Sat, 12 Aug 2006

LIGO

LIGO stands for Laser Interferometer Gravitational-Wave Observatory. There are 5 such observatories in the world; two are in the United States, and one of them is about a half hour away out into the wilds of Hanford. It is open to the public on the second Saturday of the month, so off I went to find out what a gravitational wave is.

If I understand it correctly, a gravitational wave is a vibration that moves through the stuff of space, where space isn't just nothingness, but something that distorts as vibrations travel through it. The waves are caused by events, like two stars whirling around each other many times per second until they fall into each and merge. We saw a short video which showed scientists giving their imitation of what such an event sounds like when the waves are translated as sound; one scientist made a little "whooop" sound, another described it as more of a chirp. I guess that was mainly to show that scientists can be a lovable, quirky bunch.

After the video, we went out into the hot afternoon sun to take a look at two big pipes that run for 4 kilometers at right angles to each other. The basic idea is that a laser is split at the origin, and the two beams travel the length of the pipes, bounce off mirrors at the ends of the pipes, and return back to the origin. Because the mirrors are at precisely the same distance from the point at which the beam was split, the returning lightwaves cancel each other out when they rejoin. An elaborate shock-absorbing system keeps the lasers and mirrors as still as possible, the equipment is temperature controlled, and the pipes are airless; I think this is to keep any particles from interfering with the light traveling through them.

When a gravitational wave passes through, space gets elongated in one direction and squeezed in the other. So if a gravitational wave moves through the LIGO system, one mirror will be moved ever so slightly away from the origin, and the other mirror will move ever so slightly towards the origin; then when the beams rejoin each other these differences can be detected.

A lot of numbers were floated during this explanation: the number of cycles per second that any one of those events must generate relative to the number of light years in distance to us for the generated wave to be detected; the part of a meter that the mirrors move when the long-awaited wave finally comes through (it's some pretty negative power of ten), the size of a bump on a neutron star that will create the necessary wobble for detectable waves (I think it was a centimeter), etc, etc. Since I have a hard time with numbers, I probably forgot these interesting tidbits within a few minutes of hearing them (I didn't even process the negative power of 10, my brain briefly shuts down whenever someone speaks in scientific notation), but the take-home message was that the system is incredibly sensitive. We weren't allowed anywhere near the equipment because of this. They also have to filter out noise in the data created by work going on out at the Hanford Nuclear Reservation, passing trucks, wind storms, etc. But perhaps their main problem is that the events they want to detect don't happen every day; if a wave happens to pass through town, though, they're hoping to be ready for it.

posted at: 19:25 | permanent link to this entry