

A : The simple answer : we detect gravity because it makes things accelerate. Apples fall from trees. Cats fall over. The Moon goes round the Earth (circular motion is acceleration). Planets go round the Sun. So in that sense we're detecting the gravity of the Sun by the very fact that we're going around it.

The complicated answer : how strong gravity is depends on the mass of an object. How much acceleration an object of any given mass produces is related to a parameter called the gravitational constant, G. Measuring this is very much more difficult than measuring acceleration :

Yeah, yeah, but what about if you're a windowless box, eh smarty pants ?
[Questioner did not actually say this]

Ahh, now I get you. You can't detect the acceleration inside the box since everything is accelerating at the same rate. Well... almost. In practise the box will have some finite size, so the acceleration experienced by the bottom of the box will be slightly higher than at the top. This is only going to matter near an object like a black hole.

Or, you could fire a beam of light from one side of the box to the other. The light is trying to go straight, but the box is accelerating, so it doesn't hit the exact opposite point : it appears to curve.

Should we ignore stupidity ?
Q : Should we care about harmless pseudoscience ? Do stupid people matter ?
[This was not exactly how the question(s) were asked, but I need a standard response for such matters]

Stupid people do not matter. Stupid people (or of course intelligent people with misunderstandings) with enough talent to make their nonsense go viral, on the other hand...

A : When pseudoscience goes viral, it undermines a lot of very hard work that has gone on in trying to understanding the Universe. Which is something we don't do for fame (unless you're Tyson or Cox, you won't get any) or money (even less chance of that), or because it's a cushy job (competition is just as fierce as in any other sector), or because we have any political axes to grind (galaxy evolution doesn't care if you're left-wing, a fascist, or a small turtle). Astronomy is, as much as is humanly possibly, knowledge for knowledge's sake - there are no campaigns to ban dangerous chemicals based on observations of the Orion nebula, no decisions about people's rights because of the Triangulum galaxy.

We do astronomy because we think it's worth doing. At a professional level, astronomy is funded almost entirely by taxpayers, and we want to give them the truth, whatever that may be, because that's what they pay us for. It typically takes around 7 years of higher education before you start making meaningful contributions in astronomy, let alone coming up with ground-breaking results. Moreover, it's based on exactly the same proven physics that's led to things like rockets, radios, telecommunications, microwaves, radar, satellites, electrical power... pretty much the entire basis of the modern world really. It is not a soft science. So, if a non-scientist comes along and makes a fancy video with some rudimentary, but easily correctable errors, claiming to have overturned an extremely basic fact of a subject that tens of thousands of people choose as a career for the sole reason that they think knowledge is worth knowing.... well, you can imagine how we feel about that. Instead of communicating our latest hard-won discoveries to the tax-paying public, we have to spend time convincing them about things that were established beyond all doubt centuries ago.

For a thorough analysis, see my article on quackery; for a really comprehensive review see this.

What is time ?
Q : Well what is it then ?

A : I don't know.

What's the difference between gravitational waves and tidal forces ?
Q : I have heard of "gravitational waves" in space, but not "tidal waves". Are they different ?

A : This was in reference to a press release describing the collision of galaxy clusters as a "tidal wave" (

The simplest answer is that "tidal wave" generally means a wave in the normal sense - in a fluid - whereas "gravitational wave" refers to a change in gravity.

The tricky part is that tidal forces (not waves !) are caused by a difference in gravity between two places : e.g. one side of the Earth experiences less force from the Moon than the other. This is very similar to a gravitational wave, but not quite the same. A gravitational wave, as I understand it, is a change of gravity that moves through space, whereas tidal forces - at any given distance from their source - don't vary over time.

Can the impossible happen anyway ?
Q : So if everything is bound to happen in infinite time and space , happenings which are impossible are bound to happen.

A : No. Impossible things don't happen. It's like saying that if you have an infinite number of empty rooms, you must have some monkeys. Actually you don't have any monkeys, just an infinite number of empty rooms.

There are plenty of truly impossible things. You can't fit a square peg through a round hole of equal diameter, for instance. You cannot put two 1 metre rulers next to each other (in flat Euclidean space, smarty-pants) and have them stretch a total of three metres. There are also lots of shapes which are impossible in the real world, e.g.

Granted, if the Universe is infinite, then all unlikely things - even if extraordinarily unlikely - should happen somewhere. An impossible thing may be defined as something which cannot happen even in an infinite Universe.

I personally don't believe in a infinite Universe, for reasons I explain in detail here :

What will the first aliens we discover be like ?
Q : If there extra terrestrial life forms on other planets, what do you believe the first ones humanity would encounter would resemble? My opinion ? Life forms not dependent on heat. Cold blooded beings. I imagine they would be considerably smaller than humans.

A : There's no way to know. It will depend entirely on the conditions on the planet they've evolved on.

How can photons last for billions of years ?
Q : Does anyone know how long a photon of light can exist for ? What is its lifespan and how come it can travel over billions of light years and over billions of years ? I just don't understand how it can keep going...

[Several people had answered this by invoking relativity. The proper time - the time "experience" by a photon - is zero, no matter how far it travels. While true, and tremendously interesting, it seemed obvious to me that this was somewhat beyond the questioner's level of understanding, and in any case absolutely unnecessary. There's a much simpler and equally correct explanation.]
A : Even ordinary particles of matter will keep going indefinitely provided they don't hit anything - Newton's first law of motion :
If you want to make something speed up or slow down, you have to give it energy or take energy away from it. You can't do that unless it collides with something. The question, "why does it keep going ?" would be better stated as, "why would it stop ?". In everyday life things slow down because there's always some medium around to stop them (like air). In space, that's not the case, so they keep going.

Photons don't need to have very much energy to keep going for billions of years, it's just that they're not losing any.

Don't photons get chilly in the cold of space, and lose energy ?
Q : Space is pretty cold in places. Why doesn't the photon's energy dissipate ? How can it travel through such extremely cold temperatures and yet maintain its speed and energy ?

[This question followed on from the previous one. I could have mentioned that photons lose energy as space expands, but I felt that this would confuse the issue and decrease understanding.]
A : Temperature is a property of matter. The higher the temperature of a substance, the higher the velocities of its atoms (speed of movement through space for liquids gases, vibrations around some fixed point for solids). When the atoms collide, they may lose energy as photons. The higher the temperature, the faster the motions, the greater the rate of collisions, and so the more photons are emitted.

A single atom doesn't really have a temperature. If left in complete isolation, the electrons inside it would keep orbiting the nucleus indefinitely, and if in motion, it will keep going. It needs to interact with something to lose energy. You can't really say that any individual atom in a substance has a temperature - it's the effect of many interactions throughout the substance that gives it a temperature. It's a bulk property.

"Space is cold" only in the sense that the atoms which are present have very small motions. But the space itself between them doesn't really have a temperature as such. In most of the Universe, the density of matter is quintillions of times lower than in Earth's atmosphere. So the chance of a photon colliding with anything in this very cold gas is small. To answer your question, it's best to think of space as being completely and utterly empty.

More importantly, photons don't really have a temperature either. Most photon "collisions" just result in the photons passing straight through each other. Photons are the way in which energy is lost by atoms, but they themselves don't lose energy except when they interact with other atoms. In a true vacuum, there's nothing for them to interact with so they lose no energy.

Would two black holes with the same electrical charge repel each other ?
Q : Let's say the Black hole in the Milky Way is negatively charge and the black hole at the center of Andromeda is positively charged, that would mean they would merge. Yet, if both were of the same charge, would they not repel each other... sorta like magnets ?

A : Yes, that's true - if they were charged. In practise, it's not thought to be likely that a black hole could gain a significant charge. If it was formed from charged matter, it would be extremely difficult to compress it enough to overcome gravity.

In principle a neutral black hole could gain a charge by swallowing some charged matter. But if it did that (most matter is electrically neutral overall so it's not terribly likely) it would have a charge... and therefore attract matter with the opposite charge more strongly, and become neutral again.

