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 [37]. 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 USA,
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
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 [74], Padua [77], Louisiana [9] and Perth (Western Australia) [47] are achieving sensitivity levels better than
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.