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 in the 1960s. With colleagues he began by looking for evidence of excitation of the normal modes of the Earth by very low frequency gravitational waves [142]. Efforts to detect gravitational waves via the excitation of Earth’s normal modes was also pursued by Weiss and Block [314]. 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 [310Jump To The Next Citation Point, 311]. 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 10–16 over millisecond timescales [310], their sensitivity was far away from what was predicted to be required theoretically. Development of Weber bar type detectors continued with significant emphasis on cooling to reduce the noise levels, although work in this area is now subsiding with efforts continuing on Auriga [88], Nautilus [239], MiniGRAIL [233, 158] and Mário Schenberg [159, 70]. In around 2003, the sensitivity of km-scale interferometric gravitational-wave detectors began to surpass the peak sensitivity of these cryogenic bar detectors (≃ 10–21) and, for example, the LIGO detectors reached their design sensitivities at almost all frequencies by 2005 (peak sensitivity ≃ 2 × 10–23 at ≃ 200 Hz) [315Jump To The Next Citation Point], see Section 6.1 for more information on science runs of the recent generation of detectors. In addition to gaining better strain sensitivities, interferometric detectors have a marked advantage over resonant bars by being sensitive to a broader range of frequencies, whereas resonant bar are inherently sensitive only to signals that have significant spectral energy in a narrow band around their resonant frequency. The concept and design of gravitational-wave detectors based on laser interferometers will be introduced in the following Section 3.2.
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