Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website,

Tuesday 27 May 2014

The Importance Of Being Idle

I don't usually reblog science stories, but this week there's an article so interesting, and so close to my own research, that I'd feel silly if I didn't. And also I think the various websites which have re-printed the official press release have kind of missed the point of why this particular story is so interesting, so I'm going to try and redress that here. This is all about "dark galaxies", which are the very thing that led me into radio astronomy in the first place.

This is going to be a pretty long post. If you're already familiar with dark matter, skip section 1. If you already know about dark galaxies and why they might be important, skip section 2 and go straight to section 3, where I describe the latest results.

1) Dark... what now ?

Although not everyone likes the term, a "dark galaxy" is generally reckoned to be a cloud of gas sitting quietly inside a dark matter halo minding its own business. The point being that the gas isn't forming - or has ever formed - any stars, making it dark. Technically this means it's only dark to visible light - it can still emit at other wavelengths, like radio waves, so some people prefer to call them "optically dark galaxies" or "gas only galaxies".

People who worry about such things probably don't get invited to parties very much, so I'm going to stick with plain-old dark galaxies for the rest of this post.

Firstly, the dark matter. Over the years, "being mostly made of dark matter" has become the de facto definition of a galaxy. Dark matter is pretty simple really - galaxies are rotating too fast to be stable, so without something else beside the visible matter to hold them together, they should just fly apart. The links have more details if you like that sort of thing.

This is what happens if you take the dark matter away.
There are alternatives to dark matter (like different theories of gravity) and I'll admit to holding a small degree of skepticism about it even now. However, the evidence is leaning pretty far in favour of dark matter's existence (Ethan Siegel has this excellent summary), so I can't justify my skepticism rationally. For the rest of the post, let's go with the consensus and assume it does indeed exist.

Sometimes, it can be tricky to tell the difference between a cluster of stars and a genuine galaxy (I stole this idea from Robert Minchin while he wasn't looking). What gives the game away is the dark matter - if the object needs dark matter to hold it together it's a galaxy, if it doesn't, it's a star cluster. Keep a copy of this identification chart handy if you have any doubts.

Mind you, not everything without dark matter is a star cluster.

With dark galaxies we're looking for clouds of hydrogen that need dark matter to hold them together, but haven't bothered to form any stars. If "dark galaxies" is a controversial term, then perhaps we could go with "lethargic galaxies" ?

In some ways, they wouldn't be all that remarkable. In almost all cases, the hydrogen gas extends further from the center of galaxies than their stars do (usually by a factor ~2, but sometimes much further) - so in that sense, all galaxies have a dark component. But the idea of objects which don't have any stars at all is intensely controversial.

2) Are you sure you're talking about "dark matter" and not "doesn't matter" ?

Pretty much everyone, I think, accepts the idea that some dark galaxies might exist. The sticking point is : are there just a few exotic, weird objects, or are there bajillions of the little blighters ? And that brings us to the "missing satellites" problem. This is nothing to do with China blasting satellites out of the sky - it's about how many smaller "satellite" galaxies should be buzzing around large galaxies like our own Milky Way.

Theories suggest that there should be about ten times as many dwarf galaxies orbiting the Milky Way as we actually see. Huge projects like the Millennium Simulation (below) attempt to re-create the Universe inside computers. And they do very well on large scales, like filaments and voids, but fail miserably on the much smaller scales of individual galaxies.

Dark matter (used as a proxy for galaxies) distribution in an artificial Universe.

There are lots of possible explanations for this - finding galaxies can be quite hard, we may not understand the physics of galaxy formation all that well, or possibly there are just too many problems with our whole cosmological model and we should just give up and start again. That last one probably isn't as outlandish as some of us might like, but let's assume that we don't need to resort to such drastic measures. Even so, the missing satellite problem is a big problem in cosmology, and solving it would be a seriously major breakthrough*.

* It's unclear whether the newer, spectacular Illustris simulation has any answers to this - we'll have to wait and see.

