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Gravitational waves are predicted by General
Relativity from accelerated masses. Since gravity is the weakest
of the four forces detecting the waves is no small feat. A gravitational
wave travels in a plan perpendicular to its direction of propagation. As
the two dimensional wave passes through mass it does not affect it along
the axis of propagation while it simultaneously contracts space in one
of the orthogonal directions and expands it in the other.
The first experimental attempt to detect these waves was by Joseph Weber at Louisiana State University in 1966. Originally the idea was that a piezoelectric crystal could be used as the detector. The resultant currents from the compressions and expansions of the material could be used to determine the strength of the gravitational wave. However, it was found that the size of the crystal was not feasible and as such the idea modified to employ large masses of metal, gravitational antenna. As the wave passed through the mass an acoustic wave would be generated, this wave could be monitored to determine the strength of the wave. Though the use of the Weber Bar continues there is much debate over it ability to distinguish such subtle acoustic waves.
Joseph Weber and a "Weber Bar" or Gravitational Wave Antenna
The first theoretical observation of gravitational waves came from the work of J. Taylor and R. Hulse. Their observation of a binary pulsar system and its acceleration coincided with prediction from General Relativity for the emission of gravitational waves. The Laser Interferometer Gravitational Wave Observatory or LIGO had been on the drawing board for many years before construction began in Nov. of 1994. LIGO is a joint project funded by the National Science Foundation and overseen by both The California Institute of Technology and the Massachusetts Institute of Technology. LIGO is a "brute force observatory for gravitational wave, by building a large enough interferometer it is hoped that the subtle effects of gravitational waves can be observed. More importantly LIGO allows for the determination of the source in that the direction from which the came can de determined. This analytical ability stems from two factors, there are two spatially separated detectors and and each detector simultaneously performs two perpendicular measurements. There is one detector in Hanford Washington and another in Livingston Louisiana:
This $365+ million dollar project will allows for fundamentally new physics, observations of General Relativity in the strong field limit and a window into the universe in which the wave are not scattered during propagation. The interferometer works by bouncing a laser down two perpendicular 4 km arms so that the reflected beam destructively interferes with the incident beam following it. With this setup no light should end up at the photon detector unless a gravitational wave changes the dimensions mirrors (test masses) so that the wave do not destructive interfere completely.
Above is an image of Hanford's interferometer, note two added features. To increase the signal to noise it is optimal to have the wave interfere with as much light as possible. To accomplish this the light is stored by reflecting it back and forth many times. Also not the the test mass, essentially a 2 km interferometer built into the 4 km interferometer. This allows for observation of local anomalies because of the differences between how noise and signal respond to different length arms.
Using this technology LIGO hopes to achieve spatial sensitivity to 1 part in 1021 for a given 4 km arm...or sensitivity to 10-18 meters.
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