50 60 v 1 1 6 M ay 2 00 0 LASER INTERFEROMETER GRAVITATIONAL WAVE DETECTORS

I give a brief introduction on gravitational wave laser interferometers, possible detectable sources from the ground and noise in the detectors. Gravitational wave research started in the early 1960's thanks to the pioneering work of Weber 1. Since that time, there has been an ongoing research effort to develop detectors of sufficient sensitivity to allow the detection of waves from astrophysical sources. The effect of a gravitational wave of amplitude h is to produce a strain in space given by ∆L/L = h/2. The magnitude of the problem facing researchers in this area can be appreciated when one realizes that theorists predict that for a reasonable 'event rate' one should aim for a strain sensitivity of 10 −21 to 10 −22. This means that if we were monitoring the separation of two free test masses of one meter apart, the change in separation would be 10 −21 m. Such figures should give the reader a feeling for the enormity of the experimental challenge facing those developing gravitational wave detectors. The different types of detectors can be classified into two main categories: those using laser interferometers with very long arms 2 and those using resonant metallic masses that may be cooled to ultra-low temperatures 3,4. The first have the ability to measure the gravitational wave induced strain in a broad frequency band (expected to range from 50 Hz up to perhaps 5 kHz), while the latter measure the gravitational wave Fourier components around the bar's resonant frequency (usually near 1 kHz), with a bandwidth of order a few Hz. For resonant mass antennas, the fundamental limitation to their sensitivity comes from the thermal motion of the atoms that can be reduced by cooling them to temperatures of order 50 mK; existing antennas can then achieve a sensitivity 10 −19 to 10 −20. In the early 1970's emerged the idea that laser interferometers might have a better chance of detecting gravitational waves. Detailed studies were carried out by Forward and his group 5 and Weiss 6. Since then, several groups have developed prototype interferometric detectors at Glasgow (10 meter Fabry-Perot), Garching (30 m delay line), MIT, Caltech (40 m Fabry-Perot) and Tokyo. Projects to build long arm laser interferometers have also been funded. The construction of the detectors has already been started and is rapidly pro-1 gressing. There is the LIGO project 7 to build two 4 km detectors in USA, the …