Is there any known way of reversing time ?
Q : Is there any known event or force that could reverse time on a spatial plane. (Hoping I used that term correctly). My thinking is this: Since the "Big Bang" created the multiverse, instantaneously, could the reverse happen. What got me thinking was Newton's 3rd law: "For every action, there is an equal and opposite reaction". Is there an event like a time portal, that could appear, reverse time and then move it forward at a different chronological timeframe.

A : Travelling to the future is easy : just go really fast.

Travelling into the past is much more difficult and might be impossible. The closest thing I can think of to a "time portal", which, so far as is known, is not prohibited by the laws of physics, would be a wormhole.

A wormhole is a a short-cut through space that allows you to travel from one place to another almost instantaneously - much faster than the speed of light through ordinary space. To use them for time travel, the idea is that you keep one end fixed, then drag the other off at near-light speed and then bring it back again. Time passes more slowly at high velocities, so the moving end of the wormhole will have aged less. You can use this to travel back in time, but not to a point before you starting mucking about with the wormhole.

If the speed of light isn't constant, why do we measure distance in light years ?
Q : I just want to know. Why is distance still measured in light years? I thought the speed of light was proven not to be constant and that it's affected by temperature and gravity.

A : Nope, it's a constant. Gravity causes it to shift frequency, but not speed.
In the laboratory it is possible to slow down light but AFAIK not under any conditions we know of that can occur naturally.

Can you escape the gravity of an object ?
Q : If you move far enough away from an objects, will you eventually be totally unaffected by its gravity ?

A : No. Relativity says that the speed of gravity equals that of light, which is finite. Objects which are separated by enough distance do not feel each other's gravity. However, since you can't travel faster than the speed of light, you can never escape the gravitational field of any object which is already affecting you.

You can get far enough away from an object so that its gravitational field becomes negligible, but how far depends on the mass of the object (and what you define as "negligible").

Do gravitational waves ever stop ?
Q : What I am wanting to know is at the instance of a gravity wave, does the wave have an "ending" or does it go on, til there is some sort of other event? The reason I ask is, from my standpoint, space is "endless". So, I'm not sure if a gravity wave would continue on, but weaken, over time.

A : That's exactly what would happen. Like gravity itself, its strength decreases the further it goes, but it never reaches zero.

What's the latest in the search for gravitational waves ?
Q : Speaking of gravitational waves, I have yet to hear any news about gravitational waves. Where can I get better sources of information ?

A : I seem to recall that the LIGO detector is finally at the stage where they expect to be able to detect gravitational waves :
So in principle they could make an announcement at any moment. Given that the search has been going on for decades, I wouldn't count on it being any time soon though. If they've got any sense they'll wait until they're really sure.

If two gravitational waves collided, what would happen ?
Q : Would they just cancel each other out, or create a new wave ?

A : Actually, when two waves meet, they don't create a new wave. If two wave peaks meet, they add up to produce a higher wave but only temporarily - they keep going afterwards as though nothing had happened. If a wave peak peaks a trough, they can cancel out completely, but again it's only temporary and they keep going as though nothing had happened.
It's much easier with pictures :

Have there been any radio signals from aliens yet ?
Q : Have there been any signals from space using the existing radio telescopes around the world ? Also, are we transmitting anything into space for other worlds to pick up ?

A : Signals from aliens no, with the cautious caveat of the "Wow" signal :

There have been a handful of attempts to deliberately transmit signals that aliens could detect :
Most (all ?) of these wouldn't be powerful enough for our technology to detect if we were on the receiving end, and rely on the aliens having much more sensitive equipment.

Are you sure it's not aliens ?
Q : Why do you say: "It's still premature to assume it's aliens" ? Since the universe is some 13-15 Billion yrs old, wouldn't the earlier segments of the universe have already developed some sort of technology to communicate ?

A : It's not about whether aliens exist or not - they might very well do. The problem is that we have no idea how advanced aliens are or what their capabilities might be. So you can use aliens to explain literally any unsolved mystery : fast radio bursts, where are my socks, anything. If you assume it's aliens, you risk missing discovering interesting natural phenomena that could tell you a lot about the Universe that wasn't known before.

Of course that doesn't mean that you can say it's definitely, definitely not aliens, or that SETI projects shouldn't take a look at potentially interesting targets. It's only that if you say it definitely is aliens that you run into trouble.

Why do we assume aliens use known technologies to communicate ?
Q : I don't understand why we assume any alien civilization would be using radio waves for communication. Just because we haven't discovered something else yet doesn't mean that it doesn't exist.

A : Well if they were using a method we haven't discovered yet, we wouldn't be able to search for aliens using that method. If we want to do the search at all, we have to assume they're using methods we understand.

Do gravitational waves cause winds in space ?
Q : Well, do they ?

A : No, winds in space are caused by something simpler : heat. The heat from a star is enough to blast off part of its atmosphere.

Can gravitational waves be dangerous ? They took out Praxis, the Klingon moon, in Star Trek VI...
Q : So, since I believe what you say, in that gravitational waves are so weak, the explosion of "Praxis" (the Klingon moon in Star Trek VI: The Undiscovered Country", couldn't happen ? Or wouldn't be as devastating ?

A : f I recall correctly, Praxis was destroyed by some mining accident thingy, not a gravitational wave. Still, the explosion might itself produce a gravitational wave. In the movie a huge shockwave hits the Excelsior which is some light years away. This relies on Trek physics since in the real world faster-than-light is not permitted in physics as we currently understand it.

So, let's have the Excelsior be at least somewhere in the Klingon star system. There's an interesting thing about expanding (or exploding) spheres : you won't notice any change in gravity until you're inside the debris cloud. The reason is that until that moment, there was still just as much mass "beneath" you (i.e. between you and the centre of Praxis), so you're feeling the same gravitational force from "Praxis". The fact that Praxis is now much, much larger turns out to make no difference. Except to everyone on Praxis, of course.

Once the debris stars moving past you, there is now less mass "beneath" you, so you feel less gravity. But, unless the Excelsior was very close to Praxis (like in orbit), the gravity from Praxis would have been negligible anyway, so there wouldn't be any appreciable change. Also, there's a subtle difference between "gravitational waves" and "a change in gravity", which I don't fully understand, but it turns out that spherical explosions don't produce gravitational waves at all (I am wondering if this is strictly correct, I'll get back to you on that) :

The visible blast wave in the movie is presumably supposed to be debris from the explosion. While I'm loathe to criticize anything Trek, this follows the classic movie "circular shockwave" effect, which looks cool but is totally wrong. There's no reason at all to think the explosion would be confined to a thin plane.

However, if the explosion was powerful enough to destroy a moon, the debris cloud would certainly be dangerous since the debris would be moving at several kilometres per second at the very least. The difference between reality and the movie is that the dangerous blast radius would be very much smaller because the explosion should be spherical (well, very nearly).

So, no gravitational wave destroying Praxis, and no gravitational wave from the explosion either. The movie gets right that the explosion itself would be extremely dangerous, but gets just about everything else wrong.

However, in the extreme case that for some reason the explosion of Praxis was non-circular, which would produce gravitational waves, they might be dangerous if the Excelsior was close enough. Here's a very nice link :

In summary, while gravitational waves can be dangerous under extreme circumstances, as far as we're concerned here on Earth they are probably one of the least dangerous things in the Universe.

Why do different planets move at different speeds ?
Q : I have been informed by folks here, that the Earth is traveling at around 66,000 MPH. What I was wondering is do different planets and moons in different regions of space travel at different speeds in space ?

A : Here's my attempt at a simple, non-mathsy answer.

The speed an object moves depends on the strength of gravity where it is. Gravity depends on the mass and distance of other objects. For example, if you fall over on the Earth you fall down more quickly than if you fell over on the less massive Moon.

So, Mercury (for example) moves more quickly than the Earth because it's closer to the Sun. If it moved more slowly it would fall in.

Here's quite a nice little simulator. You can alter the starting speed and direction of various different objects and see how they react.

