When an electrical charge is accelerated in a straight line or moves around an arc of a circle, the charge radiates an electromagnetic wave, similarly, when a massive object is accelerated general relativity predicts that it will radiate a GRAVITATIONAL WAVE. Although there is no celestial object we can point to as one that does produce strong gravitational radiation, there is reason to expect that strong gravitational-wave pulses might be produced by binary neutron stars, for example. In the hope that gravitational waves might exist and have hitherto gone undetected, a number of workers have built a variety of detectors designed to pick up the waves if they exist. Most are similar to the first one, built by J. Weber of Maryland, USA. Basically they consist of a very large cylinder (often of aluminium), which is squeezed by the incoming gravitational radiation. This causes it to ring like a bell, and sensitive vibration detectors (or PRESSURE TRANSDUCERS) pick up these exceedingly weak vibrations which are then recorded.

A problem with this detector is that many spurious disturbances can affect the device. For example, earthquakes half the world away are easily detected by seismographs and seismometers and are also easily picked up by a gravitational-wave detector. Even road traffic vibrations can be significant in producing spurious signals. The most promising approach for eliminating ‘local’ disturbances is to use a number of detectors great distances apart. All pulses from each are recorded, and the recordings are then compared for events that are common to all the detectors. In this way, relatively local disturbances can be filtered out. Seismic disturbances travel much more slowly (sound speed) than gravitational waves (speed of light), so only genuine gravitational-wave disturbances should produce simultaneous signals at all the detectors. However, the extreme difficulty of making these measurements has meant that no gravitational wave or pulse has been unambiguously recorded.

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