Dark galaxies could offer a way out of this if most of those missing galaxies just haven't formed any stars. And, just like black holes, that would make them fiendishly difficult to spot. I'll let Red Dwarf's Holly explain why.

Quite. But it's possible that some of those dark galaxies could have hydrogen in them - just enough to detect, but not so much that they'd start forming stars (more gas => more star formation... usually). Now, if there are only a very few such objects, then perhaps they don't really matter much in the grand scheme of things. But if we can prove that even one exists, then that allows the possibility that there could be many more, potentially solving the missing satellite problem.

Proving the existence of even one dark galaxy turns out to be darn tricky. Which leads us on to the object that got me into radio astronomy in the first place : VIRGOHI21. It's not too much of a stretch to say that if this gif hadn't been shown at my PhD interview, I probably would have ended up doing something completely different*. I stole this one off Robert Minchin again. Poor Robert... allow me to compensate by saying, "go and read Robert's awesome blog !"

This is just one particularly famous example. During my PhD I discovered 8 other objects that might be sort-of similar to this one, but we don't have such good data for these yet. Various other candidates have been proposed over the years, but none have ever quite satisfied everyone.

VIRGOHI21 is in the center, linked by a stream to the nearby galaxy NGC 4254 (the biggest, brightest blob). This is the hydrogen "data cube" for the region - for more on these, have a look here, here and here - well, throughout the whole blog, really.
Robert has a detailed post about this object, so I'll just give a short summary. Basically, it's a cloud of hydrogen in the Virgo Cluster that's rotating too fast and seems to have interacted with a the spiral galaxy NGC 4254, with a long stream linking the two. It looks as though the hydrogen cloud be a dark galaxy that's pulled some of the gas out of NGC 4254... but the interaction is the problem. It's possible that the apparent rotating "dark galaxy" was formed when some other object pulled the gas out of NGC 4254, creating the illusion of a stable, rotating cloud of gas.

And that's the crux of the matter. It's easy (ish) to prove that a gas cloud is rotating too fast and would require dark matter to be stable. It's far more difficult to prove that it actually is stable, and not just tearing itself apart.

3) Jeez Louise, get on with it and tell me about the new results already !

Which bring us on, at last, to the latest awesome press release, concerning an object called the Smith Cloud. This is an object which is interacting with our own Milky Way galaxy called a high velocity cloud - simply because it's gas that's moving more quickly than gas in the disc at a similar position on the sky.

Lots of other HVCs are known, and there are almost as many theories as to what they are - maybe just gas thrown outside the galaxy by supernovae, though for the Smith Cloud this doesn't seem very likely... it's too massive. As the authors of the study say, it would take the power of 1,000 supernovae to eject it (you know you've got something cool when you cut out the bit about one thousand exploding stars from your press release). Or perhaps they're torn off by other interacting galaxies, or maybe they're primordial gas that's condensing from the intergalactic medium. Some of course, might be dark galaxies. In all likelihood, different clouds are probably formed in different ways.

From the official press release : "If it were visible with the naked eye, 
the Smith Cloud would cover almost as much sky as the constellation Orion."
The idea that (some) HVCs could be dark galaxies is by no means new. The difficulty is proving it. In the current paper, the authors describe the result of simulations of the Smith Cloud as it orbits our galaxy. From observations, they've been able to constrain its orbit (how well, I'm not sure) and find that it passed through the disc of our galaxy about 70 million years ago. They've gone to some lengths to test lots of different models, with and without dark matter, using different gas densities, to see what should happen to a cloud this massive as it punches through the Milky Way's own gas disc.