Was Giodano Bruno a scientist ?
Q : Was Giordano Bruno not doing science when he claimed that the stars were in fact suns, despite having no conceivable way of measuring such a claim for hundreds of years ?
[This is a question about the philosophy of science, relating to the very interesting article here :]

A : A very brief background for those who didn't follow the discussion : Bruno was a 16 century Italian thinker who claimed that the universe was full of inhabited worlds. He had no actual evidence for this, just sheer philosophical reasoning. He had no way of testing his claims either. He was burned at the stake by the Church, though whether this was because of his astronomical beliefs - and belief is absolutely the correct word in this case - is a matter of controversy.

In my opinion, Bruno was probably not a scientist or doing science.
You could fairly call Bruno a martyr to the cause of religious freedom, but his cosmic worldview was neither a deduction nor a guess. It was a philosophical corollary of his heterodox belief that God and souls filled all of the universe.

However, that article states that the charges against Bruno are known, whereas a 2009 book states that the list didn't survive. It also says that Bruno wasn't even a brilliant mathematician, but a plagiarist :

So, there is still some controversy over Bruno.

One could also ask if Kekulé was doing science when he dreamt about a snake and came up with the molecular structure of benzene :

A personal example also comes to mind. During my PhD, a friend of mine was working on a particularly tricky problem. Something involving galactic dust extinction, I think. Being prone to bouts of obsessiveness, he was working on this problem rather a lot. One night he had a dream in which we were both in a dingy Cardiff nightclub. Over the noise I was shouting to him the answers to the equations he'd been working on. When he woke up, he wrote them down and found that they worked. He cited this in his thesis as, "Rhys Taylor, private communication."

I guess my take would be that it's possible to do science by irrational means. Sheer blind luck is sometimes very important. But, Bruno was both using irrational methods and coming up with untestable conclusions. And that's definitely not science, because God knows how many other people at that time were doing the same thing. The fact that he got something right is somewhat incidental, because almost everyone else who does things in this way gets it wrong. Scientists can get lucky, but being lucky doesn't make you a scientist.

Which also reminds me of Wegener's Law :

Are the constants really constants ?
Q : How certain are we today of the, well, constance of the physical constants ?

A : Pretty sure, at least as far the speed of light goes. If it had changed, we would see differences in the spectral lines of different elements. Thus far, no significant results have been reported. I know some of the researchers involved in this. The last I heard there was no sign of the very weak detections being confirmed, so they were most likely not real.

Sometimes people invoke, "if the speed of light wasn't constant then it would mean the Universe was much smaller so it's not billions of light years across at all and everything is wrong !". This is true. However, you can't alter the speed of light without causing massive other changes : radiation transport would be totally different for one thing, so star formation and evolution would be drastically different. Since even galaxies billions of light years away look basically similar to ones nearby, there's no sign that the speed of light has drastically altered. Small changes, however, are not ruled out.

What's the opposite of a black hole ?
Q : I have heard that "white matter is at the opposite end of a black hole. What is white matter. When I type in "white matter" I got The Beattles White Album... I thought that was odd, to say the least.

A : I never heard of white matter before. Google tells me it's some brain thing, which I know nothing about.

A white hole is the theoretical opposite end of a black hole. Just as you can't escape from a black hole, you could never enter a white hole. The idea is that a black hole curves space into a tunnel which opens somewhere else (maybe somewhere far away from the black hole, maybe in another universe entirely). So any stars or planets which get swallowed by a black hole get spewed out as hot plasma from a white hole somewhere.

A white hole would look like an intense source of energy apparently coming from nowhere. Claims that they've been spotted turn up from time to time, but thus far there's no definitive proof that they exist. It's believed that even if a black hole did produce a tunnel in space, it would quickly collapse.

So black holes probably aren't really "holes" in the normal sense. If you fall into one, you don't get shot out somewhere else - you just die, quickly and painfully, ending up as a very, very small lump in the centre.

What would happen if a black hole collided with a white hole ?
Q : I'm wondering what were to happen if a white hole from one region of space came in contact with a black hole from another region of space.

A : Very difficult question. The rest of the internet seems to want to dodge the question completely by saying that "white holes don't exist". That may be true, but it's still an interesting hypothetical question.

After thinking about it, I would tentatively suggest that a black hole and a white hole could never collide. One is trying to draw everything in, the other is pushing everything away. Both go to infinity at the center, which is another way of saying, "the maths is broken, cap'n", but before that happens everything will stay sane.

With a black hole, nothing that falls below the event horizon can ever escape - or in other words you're safe as long as you keep your distance. With a white hole nothing can ever fall below its event horizon. So the white hole pushes the black hole aside before everything gets wacky. I'm not too certain about this answer though.

What causes inertia ?

A : After looking into this, I have to admit defeat. The internet is full of pseudoscience, possibly valid science but totally obfuscated in dense mathematics, and people saying, "it just is, so there". I can't make head nor tail of it. Sorry !

If black holes and white holes have the same strength gravity, doesn't that mean they're identical ?
Q : But the white hole would have the same gravity field as the black and, would therefore, behave the same surely ? How could matter, and energy, escape a white hole with the same gravity field as the black hole ?

A : A black hole has extreme gravity. In general relativity, gravity is the curvature of space. A very nice explanation is here :

Gravity is like a depression in a rubber sheet, things roll around it but ultimately fall in if they're not moving fast enough. Anti-gravity (which is what you'd get from a white hole) would be like pulling the rubber sheet up into a hump. So things would again roll around it, but ultimately get pushed away unless they're moving fast enough. Another way to think of it would be to say that a white hole has negative mass.

Black holes and white holes take this to extremes. In a black hole, you can't escape even if you travel at the speed of light. Since that's the fastest it's possible to go, if you get too close to the hole there's no way to escape. For a white hole, the only way to enter the hole would be to travel faster than the speed of light. Since you can't do that, nothing ever gets in.

IIRC, a white hole could theoretically form when a black hole forms as it curves spacetime into a tunnel which opens somewhere else. Where it might open gives everyone a philosophical headache. Somewhere else in our Universe ? Another universe entirely ? It is also thought that the tunnel would be extremely unstable (I guess because you'd need exactly equal amounts of positive and negative mass), and anything trying to enter it would immediately cause it to collapse.

If it did exist, a white hole would look like an incredibly bright object with no apparent energy source. Thus far, while a few candidates have turned up from time to time, there have always been more conventional explanations that could explain them.

EDIT : This answer is totally wrong ! I'm preserving it only to show what a stupid mistake I made. White holes aren't anything like what I've described, they're far more complicated than that. It turns out that white holes do indeed have gravity just like a black hole, so things can orbit them just as they can a black hole at a safe distance. Rather than being a sort of anti-gravity, white holes are actually black holes running in reverse :
The upshot is that because general relativity is Bloody Complicated, white holes still have strong gravity like a black hole but are nonetheless completely different objects. Frankly this is too mind-warping for me and I can't find a good maths-free explanation. So I'll be a lot more cautious about answering questions about white holes in future.

Where does matter falling into a black hole go ?
Q : I mean it's got to go somewhere, right ?

A : Yes - it goes into the hole. "Hole" does not equal "tunnel", it's most probably just a pit. Throw stuff into the hole in a bin and it stay in the bin and nothing else happens.

How can we prove there's life on other planets ?
Q : How do we prove there is the possibility of life on other far away galaxies? Since they are an older society, they would have already figured out "fire" and how best to use solar power and could very well be far superior in technology. On Earth, we have smoke stacks spewing CO2. What if other planets had a different chemical that they used, how would we be able to detect that? Hoping I made sense with that question.

A : The way to do that would be to look for oxygen in the exoplanet's atmosphere. Oxygen isn't stable, it's only because of photosynthesis that 20% or so of our atmosphere is oxygen. There are other chemicals we could search for (such as methane) but a 20% oxygen atmosphere would be pretty solid. Detecting trace gases like CO2 would be much harder. To do this, we would need to use spectroscopy : splitting light into its component colours and looking for signatures of specific chemicals :

If I recall correctly, the great sensitivity needed to do this even for relatively nearby exoplanets requires next-generation giant ground-based telescopes or planned future space missions. It's not easy, because planets are so faint and also overwhelmed by the glare of their parent star.

