Physicists in Japan have built the world’s first underground gravitational-wave detector. Preliminary tests have shown that the reduced environmental noise in the underground site means that LISM – Laser Interferometer gravitational-wave Small observatory in a Mine — can perform as well as existing instruments.

Gravitational waves are ripples in the fabric of space-time that are produced when massive bodies accelerate through space. However, the waves are very weak – even for events as extreme as supernova explosions or collisions between neutron stars and black holes – and are therefore extremely difficult to detect.

Gravitational-wave interferometers are designed to detect these very weak waves by using lasers to monitor the movement of test masses placed at the ends of the perpendicular arms of the interferometer. The arms in LISM are only 20 metres long, which is relatively short compared with the 3 kilometre arms of the VIRGO detector in Italy, and the 4 kilometre arms of the two LIGO detectors in the US.

When a gravitational wave passes through the detector, it causes the distance between the test masses to increase in one direction and decrease in the other. However, the changes caused by any gravitational wave are extremely small – only about 10-21m – so the detector must be very sensitive. But extremely sensitive detectors are easily disturbed by environmental noise – such as seismic motion and temperature variations – in its surroundings.

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