What they find is that without dark matter, the gas density needs to be very high for anything to survive that even remotely resembles the cloud we actually see - and then it would end up having more gas than we observe. With dark matter, however, they're able to reproduce something that not only looks quite remarkably similar to the observations, but more quantitatively, also has the correct mass and density of gas. Here's their figure :

Contours indicate the dark matter.
Impressive stuff, especially since they have the comparison simulations without dark matter that just don't work. As far as I'm aware, the paper hasn't been accepted for publication yet, but it looks extremely interesting to me. What would be really exciting about this is that it implies that at least some other HVCs could also be dark galaxies. That would really overturn a lot of current ideas and could, potentially, totally knock the missing satellite problem on the head.

But let's be cautious, and remember VIRGOHI21. When this object was announced to the world, the controversy was intense, and the debate more than a little heated - in the end, few people still think it's a likely dark galaxy candidate. To me, it looked like a very plausible candidate initially, but as more observations were completed and more simulations run, it looked less and less plausible.

I shall certainly be awaiting the final version of the Smith Cloud paper with baited breath (or in reality, with a nice cup of tea). The devil's in the details, of course... and it's always possible that this object is special, and not really representative of HVCs in general (it is, after all, a lot more massive than most of them). The authors themselves make no claims in their paper for anything as grandiose as solving the missing satellites problem - even if it is a dark galaxy, it doesn't mean there are more out there. Ultimately, though, if this object fits the bill, then it certainly makes dark galaxies look like a perfectly valid way to solve the problem.

My biggest concern is a philosophical one : the authors have found a model that works, but that doesn't mean there isn't another model they haven't considered. Such things have been known to happen in the past. Also, I'm not an expert on simulations, so I can't tell if you what they've done is sensible or bonkers. Actually they don't give very many details about the simulation setup, or much description of exactly what happens during the simulation. Some of those details could be very important.

For example, how high does the gas density get when the cloud and disc collide - high enough for star formation ? Would the simulation have allowed star formation to happen, or was it turned off to save CPU hours ? Or does the complex hydrodynamics actually make the gas density decrease in some way ? The simulation reproduces the observed cloud, yes, but that's all we're shown. It would certainly be nice to watch how the cloud evolves.

Make no mistake, I'm excited about this paper. But I also don't want to leave readers with a feeling that any minute now someone will reach an exciting or uninteresting but definitive conclusion - science isn't like that. It's a process - people have been observing the Smith Cloud for years, and it's very unlikely we'll get a decisive result anytime soon, if we ever get one at all.

So, what would it take to prove a dark galaxy, definitively ? It would be difficult, but not impossible. The gas would have to show all the features of ordered, stable rotation we see in normal galaxies. It would have show no any signs of interacting with another object - otherwise there could always be some doubt that it was formed by the interaction, somehow. It would have to be extremely isolated, to avoid any suggestion that it formed by an interaction in the past. The Smith Cloud research is very exciting, but the platonic ideal of dark galaxies has yet to be found. Until then, the hunt goes on.

Saturday 24 May 2014

D U N E : A Pug's Tale

Ah, Dune... a novel of such epic proportions and complexity that it's probably unfilmable. Not that that's stopped people from trying, however.

"They tried and failed ?"
"No, they tried and no-one liked it very much."

Unless you read the book, David Lynch's 1984 movie is, at best, a broken masterpiece. Rich, dark and with a soundtrack to match, it also just doesn't make any sense. And why should it ? About half the book consists of what characters think other characters are thinking - and what they think they're thinking isn't always what they're really thinking, I think. You can't really film that.

However, it adds at least one detail to the Dune story that puts a rather different slant on the whole shebang. I'm talking, of course, about Paul Atreides' pet pug. Knowing pugs, this alters the tone of the entire movie.

Dune is not hard science fiction, but pugs are pretty consistent with the established universe. Like Paul's tiny Bible*, pugs are small and therefore well-suited to space travel. They can withstand high accelerations due to their natural crumple zone. And, although set at least ten thousand years in the future, the survival of the breed would be assured by the Bene Gesserit sisterhood's enduring eugenics program (were they trying to breed a Kwisatz Haderpug ? we'll never know).

