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.

Lisa is an equilateral triangle with an identical spacecraft at each of the three vertices and sides measuring 5 million kilometers in length. Lisa will be launched into a solar orbit such that all three spacecraft have independent orbits but maintain their triangular orientation. The triangle of spacecraft will be located in the same plane as earth's orbit and the same distance from the sun as the earth, but about twenty degrees behind the earth. The plane of the equilateral triangle is set at an angle sixty degrees from the plane of earth's orbit.

Lisa is a giant interferometer. Each of the three spacecraft has two detectors on it (for precision measurements as well as insurance in case one fails).

Lisa's primary goal is, of course, to detect and study gravitational waves. Particularly those emitted by massive black holes in binary systems. Secondary to that goal, however, is a broad project to determine the numbers and distribution of compact binary systems in the Milky Way galaxy.

Well. First of all, earth's gravitational field, small though it may be, does offer slight disturbances and is constantly changing due to tides and weather systems. Secondly, there are different frequencies at which gravitational waves may be detected and one range, the one including massive black holes, is too low for terrestrial detectors like LIGO. Thus, according to Lisa's proponents, that little distance out into space is crucial. This nifty graph shows the frequency regions that both LISA and LIGO will cover.

Nope. Lisa is still on the launch pad. She won't actually go into space until approximately 2009. Much of the technology involved is still under development because such sophisticated instruments are required to detect the incredibly, astoundingly miniscule effects of gravitaitonal waves.