Sunday, 7 September 2008

Think the LHC is big? Think again.

The LHC is a massive piece of machinery. With a circumference of 27km, it's the largest machine mankind has ever created and has had lots of press coverage on that detail alone. But it might not be holding on to that title for long.

Meet LISA. LISA stands for Laser Interferometer Space Antenna and is poised to dwarf the LHC when it's launched sometime in 2018. But before I tell you how big LISA is, I need to tell you what LISA does, so you can understand the need for such size.

What does LISA do, you ask? LISA, as the name implies, is an interferometer. Interferometers were made famous by two guys , Michelson and Morley, in the late 1800s when they attempted to find evidence for the luminiferous aether (they found none, and Michelson won the Nobel Prize for this and subsequent research). A classic interferometer is set up like in the picture to the right: A beam of light (like a laser) is intercepted by a half-silvered mirror, which splits the beam in two; half is reflected at a 90° angle and half passes through. Each half then contacts a regular mirror, and get sent back towards the half-silvered mirror, whereupon the beam that was originally reflected passes on through and the one that wasn't gets reflected at a 90° angle. The result is that both halves reach a detector at precisely the same time (since their paths were precisely the same length). The detector can determine how long it took each beam to arrive based on the interference pattern.

LISA uses such a setup to search for gravitational waves. Gravitational waves arrive as a result of Einstein's theory of general relativity. Massive objects warp the curvature of spacetime, and as such objects move around, they can cause the spacetime to ripple like water in a pond. These ripples are gravitational waves. If one could detect gravitational waves, then it would serve as further proof of Einstein's ideas.

How does one go about detecting gravitational waves? With an interferometer, of course! One consequence of gravitational waves is that they cause space time to ripple; to an observer, this would appear as a stretching and shrinking - an oscillation - of space. If a gravitational wave were to pass by an interferometer, the oscillation of space would cause one beam of the interferometer to take slightly longer to reach the detector than the other, resulting in a change in the interference pattern. A sufficiently precise interferometer, then, serves as a gravitationalwaveometer (yeah I made that word up).

Such a device already exists - LIGO, the Laser Interferometer Gravitational-Wave Observatory - consists of three laser interferometers in separate areas of the US (three are used to make sure changes in interference are caused by gravitational waves, which should be recorded by all three, rather than local disturbances). These devices are very large (2-4km in length for each arm of the device), but not LHC-big. And how have the results been so far? Well, lousy. You see, gravitational waves are small. Very very small. Theoretically, they should cause a change in the distance between the arms of the detectors by 10-17m, a fraction of the width of a proton. It is incredibly difficult to measure such a minuscule change, and so far, LIGO has not put out any conclusive data. Perhaps a more precise machine is needed. Or perhaps...a bigger one.

At the scale of 4km, gravitational waves cause a difference in 10-17m, but as the measured distance gets bigger, so does the discrepancy caused by gravitational waves. A bigger device would be able to detect gravitational waves much more easily. Enter LISA.

LISA is poised to be, not just the biggest laser interferometer ever built, but the largest, well, ANYTHING ever constructed. How big is LISA?

Each arm of the interferometer will be some 5 MILLION KILOMETERS long.
That's 5,000,000km.

That is HUGE.

Now, in the LHC's defence, the lasers will pass through empty space rather than down a long tunnel, so the actual physical parts of LISA are not 5 million km long, but still. The ends of LISA need to be placed in space with the utmost precision, and over such a distance, that is quite the task. That's not even factoring in solar winds, and light pressure, which can have a tangible cumulative effect at such distances, and can throw off the lasers.

But at 5 million km per side, LISA is one impressive machine, which might finally give us evidence for gravitational waves and prove Einstein right once again.

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