Here's a very nice, very readable article on the challenges of looking for life based on the chemistry of planetary atmospheres :

What is the escape velocity of Jar Jar Abrahams and is there any way to reduce it to zero ?
Q : You heard me. Make with the Jar Jar !

A : The escape velocity of JJA is small because his mass is small. Alas, his movies are transmitted at the speed of light, making them impossible to escape according to current physics.

How do you know black holes exist ?
Q : Well ?

A : For now, through a combination of evidence and theory. The observations show that objects exist above a certain density. The theory says that objects above this density cannot avoid collapsing into black holes. Cygnus X1 is a nice example of a good stellar mass black hole candidate :

Sagittarrius A* is probably the best candidate supermassive black hole since we can observe the orbits of individual stars around the compact object and thus determine its mass very precisely (we can also estimate it from X-ray emitting gas) :
According to current theory there's probably nothing else it could be AFAIK. Still, one should never rule out observations detecting something not predicted by theory - it wouldn't be the first time.

But the really nice thing about Sag A* is that with the development of the Event Horizon Telescope, it's so large that we should actually be able to image the event horizon and finally see the black dot at the centre of the accretion disc. That would be really solid evidence, bordering on actual proof.

How long will it take for all the heavy elements to decay ?
Q : If the last star in the universe were to explode today, how long would it take for all the heavy elements (like uranium) to break down in to lesser elements ? And I am excluding Black Holes which might still exist and be responsible for creating new stars in nearby nebulae. Let us assume these are gone already too.

A : I won't be infinte since there are a finite number of atoms. The numbers keep reducing until there's only one left, which, eventually, would also decay. It's very hard to put a number on this but my guess would be hundreds of trillions of years.

There are also some theories which say that protons themselves are not stable. So if you wait long enough you don't even need to worry about radioactive decay because protons and neutrons won't even exist. Timescale estimates for that vary wildly, from as little as 10^40 years to 10^200 years.

Is it possible stop microscopic black holes from shrinking ? I am totally not plotting to destroy the world.
Q : I asked a question if a man made black hole could be created in space, to make it grow large you could place it near a star, or maybe inside a moon for it to consume matter quickly to not disappear, and or evaporate. But Hawking radiation is a problem with this, and someone said on a answers forum "You'd have to precisely aim the mass of a mountain range at it in 10^-20 seconds (say), to keep it from evaporating, and anything that approached it would be exposed to super high temperatures blasting away from it".

Also another person also said on the answers forum "The smallest microscopic black hole that you can theoretically make is has a mass of 1 Planck Mass, and a diameter of 1 Planck Length. Unfortunately, this black hole will also evaporate in about 1 Planck Time, so you won't even be able to feed anything into it at the speed of light before it disappears". So how to get around this problem, if the microscopic black hole instantly has matter to feed on if you created the microscopic black hole in either a moon, or a planet, to get larger in mass, would this fix the problem. But would the radiation push the moon matter away from the microscopic black hole, so would the black hole not be able to grow big this way.

A : I would say the fact that microscopic black holes don't grow any larger is something to be thankful for, rather than a problem to be avoided. :)

I suspect there is a threshold for the black hole's mass below which Hawking radiation is so strong that it becomes literally impossible for the hole to grow any more. At the atomic scale, radiation pressure is going to push any infalling matter away, delaying it so that the hole completely evaporates. At that size (I can't give you a numerical estimate) there is no hope - the initial mass of the hole simply has to be larger.

Your best bet for creating a black hole which stands the greatest chance of growing would be to start inside a neutron star where the density is fantastically high. That will give you the maximum rate of mass accretion into the hole, so you can start with the smallest hole possible. You can't do this on Earth though, because without the tremendous mass of the neutron star to pressurise it, neutron-degenerate matter is extremely unstable and explodes.

Can something turn into a black hole simply by going really fast ?
Q : I remember reading that if a particle goes fast enough, it would theoretically collapse into a black hole because it would contain so much kinetic energy.

A : I'm not at all expert in relativity, but the internet is providing me with some contradictory answers. First, a straightforward Google search says categorically, no, you can't make a black hole just by going faster. Which makes intuitive sense to me because you don't really add more mass by going faster - relativistic mass is not the same as rest mass.

But on the other hand, some normally impeccable sources say that energy does act in the same way as gravity :

So I'm thoroughly confused. Can anyone help clear up this mess ? UPDATE : Jonah Miller to the rescue ! He adds :

Since there is no preferred inertial reference frame (and indeed we can always find one where a non-accelerating spaceship is not moving), moving really fast alone definitely does not translate into becoming a black hole. Mass and energy are the same, but the subtlety, I think, is that the relevant quantity is the energy-momentum tensor, which is a complicated combination of energy and momentum that explicitly stays invariant when you move between inertial (and even non-inertial) reference frames. I am extremely confident about that.

But acceleration?

I am less confident about the following.

But... that's perhaps not a totally satisfying answer, so let's ask a different question. If we were to want to go at high speed relative to our current reference frame, we'd have to accelerate. Could that process form a black hole? I think... sort of.

Suppose we wanted to accelerate up to some very high speed within a time of one second. Then we'd need to output gamma m c^2 of energy per one second to do it. That means we need gamma m c^2 of energy NOW in a single packet, which might come from mass. The packet of energy we need may well be massive enough to form a black hole.

On the other hand, if we accelerated adiabatically, we'd need to expend that energy slowly and we could take it (and expend it via acceleration) in chunks. Also there is a way to mimic a black hole with motion: constant acceleration can create an effect analogous to an event horizon.

What do you think of this ridiculous hour-long video ? Doesn't this nebula look just like a finger ?
Q : What do you think of this ridiculous hour-long video ?

A : [First, for the record, I'm generally fine with reading short articles no matter how ridiculous. But if you're going to ask me to spend a full hour of my time doing something else, please remember I'm under no obligation to do so. I'm doing this entirely voluntarily. And if that video turns out to be so awful that I hate every second of it, then forget it - I'm not going to waste an hour of my free time doing something I hate. I do, however, absolutely guarantee to give my honest opinion if I do watch it, but be warned that if I honestly think it's a big pile of poop, I'm going to tell you as much. You did ask for it, after all.]

This one comes from a long and silly discussion regarding the difference between pareidolia (seeing things which aren't there) and image recognition (seeing things which are there).

The questioner professes to be a geometry teacher but apparently doesn't understand the difference between one thing resembling another and actually being the thing it looks like. A nebula that looks like a finger is exactly an example of pareidolia because it isn't really a finger. It only looks like one at a very specific wavelength and (likely) only at a very specific angle. Even if it did look like a finger from all angles and at all wavelegnths, it still wouldn't have any significance. Which I explain at length here :

The video turned out to be so unbearably stupid that I gave up after 15 minutes. It claims that a Renaissance painting resembles the Orion nebula. It doesn't. It was equivalent to saying, "hey, that turnip looks a bit like a cat !". The questioner than linked to a website claiming that the Orion nebula looks like a brain. Again, it flat-out doesn't. The image of the nebula had clearly been cut to fit and even then it wasn't doing a good job.

The questioner further claimed that wavelength doesn't matter to geometry. This leads me to conclude the questioner is actually properly delusional, because this is objectively wrong. It's a bit like pointing to a moose aand saying, "nice elephant !". It just doesn't make any sense at all. I don't get it.

Does my stupid website prove that gravity doesn't exist and the world is flat ?
Q : Why didn't you watch this 25 minute video ?

A : Because I read the text on the website and that was enough. Frankly, ordinarily I could happily watch Kate Upton in zero gravity for hours on end, but even that wasn't enough to persuade me to watch this obvious effluence.

