3.1 Initial detectors and their development

The earliest experiments in the field were ground based and were carried out by Joseph Weber of the University of Maryland about 30 years ago. With colleagues he began by looking for evidence of excitation of the normal modes of the earth by very low frequency gravitational waves [30]. Weber then moved on to look for tidal strains in aluminium bars which were at room temperature and were well isolated from ground vibrations and acoustic noise in the laboratory [104, 105].The bars were resonant at ≃ 1600 Hz, a frequency where the energy spectrum of the signals from collapsing stars was predicted to peak. Despite the fact that Weber observed coincident excitations of his detectors placed up to 1000 km apart, at a rate of approximately one event per day, his results were not substantiated by similar experiments carried out in several other laboratories in the U.S.A., Germany, Britain and Russia. It seems unlikely that Weber was observing gravitational wave signals because, although his detectors were very sensitive, being able to detect strains of the order of 10–15 over millisecond timescales, their sensitivity was far away from what was predicted to be required theoretically. Development of Weber bar type detectors has continued with the emphasis on cooling to reduce the noise levels; currently systems at the Universities of Rome [70], Padua [73], Louisiana [2] and Perth (Western Australia) [41] are achieving sensitivity levels better than 10–18 for millisecond pulses. At present (beginning of 2000) these are the most sensitive detectors of gravitational waves which are in operation. Bar detectors have a disadvantage, however, in that they are sensitive only to signals that have significant spectral energy in a narrow band around their resonant frequency. An alternative design of gravitational wave detector based on a laser interferometer, overcomes this limitation and is introduced in the following section.
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