*This always bothered me. In the Duniverse, huge space fleets aren't a problem (though they are expensive). Compressing an 1800 page book into something the size of your thumb doesn't seem like much of a saving.

One may further infer that the survival of the pugs was guaranteed by the Buterlian Jihad - the war against the thinking machines. Because if there's one thing in the Universe that's definitely not a thinking machine, it's a pug.

Paul is clearly quite attached to little Scruffles, because he took him with him from the safe (though somewhat damp) environment of Castle Caladan to the desert furnace of Arakis. What, then, was life like in the Atreides household during Paul's youth ?

We will have to assume that Scruffles was much like any other pug. And that means that just occasionally, Duke Leto would have raged around the castle complaining that everything was covered in poop. Lady Jessica would have forbidden Paul from learning the Weirding Way "until all that *!"£@ing pug hair is cleaned up, young man !". Thufir Hawat would likely have complained that even Mentats can't concentrate "with that ugly little runt barking at leaves all day."

There is, however, something very wrong with the scene where the Atreides family leave Caladan to board the Guild heighliner, and it's that the family pug doesn't do anything. In the movie I can only assume this was done by lacing Scruffles' food with horse tranquillisers. In the narrative, I expect Paul  (or possibly Gurney Halleck) spent his last few hours on Caladan chasing his beloved pug around the castle grounds. Because if God created Arakis to train the faithful, then surely God created pugs to train the patient.

We don't see the pug again until the Harkkonen attack. In the absence of Sigourney Weaver - although honestly even if she were available it would be a tough choice - there's only one man Paul trusts to guard his childhood pet : Patrick Stewart, aka Gurney Halleck, weapons master of the Atreides. And thus do we see Patrick Stewart charging into battle, blaster in one hand and pug in the other.

We don't really know what happens to Scruffles after this. However, pugs are naturally suited even to avoiding the mighty Shai-Hulud, since they're lightweight and blessed with about as much sense of rhythm as William Shatner. We can only infer that the furry little idiot survived through many adventures....

I don't know who made this image but I'd like to thank them.
... because we do know the pug survives. Right at the end of the film, when Paul has brought the empire to its knees and freed the Fremen from the Harkonnen yoke, we can just about see an attendant doing their best to stop little Scruffles from interrupting Paul during his big speech.

Is it definitely the same pug ? I don't know, but how many pugs do you think there are on Arakis ?

Tuesday 20 May 2014

Ours Is Bigger, Probably

Pretty soon, Europe will be blasting the heck out of a Chilean mountain so that they can build a great big telescope there. This extremely large telescope will be so large, in fact, that they'll call it the Extremely Large Telescope. At 40m across its extreme largeness is beyond dispute. However, some articles claim it will be the world's largest telescope, so here's a friendly reminder that (despite being 40m wide) compared with some other facilities, this will still only be an Extremely Little Telescope.

Nonetheless, other names considered included, "The Really Quite Enormous Telescope",
the "Too Big to Fail Telescope", and, of course, the "Telescope of Devastation".
Because 300m > 40m. I've counted.
Now, obviously, radio telescopes and optical telescopes  work very differently, and the challenge of building a 40m diameter mirror is quite different to building a 300m diameter metal bowl. But a telescope is a telescope, and calling the ELT the world's largest is a bit like calling Snowdon the tallest mountain - sure it is, in Wales. Omitting that qualifier makes the statement.... well, wrong.

The ELT will be the world's largest optical telescope. And it will be mindbogglingly awesome. But the more general question of "which is the world's largest telescope ?" is slightly more tricky - and much more interesting - to answer.

OK, Hubble isn't a giant telescope, but let's face it, it's pretty darn neat.
Sphinx model came from here. Anyone who responds with claims about mystical pyramid energy will be given a stern glare.