I offer some thoughts on why some ideas aren't even worth considering here :

I have great respect for people who try and persudae people that their objectively wrong ideas are, well, wrong, but there comes a limit when I refuse to get involved. I'm just not convinced some people will ever see reason even if it bites them in the groin. The best I can say for them is that they are genuinely stupid - they literally lack the intellecutal capacity for basic reasoning.

Could there be black holes made of anti-matter ?
Q : I was just thinking, if there is such a thing as an anti matter black hole ? I understand that the amount of anti matter required is mind blowing but could it exist in some distant past of the universe ?

A : There's no particular reason one couldn't form as far as I know (except that there probably isn't enough anti-matter but that's besides the point). Anti-matter is believed to cause gravity in the same way a regular matter. It shouldn't also be affected by gravity in the same way. AFAIK, you wouldn't be able to tell if a black hole was made from regular matter or anti-matter.

Apart form looking for aliens, what will China's new 500m telescope do ?
Q : The only thing the media are saying is "aliens".

A : The obsession with aliens annoys the heck out of me. Yes, looking for aliens is cool, but we aren't going to get results on that anytime soon. Meanwhile, there's a tonne of productive research to be getting on with. Pushing the aliens card so strongly is selling the public short by insisting they won't believe anything else is worthwhile. As for the research :

It's basically a more limited but more sensitive version of Arecibo. It's more sensitive obviously because it's bigger. It's more limited because it will operate over a much smaller range of frequencies and won't have a radar transmitter installed. Like Arecibo, it will still search for pulsars, measure the gas and chemistry of distant galaxies, but it won't be studying the atmosphere or asteroids.

What do you think of this UFO report ?
Q : Here it is :

A : Well, "we had the recording, but we don't any more" doesn't exactly fill me with confidence. Maybe they'll be something to be comment on after the FOI request but until then there's no actual information here.

What do you think of this 80 minute documentary about UFOs ?
Q : Here it is :

A : To be honest I'm more or less done with UFOs at this point. I spent far too much time reading about this stuff already and I never found anything really convincing. The few cases where I thought there might be something in it were just nowhere near enough to convince me this is something worth pursuing. I'm basically at the point where I won't accept any evidence short of a flying saucer landing on the White House lawn, because >99% of the "evidence", in my view, turns out to be utterly useless. I'll still entertain short articles but 80 minute documentaries are out, sorry.

Which is not to say I think no-one should investigate this, just that it's not for me.

Well could you watch the first 15 minutes at least ?

I watched the first 18 minutes because nothing actually happened in the first three minutes. Nothing here I haven't seen a million times before, so I'm going to watch War And Peace instead.

Do black holes rip you apart because they're made of anti-matter ?
Q : A black hole is made of antimatter right? That's why you get ripped up when you go inside cause they cancel each other out. I know its just a hypothesis but it makes sense.

A : No, it's made from ordinary matter. You get ripped apart because of the intense gravity. Anti-matter does not rip things apart : colliding matter and anit-matter annihilate into energy.

If we can see back in time, can we also see forwards in time ?
Q : While I know the fun of looking back in time teaches us about the history of our universe, is there anyone looking forward, to see what kind of obstacles might be in front of us ? Like the collapse of the universe ? Is that even a possibility ? What kind of technology would we need ?

A : There's certainly no easy way to do this as there is with seeing into the past. Travelling into the future turns out to be a straightforward matter of travelling very fast :

(and by straightforward I mean, "with access to more energy than has ever been produced in the entire history of mankind")

But to send a signal or anything else backwards in time (i.e. for someone to look into the future) turns out to be incredibly difficult at best, widely considered impossible. There are a few exotic possibilities, though none (AFAIK) allow you to travel back to a point before the time machine was constructed (and by "time machine" I mean anything which permits backwards time travel, e.g. a wormhole, particular arrangements of cosmic string, etc.). So seeing into the future, at best, won't be possible until we build a time machine. A very nice, quite comprehensive summary is here :
See also :

Even if time machines do turn out to be possible, they're fraught with paradoxes. The essence of it being : if I see what's coming and change my actions, where did the information come from ? E.g. I peer into the future and see myself getting run over by a bus. So I stay home that day and don't get run over by a bus... but what was the future I originally saw ? Was it real ? Or, more dramatically, what if I go back in time and kill myself - if I'm dead, how was I able to go back in time ? #MassiveHeadache

Would it take infinite time to cross the galaxy at the speed of light since time stops ?
Q : I saw this meme saying it would take 100,000 years to cross the galaxy. That's obviously wrong, right ?

A : You're wrong that it's wrong because the meme is right. It would indeed take about 100,000 years to cross the galaxy at the speed of light. Although it does depend on your point of view (whether you're the astronaut or someone back on Earth), no-one will see it take an infinite amount of time.

The time the astronaut experiences is always less than the time someone back on Earth experiences. An observer travelling close to the speed to light sees the outside universe going by extremely quickly, so from their point of view it takes almost no time at all. To an external observer it would take them 100,000 years, but they would see them ageing much more slowly. Their clock would appear to slow down, but their speed would not.

When black holes merge, do they form a wormhole ?
Q : Earlier we knew that when two black holes combine they form a worm hole and now , we have found a binary system of black holes also ... Are there any conditions concerning these two different things like if these conditions will get satisfied then a worm hole or a binary system will be formed ?

A : Wormholes are theoretical - there's no good observational evidence that they exist. It's though that (at best) if a wormhole did form naturally from a black hole, it would be incredibly unstable and collapse if even slightly perturbed. So merging black holes is likely to destroy wormholes rather than creating them, if they even exist at all. It just creates a bigger black hole.

That said, the recent detection of gravitational waves shows the small black holes do merge, but how the black holes get close enough to do this is a controversial area. When they're really close, the gravitational waves themselves carry away huge amounts of energy, so the holes get closer. At larger distances, friction with the surrounding gas can help slow them down, causing them to move closer. But there's a range of distances at which neither of these effects are thought to be very important - yet there's pretty good evidence that mergers happen even so. It's an area of ongoing research. 

Do black holes convert energy into mass ?
Q : E = mc^2
Nuclear weapons
Nukes release mass into energy yes.
Blackholes squish energy into mass ?

A : No, black holes just squish mass into really dense mass.

Can you do science without the scientific method ?
Q : If you use the scientific method to come to an infinitely-wrong conclusion, is it junk science? And alternately, if you construct a more-coherent story line that comes to the right conclusion, but without using the scientific method, is that even science? Would you rather be right or scientific? (not that those two outcomes are typically contradictory).

A : I would say the key point is how you test it. If you test things as objectively as can you can by the standard scientific approaches and still get it wrong, it isn't junk. Just about everything is eventually disproven, that's just the nature of the beast. If you come up with a conclusion that happens to be right without testing it scientifically, you have really no idea of knowing whether it's right or not - so you'd just be making wild claims, even if they were eventually proven correct.

Should the Arecibo radar be subject to ethics review boards to prevent us accidentally signalling our presence to aliens ?
Q : I can almost see +Rhys Taylor rolling his eyes already. [Original article :]

A : Back when there was that ill-fated project to send twitter messages to the supposed source of the "Wow" signal, I sat in on one of the initial meetings. I asked one of the radar operators if the signal would be detectable to an alien equivalent of Arecibo at the distance of the target stars (~100 light years, IIRC). The reply was an unhesitating, almost contemptuous, "no". Given that Arecibo is a 1 MW transmitter, I'm highly skeptical of the claims for searches of ~10 W laser signals in the article. Anyway, any civilization advanced enough to actually visit us will already have detected us without our sporadic, non-repeating, pathetically weak radio signals.

Will the new FAST radio telescope be better at signalling/detecting aliens ?
Q : I hope that the numbers will go up (a bit) with the new Chinese radio telescope. They say here it will be ready in September... do you have any more info ?

