The basic idea of interferometry is that you have mirrors suspended at right angles to each other that aim laser beams through a slit to create an interference pattern. If those mirrors are shifted even a tiny tiny bit, the interference pattern will shift as well. The origins of interferometry date back to the late nineteenth century and two gentlemen named Albert Michelson (left) and Edward Morley (right). This was back in the day before the invariance of the speed of light was known. Michelson and Morley were trying to prove using this setup of mirrors and light beams that the speed of light is slightly different in different directions, due to a so-called "aether wind." They were disappointed on this front. Instead they proved the opposite--that light moves at a specific speed and only that speed. The way this gizmo works is the light is sent from a simple laser through a beam splitter that sends half of the light in one direction and half in the other (Figure 1). Then it is reflected straight back from the two masses and picked up by a photodetector (Figures 2 and 3). The wave nature of light states that the two beams should interfere in a predictable way based on the distances they have traveled. Michelson and Morley predicted that if there was an aether wind coming from, say, the right side of the setup the interference pattern would be shifted slightly and thus the light would appear to change its velocity. The interference pattern they saw, however, was perfectly predictable and unchanged in every direction. Which, in itself, was still a monumental discovery. Incidentally, for his work in determining the absolute nature of the speed of light Albert Michelson (the distinguished gentleman on the left) went on to become the first American to receive the Nobel Prize in Physics, in 1907. I wonder what happened to Morley?

The conclusive proof of general relativity that physicists are still looking for is the detection of the predicted gravitational waves. Theoretically emitted from highly massive bodies like black holes and neutron stars, the disturbances predicted by these waves are so slight that highly sensitive instruments must be used. The lasers and mirrors of interferometry turn out to be the best hope for detection.

LIGO has two sites, one in Livingston, Louisiana (below), and one in Hanford, Washington. The image to the right shows the Hanford facility in all the desolation Eastern Washington State has to offer. The purpose of having two separate detectors is to confirm results. If only LIGO Hanford detects what might be a gravitational wave, it's not. Both detectors must detect identical signals at the same time for the possibility to exist that the signal resulted from a gravitational wave. They are so faint and there are so many other factors that can cause slight vibrations that the researchers on this project want to make sure that they rule out all other possibilities before claiming that they have finally detected the elusive gravitational wave.

Both LIGO detectors are giant laser interferometers. The two arms that make up the interferometer are each four kilometers long. LIGO Hanford actually has two interferometers in one because they built a second detector with half the arm length right inside the main interferometer. This is helpful because the length of the arm is directly proportional to the amplitude of the received signal. Therefore not only must the signals at LIGO Livingston and LIGO Hanford match but the second signal at Hanford must match at half the amplitude of the other two.

No such luck. LIGO uses ordinary infrared lasers with a wavelength around 1000 nanometers, or 10^-12 meters. You'd think that to detect a distortion on the order of 1/1000 fermi (10^-18 meters) that you would need some sort of gamma ray laser with a miniscule wavelength. Not so. First of all, a gamma ray laser is rather dangerous because not only can it go through anything, it can kill you quite easily. Moreover, we can determine the wavelength of the infrared laser very precisely so that detecting changes in its interference pattern is not so difficult.

The LIGO-Livingston site actually has a gift shop selling t-shirts, Slinkies, books, hats, and my personal favorite, hand-thrown stoneware mugs with the logo "Laissez les bonnes ondes rouler!" (Let the good waves roll!).

Why yes, in fact, there are. There is some collaboration going on between all ground-based laser interferometry groups for the same reason that LIGO has two sites. If all of the ground-based projects detect the same signals then there is that much more certainty in their results. Here is a list of the other projects and their specs:

VIRGO--VIRGO is a 3 kilometer interferometer located near Pisa, Italy and sponsored by Italy and France.

GEO--Located just outside Hannover, Germany, GEO is run by Germany and Britain and has arms 600 meters in length.

TAMA--A prototype project to improve methods for larger interferometers, TAMA is 300 kilometers of interferometry wizardry in Japan.