|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.|
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.
* 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).
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.|
* 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.|
|Because the world is not enough.|
|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).
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.|
|LIKE A BUG !!!|
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.|
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.