The diagram above shows some notable present and planned classical telescopes - big reflective (single-dish) mirrors that focus light onto a receiver. Several other very large optical telescopes are also planned (e.g. the Giant Magellan Telescope, which will consist of seven massive mirrors stuck together, and the aptly-named Thirty Metre Telescope, the American rival to Europe's ELT). There are also several other large ground-based telescopes already operating (e.g. the Very Large Telescope) and while JWST will be the successor to Hubble, it's worth pointing out that the now inoperable Herschel is currently the largest single mirror* ever flown in space at 3.5m diameter.

* Arguably. Keep reading.

Many radio telescopes work in the same way as optical telescopes, it's just that the light they collect is a at a longer wavelength. So there's no need to make their mirrors smooth and shiny - they don't care about visible (or optical) light, and the radio waves aren't affected by small bumps in the surface. That makes it quite a lot easier to build really enormous radio receivers.

In fact, the Arecibo reflector is pretty much transparent to visible light. To save weight (and, interestingly, also to let enough light through so that the plants underneath don't die), each metal panel has lots of little holes, and you can easily see through it from underneath (just like the protective grille in a microwave oven, the radio waves are too big to pass through the holes).

See more.
And that raises an important question - does the reflecting surface have to be solid and continuous ? If you insist that the surface has to be like, watertight - then the world's largest telescope is probably the 10.4m GTC in La Palma. And by that rather contrived definition, then yes, the ELT will indeed be the world's largest telescope.

A more reasonable interpretation would be that the surface must be continuous at the wavelength it's observing. In that sense, Arecibo is a clear winner, and has been for over 50 years. But all glory is fleeting, and in a few years time the massive FAST will dwarf even Arecibo. This 500m behemoth will have the extra complication of a deformable dish - thousands of cables will pull the spherical reflector into a paraboloid, allowing it to point at different parts of the sky. Whether this crazy scheme will actually work remains to be seen.

And yet, by taking a few more liberties, even FAST won't be the largest telescope. If you don't mind your reflector having some bloomin' great gaps in it, then the remarkably obscure Russian RATAN-600 claims the prize. This crazy instrument is almost like what you'd get if you built Arecibo without bothering to find a sinkhole for the reflector - basically it's a huge ring of reflectors, 576m across, which focus the radio waves onto a central receiver.

My professional opinion is that it's freakin' weird.
But we can't stop there. If we're allowing gaps, we may as well allow arrays of telescopes and not just single dishes. Then we can get very large telescopes indeed. The 27 antennas of the so-called Very Large Array form a telescope up to 22 miles (35 km) across using a technique called interferometry, and it's pretty common to connect multiple telescopes spread across the entire planet*. And yet those wonderful, crazy Russian scientists refuse to let a little thing like the size of the planet stand in their way...

* There's a price to be paid, of course - you can use multiple telescopes to get very high resolution, but you lose sensitivity by having so much empty space (especially to low density material).

I suppose it is fairly large.
Radio Astron takes interferometry to extremes. This is a 10m diameter Russian radio telescope... iinnnn sppppaaaaacce ! 10m is too large to launch a conventional reflector, so, like a giant umbrella, it was folded up during launch and unfolded once in space. Working in conjunction with ground-based telescopes, it can effectively form part of a telescope 350,000 km across. Given the existence of a telescope larger than the Earth, the question of the "world's largest telescope" seems a tad petty.

Because the world is not enough.
Then again, up until now we've been biased by assuming that telescopes have to collect light emitted by distant objects. This isn't always the case. Neutrinos are particles (so not even part of the EM spectrum at all) of almost zero mass that almost never interact with anything - most of them pass straight through the Earth unhindered. Building a camera to detect them isn't really an option. So neutrino telescopes work in a completely different way : they detect the light emitted on the (very) rare occasions when a neutrino does happen to interact with, say, a water molecule.