A : AFAIK FAST will not even have a transmitter of any kind. If it ever does get one, I would not expect the numbers to increase that dramatically - maybe a factor of a few. Arecibo's 1 MW transmitter is quite capable of imaging Saturn, so I doubt there would be a need for a dramatic increase in transmitted power unless they were deliberately trying to do active SETI. What the technical challengers of having a more powerful transmitter would be, I'm not sure. FAST has a much smaller receiver cabin than Arecibo - last I heard they need to lower the entire platform to change receivers (Arecibo has them on a rotary floor, kindof like a revolver only with more science). I don't know if it would be feasible to install a transmitter in that sort of limited space/capacity. Arecibo's transmitter is not the sort of thing you'd want to be taking out every time you want to observe something else.

As far as regular SETI goes I would expect a fairly significant increase in sensitivity. The dish size increase is not quite what you might expect. Arecibo typically only uses 225 m of the 305 m dish (otherwise it could only point straight up). FAST will typically use 300 m of the 500 m dish, so that's an area increase of only 1.7. However, it's planned to have a 19 beam receiver compared to Arecibo's current 7 (though a 40 beam receiver is planned/under construction), so it should be able to survey much larger areas more quickly (numbers depends on the performance of the receiver instrument, which I don't know). It will also have about twice the area coverage that Arecibo has.

I believe the plan with FAST is for first light in September, which right now looks entirely feasible given the extremely impressive construction pace. My guess would be that initial science results will follow within a few months, with the first approximately design-spec stuff coming sometime in 2017.

How about these space facts, eh ?
Q : Isn't space COOL!

A : Awww, man, I hate stuff like this. Totally random assorted nuggets of trivia that are usually just wrong. And this list is no exception.

"the Eridanus supervoid is very empty of ordinary matter. Dark matter likely is very abundant here, which would prove as an explanation."
That doesn't make any sense as an explanation. Dark matter is abundant in normal galaxies; it's thought to be a major reason why they form. In standard cosmological models voids are devoid of both dark matter and normal matter.

"Everything in the universe will return to its position it is in today in 10^10^10^10^2.08 years, according to the Poincare-Recurrence time."
That assumes the Universe is either static (it isn't) or oscillating (for which there is little or no evidence). Currently the data favours the Universe expanding forever, which means there's no chance matter will ever return to its original position.

"Black holes will live longer than the universe."
If there's no Universe, there can't be any black holes. Black holes may well live longer than anything else in the Universe, but they can't live longer than the Universe itself.

"Every galaxy in the observable universe is headed for the Great Attractor, minus the ones being pulled away from the attractor due to the expansion of the universe."
Since the number of galaxies which are being pulled away due to the expansion is much, much greater than the ones which are not, the first half of that sentence makes absolutely no sense. It's like saying, "Everyone in the world is currently in Wales, apart from the ones who aren't."

If nothing can escape from a black hole, why do they have jets ?
Q : If nothing, not even light escapes, where are the jets generated... and where are they escaping from ?

A : Nothing can escape from within the event horizon of a black hole. Outside that distance things can still escape provided they have enough speed.

The formation of the jets is not well understood, but a huge amount of energy can be released by matter falling into the black hole - enough to drive some of it back out into the jets. The jets are likely escaping from the discs of accreted material surrounding the hole.

Does centripetal gravity affect different masses differently ?
Q : Does spin generated gravity affect objects equally, regardless of mass, or does it accelerate objects of different masses at different rates ? I think it affects all masses equally, however I also recall that centrifuges are used to separate masses that are intermingled, like how a uranium centrifuge is used to separate the heavier U-238 from the lighter radioactive U-235, which would make me suspect that spin-generated gravity does not affect all masses equally, but there are many other factors that could be at work in the case of centrifugal separation. am not aware of any other sources that could answer this question.

A : I would think that things in a centrifuge are separated just because of their different densities, same as under normal gravity just much stronger. Centripetal acceleration doesn't depend on mass, though force does - same as gravity. The force experienced is directly proportional to the mass, but acceleration is always inversely proportional to mass (F = m*a), so they exactly cancel.

Have we reached the end of physics ?
Q : I've read this article, and... hey, wait, I never asked for your opinion ! STOP IT !!!

A : Saying, "the end of physics" is a bit like saying "Voyager has left the Solar System". This is flagrant clickbaiting.

"for the first time in the history of science, we could be facing questions that we cannot answer.” There's never, ever been a period where that wasn't the case. It's a sort of defeatist humblebrag to state "we've got this huge mystery we'll probably never solve". For one thing, the multiverse is but one possible explanation. For another, whether you can learn everything or not is not the point. The point is to try.

The infinite multiverse, like any theory of an infinite universe, lets you get away with scientific murder. You don't have to explain anything because everything happens an infinite number of times, so obscenely unlikely events become certainties. Instead of searching for a deeper answer, it's all, "nope, it's a statistical fluke". Maybe it's even true, but it's not helpful.

If aliens visited Earth, what would be the hardest universal human behaviour to explain ?
Q : Well, what ?

A : It's impossible to answer without know what the aliens are like. They could be every bit as batshit crazy as the rest of us.

Are there any known lifeforms which can survive in space ?
Q : Is there any other type of life which exist in space. I got this question because some persons say that earth got its components like water, oxygen and many other things from space. What if those components were from a planet which have those components and life forms in it.

A : There are organisms known which can survive in space, for example, tardigrades :

We also know that not just water but also more complex chemical compounds like amino acids (which are important for life) can be found in space :

But what we don't know is whether other planets or comets would have been more suitable places for life to begin than the early Earth, or if any lifeforms could survive crashing into the planet. Some people think that bacteria might have seeded the Earth if they were deep instead meteorites, which is one type of controversial "panspermia" theories. It could also have worked the other way around, with bacteria from Earth (presumably unsuccessfully) seeing other planets in the Solar System.

Are black holes gateways to other dimensions ?
Q : I really wonder about black holes! Are they an actual cosmic doorway maybe to another universe or another dimension!?? You never know! And sadly I don't think we will ever find out!?

A : We can't know for certain without visiting one. And even then, anyone who fell through wouldn't be able to tell us about it unless there happened to be another black hole to send them back to our Universe. Which is unlikely as there's no evidence for any white holes.

All we have to go on are our theories and (somewhat limited) observations. Currently it doesn't seem likely. It's possible that wormholes do form inside a black hole, but they would be extremely unstable and not likely to last long enough for anyone to pass through them. For smaller, stellar-mass black holes you'll get ripped apart by tidal forces anyway, though for supermassive black holes you might have a chance.

So we really don't know, but current theory says probably not. Black holes could simply be pits : throw something in one and it ends up in the bottom of the pit, feeling very unpleasant and not doing anything very interesting.

Could Sagittarius A*, the supposed supermassive black hole at the centre of the Galaxy, actually be a white hole ?
Q : I read this article on the internet. Is is true ?

A : Every so often, evidence for white holes comes along, then it fades away again. I would be very skeptical about any such claim. They're theoretical concepts but in a very different way to black holes - it shouldn't be possible that they form at all. Not everyone agrees with this though, because we're dealing with things like time dilation and the reversal of space-time.

(If you only visit one of those links, I recommend the second one)

I'd always thought that white holes had anti-gravity and that's why they push stuff away. As the third link makes clear, it is much more complicated than that. So my initial answer of, "Sag. A* can't possibly be a white hole, things are orbiting it" won't do. Darn. That would have been too easy.

Still, that link does have some misconceptions I'm more confident about :
1) As the link in this point makes clear, this isn't so unusual. Black holes are known to be messy eaters. Given the complex environment of the hole, I'd want to be really sure we knew just how much mass should be accreting.
3) The black hole doesn't "energise" anything directly. It's just gravity with a tiny, negligible amount of Hawking radiation. But material orbiting in an accretion disc can be extremely hot and a powerful source of X-rays, which can ionise the surrounding region.
4) Again, black holes are messy eaters. The density of material being particularly high near the centre, it's not surprising we see stars being born. It's certainly not surprising we've never seen a star being eaten - although the density is much higher than in our locals stellar neighbourhood, it's still very, very low - too low to expect to see stars colliding either with each other or the hole. The radius of the hole is ~0.08 AU whereas the nearest star is typically ~1000 AU away.
5) A former colleague of mine was saying ever since the G2 cloud was discovered that nothing was going to happen.
7) Urrgh, this is complicated :
8) We can't observe singularities. However within the next few years we should be able to observe the event horizon of Sag A* :
And white holes have singularities too.