Yes, that's a boat. Yes, there was a missed opportunity for a Bond movie here.
The spheres house the sensors that will detect the light emitted by neutrinos colliding with water molecules (when the entire tank is filled). 
The Super-Kamiokande neutrino telescope seen above holds 50,000 cubic metres of water in a cylindrical tank about 40m tall and 40m across. For more "normal" telescopes, the area of the reflector determines how many photons of light you can detect. For neutrino telescopes, it's volume that matters, not area. Which makes them fundamentally different, so we can't really compare them to classical telescopes. That would be completely unfair, so let's do it anyway.

If we emptied all the water of Super-Kamiokande into Arecibo, it would only fill the reflector to a depth of about 8m. Assuming it somehow didn't just drain away, which it would.

Critics agreed that budget cuts to the remake of Goldenye were a mistake.
So, can radio astronomers point at laugh at all the other astronomer's puny instruments ? Possibly, but it would be inadvisable. For one thing, there's another, much bigger neutrino telescope : IceCube. Instead of building an enormous tank filled with liquid water, this is an even more audacious project using the ice of Antarctica. Buried over a mile below the surface are 86 boreholes each filled with 60 detectors, enclosing an entire cubic kilometre (1 billion cubic metres) of ice. That's a mass of 900 million tonnes - easily enough to crush Arecibo a like a puny little bug.

Still, although its mass is much, much greater, a mere cubic kilometre is paltry compared to the 350,000 km baseline of Radio Astron. For now, then, radio astronomers can laugh at other scientists as long as they're very very careful about it. Neutrino telescopes may have the most physical mass, but radio telescopes have the biggest reflectors and the largest diameters. They're likely to retain the prize for biggest mirrors for the foreseeable future, but another exotic type of astronomy has the crown for largest diameter firmly in its sights.

Gravitational waves are nothing less than ripples in space itself, produced by any moving object. These distortions should be detectable using a system of lasers and mirrors - roughly speaking, if a wave passes through, the length between the mirrors changes, altering the time it takes the laser to travel along its path. The path of the laser beam needs to be as long as possible since the changes are damnably small. Current detectors have laser paths a few kilometres in length.

LIGO detector in Louisiana.
Much to the disgust of other astronomers, none of these facilities have ever detected anything. In fact, it's the radio astronomers - yet again - who can point and laugh, since the only convincing evidence for gravitational waves comes indirectly from observations of binary pulsars (first measured, by the way, at Arecibo), whose orbital decay is in perfect agreement with the predictions of general relativity assuming they're emitting gravitational waves.

However, unless something is staggeringly, astonishingly wrong with the theory, given enough time it will be possible to directly detect gravitational waves. Fed up of other astronomers calling them "gravy waves", the GW community has hatched a foolproof plan to massively increase sensitivity and make absolutely sure if the pesky things are real or not. And that involves another giant space telescope - one that makes even Radio Astron look just a little bit pathetic. If it works.

The LISA gravitational wave observatory will use lasers to create a telescope ~5 million km across. This is a no-lose situation. If it works, we'll have a whole other way of studying the Universe. If it doesn't - I mean if the telescope itself functions correctly but fails to detect anything - it will be the greatest non-detection since the aether, and a scientific revolution is sure to follow. And, best of all, until it's built the rest of the astronomy community can continue to poke fun at the gravy wavers with all their oh-so-interesting noise measurements.

So, there you have it. The world's largest telescope is a tricky question to answer, and maybe even irrelevant. Already we have telescopes as big as or even larger than the world, in some sense, with more on the way. And that's just artificial telescopes. Arguably, even more exotic techniques like pulsar timing arrays and gravitational lensing use entire galaxies to form natural telescopes. But that's another story.

Sunday 18 May 2014

Hercules, Probes, a Talking Moose... it's funnier without the commas

Sometimes I don't bother posting anything because I'm lazy. Well, Game of Thrones isn't going to watch itself, is it ? You're goddamn right it isn't. Other times, as lately, I don't post anything because I don't have time to.