Why do you want physics to be broken ?
Q : Why do you want physics to be broken ? Most people who want physics to be broken are people with an existential interest in fantasies that simply can't come true in a universe as defined by the current physics.

A : [Edited version of my original answers. It is indeed true that most cranks have minds which are so open their brains fall out, but this hardly means that scientists should consider their theories inviolable ! This "scientists are dogmatic" attitude is something we should actively be fighting against, not encouraging. Anyone who thinks current science has solved all the problems or is impervious to attack simply does not understand how the scientific method works.]

And most scientists also enjoy breaking physics. Scientists love revolutionary discoveries because a) it makes us feel important and b) we get to learn something new instead of just validating the existing model. E.g.,

The vast majority of scientists would agree that we don't have all the fundamental pieces of the puzzle, for instance, we don't know what dark matter/energy are, we can't reconcile quantum mechanics with general relativity, etc. So it's entirely fair to say that physics is already broken. In fact it's usually broken, thus far we just keep improving theories. Nothing wrong with that, it's the nature of the beast. And it doesn't mean that the theories we currently have don't do a fantastic job in most regards.

I'm an observational astronomer by training, so I take whatever the Universe throws at me. Sometimes that means it's dull and boring, occasionally it throws up anomalies we can't explain. I'm also involved in numerical simulations. Specifically, I study extragalactic neutral hydrogen. A few years ago I found some dark hydrogen clouds in the Virgo cluster that aren't easily explained. Currently I'm running simulations to test three proposed scenarios to test which (if any) is the most likely. Thus far, one of the more controversial solutions is emerging as the clear winner. But I really, honestly do not care which solution works best as long as we find something that does work.

One can go from being a genuine skeptic to a denier by adopting too extreme a position in either direction. It's as foolish to cling too strongly to the idea that we've solved a problem as it is to the idea that we haven't solved it. While I'd agree that there are a lot of quacks out there screaming LOOK AT MY PIE CHART IT DISPROVES PHYSICS (believe me I get those kinds of emails), it's equally important to remember the history of science is not one of merely establishing facts with ever-greater precision, but also of radical, drastic revisions to its interpretations of the world. The trick is to walk the middle ground, not to go nuts over every tiny anomaly, but not to fall victim to hubris at every vindication either.

A tremendous amount of work goes into getting results one way or the other. The Universe is the way it is, it's not obliged to agree or disagree with our theories. Verifying them or disproving them are both "victories" of a sort. But for me personally, the really exciting stuff is when we find something we really can't explain. Knowing that existing theories are still working is great, but doesn't help us solve the known problems.

How do we know black holes are spinning ?
Q : How can we determine that the black hole is spinning ? Does black hole spin on its own axis ?

A : Most black holes are probably spinning because most objects in the Universe are spinning. Obviously we know the Sun is rotating because we can watch features on its surface move. We can do this to some extent with other stars, but mainly this can be estimated from the spectra of the stars. Lines in the spectra look wider than they otherwise would due to the rotation.

Since black holes are collapsed stars, they must also be spinning, and because they are small they have to be spinning fast. Which is easily demonstrated by spinning on a chair and pulling your legs in.

To actually measure how fast a black hole is spinning we need to measure the material around it. There's no way to measure the spin directly since the hole has no solid surface. But we can estimate the spin from material close to the event horizon.

Another, weirder effect of the extremely strong gravity and spin of black holes is frame dragging - the distortion of space by the black hole's spin. This also has been observed, confirming the rotation of the hole :

Do all astronomical imaging systems have the same contrast range ?
Q : Contrast between two spots in an image is the same no matter what number of pixels devoted to it. Am I wrong about that ? Aren't all imaging systems limited to the same contrast range ?

A : I think that would only be true if you had identical receivers performing the same observing setup (but with different integration times), the sources were both above the noise level but below the saturation limit, and the images were created using the same algorithm. In that case you'd measure different absolute fluxes but the relative contrast would be the same. However, what should be more resilient is the inferred intrinsic luminosity of the source after calibration - as long as it's a point source to both receivers, that should be identical (if it's detected and not saturated) regardless of the receiver size, integration time, observing and imaging setup.

But, in general, this isn't the case. Telescopes are different sizes with different angular resolutions, so a point source to one telescope may be resolved to another, so the flux is spread out over more pixels. Or velocity channels for that matter. Instruments can have different dynamic range sensitivities. Images can be reconstructed in ways which favour point sources or extended emission, from the same raw data (number of pixels in the image doesn't usually relate to number of receiver pixels). Choice of observing setup can strongly affect the end result, especially (as in the OP) where there's very bright emission filling the entire beam and the data must be calibrated accordingly or things just don't work at all. In a word... no. :)

How could we detect dark matter with an unlimited budget and/or resources ?
Q : What could a Kardashev type I, type II or type III civilisation build to detect dark matter/test its existence ? What could modern astronomers build, given an unlimited (say, trillion-dollar) budget, to detect dark matter ? A giant Ice-cube type neutrino detector using the entire surface of Ganymede ? A particle accelerator running around the moon ?

A : Let's take this on a scale of ascending technological advancement, starting with our own puny civilization.

I'm no expert in the direct detection experiments but offhand I'd say yes : bigger, more sensitive detectors would be the way to go. Not necessarily on the ice moons though : there's plenty of ice on Antarctica for expanding IceCube ( and we could expand other detectors relatively easily (e.g. LUX uses just 300 kg of xenon - let's use more ! Whether a larger particle accelator would help I wouldn't like to comment. Still, giant space projects are probably more in the realm of a type-I civilisation - unlimited budgets won't remove the need for a massive space infrastructure that would take decades to develop.

But there are also improvements in the indirect detection methods that could us much stronger evidence. For instance, one possible experiment with the Square Kilometer Array will be to measure how the velocities of galaxies change as the Universe expands ( So with some serious hardware upgrades possible in the next 10-20 years (i.e. the sort of problem that just needs money thrown at it) we might be able to measure how the dynamics of individual galaxies change in real time. For extreme objects, that might just give us the capabilities to distinguish between dark matter and alternative theories of gravity. We'd also need more advanced numerical modelling to give us precise predictions : so a bunch more supercomputers. Couple that with more radio telescopes - say, ten Arecibos or FASTs, which would give us enormous sensitivty and sky coverage (and resolution if linked as an interferometer). That would give us the prospect of detecting objects that just couldn't exist without dark matter : the sky remains only poorly explored at radio wavelengths. Telescopes could also play a role in more direct detection methods, such as searching for signals of self-annihilating dark matter expected in galaxy cores in some models. But really there's not much we could do today with an unlimited budged that we're not doing already, except bigger and faster.

But what could we do if we did have the much greater energy and resources of type I-III civilizations ? One option would be to do tests of general relativity in the very low acceleration regime where alternative theories say gravity works differently. The famous "Pioneer Anamoly", where the Pioneer spacecraft drifted slightly off-course, now seems to be exlained by the uneven heating of the probe by the Sun. But the principle is sound. It would be trivial for a type I/II civilization to send thousands of probes throughout the Solar Sytem and monitor their positions as they get further from the Sun, where the gravity from the Sun is far less than near Earth. Probes would be sent in different directions at different speeds, with different sizes, masses and designs to account for thermal effects. Although technically possible to do this for our own civilization with an unlimited budget, it would be far easier and more reliable for a more advanced civlization : they'd deep to know about the distribution of matter in the Solar System as precisely as possible. For example, powerful deep space radar and imaging might give them a much better idea of the distribution of comets in the Oort Cloud, helping to distinguish the effects of different theories of gravity from the tiny effects of close encounters with bits of rock and the like.

Another option would be a relativistic probe that exploits the effects of time dilation. Accelerate a ship to very nearly the speed of light and the crew could watch galaxies interact and collide in a few minutes - the ultimate way to test their predictions. Of course they wouldn't be able to relay this information back, since for the rest of the Universe billions of years would have passed, but this question postulates a species who are clearly going to put this "£@*ing dark matter issue to rest ONCE AND FOR ALL, DAMMNIT.

Perhaps the grandest method, only available to a no-fooling-around type III species, would be redistributing matter. Such a species could start moving stars to different parts of the galaxy. At least some alternative theories of gravity, such as MOND, predict that the distribution of matter is important to its motion, not just the mass. If there's really dark matter holding our galaxy together, moving the stars to different orbits should result in different motions to if this is due to the nature of gravity. It would taken tens of millions of years, but that's faster than waiting for the relativisitc probe to return.

Are smartphones as good as telescopes ?
Q : Each tiny phone can take images that have the same value as any big mirrors. The matter is how you look at a picture, as each ever taken has a complete information of all frequencies in it. I have a way of doing it, so the answer is yes, all the frequencies including invisible are present in any imprint, so a radio signal can be converted to visual what is nothing new as TV works that way. besides it can all be seen in a still image that is still only in our comprehension while containing huge amount of motion. Problem is that people make machines specified for different frequencies without knowing that they are always all there. Receivers as you know may be specified by manufacturer, but they all have much wider potential

A : No. Just no. Seriously, no.

Receivers are only sensitive to specific frequencies. Neither your phone camera nor photographic film can even detect radio wavelengths, and if they could, they would be nowhere near as sensitive as a large telescope like FAST. No amount of clever processing of the data will change that. You'll never get the same information from a smaller receiver - it's fundamentally impossible. A smartphone camera isn't even sensitive to radio wavelengths at all because that's not how it works. No amount of image processing will ever recover this. Anyone claiming they've found a way to do so should patent it and make millions ASAP... :)

Is there sound in space ?
Q : A friend of mine is convinced that there is sound in space. There isn't, right ?
This question wasn't sent to me. I found it in Randall Munroe's excellent "What If ?" book, which I would highly recommend to anyone. However in this case I have to find a fault, as Munroe relegates this question to the "weird and worrying" section - questions which don't get answered but are just stated for the record. And while their weirdness is normally self-evident and amusing, that's not really the case here. In space no-one can hear you scream... but there's more to sound than screaming.

A : Right ! Well, sort-of. Actually not really. No.

The smart-alec answer would be that planets are in space, and there's certainly sound on other planets as long as they have an atmosphere. That's clearly not what the question was getting at though. Obviously it's about sound far away from the atmospheres of any planets.

Sound waves are just propagating variations in the density of any medium, be that solid, liquid or gas. Like with ordinary water waves, the air (say) itself doesn't move much, it's just that the molecules bunch up for a little while and then move apart again as the wave passes, before settling back into their ordinary density level. It's this density wave that moves, not the molecules themselves. This can happen in solids and liquids as well as gases, as anyone who's ever dunked their head underwater in the bath can confirm.

So all you need for sound is... well, stuff for it to travel through. And "space" is misnamed, because it's NOT totally empty. Oh, it looks empty, but that's only because most of the material there emits at wavelengths we can't see with our eyes. Actually you can see this indirectly, at least on a dark, clear night : the dark band across the Milky Way isn't dark because there are no stars there, but because there's intervening dust that blocks the view. Look at it in other wavelengths and it's actually a bright, interesting place.

So there IS stuff in space for sound to travel through. In numerical simulations, sound speed is an important parameter in calculations of fluid dynamics - it tells you how disturbances travel through the gas. So in that very real sense, there most certainly is sound in space, and it plays a very important role in determining the evolution of gas structures and ultimately star formation. We can even, in a few rare extreme cases, witness waves moving through interstellar material in real time, such as in this movie of the Crab pulsar.

But don't go nuts. "Space" is not a totally stupid name, because although there is stuff there it's incredibly thin : typically quintillions of times thinner than ordinary air, and thinner than the best vacuum ever created in a laboratory. So calling it empty is pretty reasonable by our standards. Of course if exposed to this near-vacuum for more than a few minutes you'll die, but what if only your ears were exposed ? Or, better, what if you used a microphone that was equal in sensitivity to your ears ? Would you hear anything ?

Although human senses don't fare very well compared to other animals, even so they're still incredibly sensitive. Your eyes can, astonishingly, detect a single photon of light - only your brain's processing normally puts a slightly higher limit on this to give you a cleaner image. But individual photons can stimulate the movement of individual electrons relatively easily. And while your ears can detect variations as low as one billionth of normal atmospheric pressure, those density variations have to be able to move your eardrum - and individual atoms and molecules just can't do that. You certainly wouldn't be able to hear anything in space, which is easily demonstrated with a small vacuum and a loudpseaker. Even at levels way higher than in space, we wouldn't be able to hear anything.

Of course, there are also grey areas. Even though even intergalactic space isn't totally empty, most of those spectacular pictures you see of nebulae are still thinner than our best laboratory vacuums. But sometimes that density has to increase dramatically, otherwise stars and planets would never form.

So the answer is yes, no and maybe all at the same time. Yes there is sound, but no it's not audible except maybe for some rare exceptions. Randall Munroe, you've made a rare error here - this question shouldn't be consigned to the "weird and worrying" section at all !


  1. How firm is the data indicating that the expansion of the universe is accelerating? Is this something that with new data or better analysis might be found not to be the case 20 or 30 years from now?

  2. Great question ! Added my answer to the Cosmology section. For your convenience I'll also provide it here :

    I'd say it's pretty solid. Maybe not 100% certain, but good enough for government work. :)

    The evidence that the expansion comes from supernovae explosions - specifically, type Ia supernovae. These happen when a white dwarf gains enough mass from a companion star that it re-start fusion. The point at which this happens is thought to only depend on mass so the resulting explosion should always be of the same energy. Knowing that energy, we can work out the distance to the supernova pretty accurately.

    The benefit of using supernovae is that we can measure both their redshift and distance even in distant galaxies. Because light travels at a finite speed, the further away a galaxy is, the younger (and therefore smaller) the Universe was when light left that galaxy. After accounting for this, the supernovae data indicate that the acceleration of the Universe is increasing - which is something nobody was expecting.

    There was an alternative interpretation of the supernovae data : we live in a void, a region of the Universe which happens to have much less matter than the rest. Since there's less matter inside the void, there's less gravity to slow down its expansion, which would look like acceleration.

    This led me to a rather interesting paper-chase. Here's an original press release from 2009 categorically stating that we don't live in such a void :
    And here, on the exact same day, is another wesbite interpreting this to mean that we DO live in a void and dark energy is wrong !

    More recent, independent evidence futher supports the notion that we don't live in a void. As far as we can tell, the expansion really is accelerating.

  3. Dear Rhys, by what factor is Galactic astronomy cooler than extragalactic astronomy? Is it 7, 17, or 70?

    1. Since Galactic astronomy is a mere sub-branch of extragalactic astronomy, it is obviously the less cool of the two. Exactly how much less awesome it is will depend on the number of galaxies. Clearly, you're talking about powers of ten. There are definitely more than 10^7 galaxies, as you well know, so I expect that one was a red herring. 10^17 is a plausible number given that the observable Universe is probably not the whole Universe. However, if inflationary theory is correct then the whole Universe is likely to be very much larger indeed, in which case Galactic astronomy could easily be a factor 10^-70 cooler than extragalactic astronomy.

    2. A mere sub-branch?! I seriously laughed at that one. Based on this backward reasoning, we should invert your numbers and deduce that Galactic astronomy is 10^70 times more awesome than extragalactic astronomy. Sounds good - we should've settled this back at Arecibo. :)

    3. Bah, you're just jealous because your sample size is 10^70 times smaller.

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