First, my parents visited. Hurrah ! I had all of two - count 'em TWO visitors in two years at Arecibo. And that's an average. "An average ?" Yes, an average - one person came specifically to visit me (although that was tacked on to an observing trip somewhere else in America-land), one person I'd never met before but was planning to work on AGES, so doesn't really count, and the other I've only met at conferences, and - surprise surprise - was there mainly for a conference, so also doesn't really count.

The excuses, "it's too far", "I can't drive", "it's too expensive" are all perfectly valid for Puerto Rico, but don't pass muster with Prague. And since another visitor is lined up for the end of the month, my visitor rate in six months has already exceeded that from 2.5 years in the Caribbean.

With my parents, we mostly explored things I've already seen before, so I won't repeat them. Perhaps the highlight was wandering through the Wallenstein Gardens, which I had not done before, and discovering a pure white peacock. It was displaying the moment we entered, but didn't do it again. My mission is now to keep going back there and get a photograph of the peacock in full "look at me I'm a giant Elizabethan ruff" mode.

There were also regular, crappy peacocks.
The other noteworthy thing about the gardens is that Prague has again failed at dragons. This time it's a statue of Hercules fighting a small though ferocious-looking dragon. Nothing wrong with that, except that it's abundantly clear that Hercules has really got the upper hand. Specifically, he looks like he's about the knock the poor thing's head off. It doesn't stand a chance.

We also visited the senate halls, which are resplendent and made a nice change from all the churches, and Petrin hill, because you can't really go wrong with Petrin hill. This time we went inside ┼átef├ínik Observatory, which has quite a nice public exhibition. It even has a little panel about Arecibo, and I learned that "Sondy" means "probes" in Czech. As in "meziplanetarni sondy" - interplanetary probes. Which I think must be very funny if you're a planetary scientist and your name is Sondy.

The week after that I was back in Cardiff giving a seminar. For seminars these days I'm trying to up the ante on the theatrics, which means a lot of preparation time - pretty much every spare evening beforehand. The title is, "The Arecibo Galaxy Environment Survey : Science with a Lonely Smurf and a Talking Moose", and it includes a physical data cube and three properly 3D movies. As in the old-school 3D glasses. None of this fancy full-colour nonsense.

I've been wanting to do this for years.

Full colour versions can be seen in this video.

There's not a lot else to say about Cardiff other than that May is bluebell season, which doesn't sound so impressive unless you realise that they grow by the million. Add in a large friendly dog and we have ourselves a winner.

We also went to the sand dunes at Merthyr Mawr - yes, Wales has sand dunes, believe it or not. The Czech Republic does not have any coastline, of course, despite some laudably hair-brained schemes to overcome this particular difficulty. The problem is I've been spoiled by the ridiculous warm waters of the Caribbean, and Merthyr Mawr is more on an estuary than the actual sea. You can see the other side, for heaven's sake. Which left the whole experience somewhat unsatisfying.

Once back in Prague I gave the same seminar in Ondrejov, where the other half of the institute is located. The 3D movies were just as popular, though overall the reaction was a lot more muted than in Cardiff. Several people fell asleep. Well, I was battling a cold at the time, and it's very difficult to speak with any enthusiasm when you can't actually hear even how loud you're speaking. It probably also doesn't help that the galaxy group is a lot smaller in Prague and there were very few native English speakers in the room.

This done, I pointedly spent the rest of the week watching Netflix and DVD box sets, because I'm sick of doing science in the evenings. I don't like ad-libbing in front of large audiences, hence I end up repeating the entire 50-minute talk to an empty room again and again and again.

Anyway, that's how I spent the last two weeks. No more seminars in the foreseeable future - the next conference is in Vienna, but mercifully that's a poster presentation. And a jolly good thing too, because short of buying an Occulus Rift, or baking everyone a cake, I have absolutely no idea how I raise the stakes for the next talk. Fireworks ? Cheerleaders ?* Random pictures of cats ? Further research is required.

* If nothing else, a letter-by-letter chant for "extragalactic neutral atomic hydrogen" would probably obviate the need for an actual talk.