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 the strongest astrophysical sources, such as supernova explosions or collisions between neutron stars and black holes – and therefore extremely difficult to detect.

When a gravitational wave reaches the Earth it should expand space in one direction and contract it in the perpendicular direction. VIRGO relies on laser interferometers to detect the effect of these changes on pairs of widely separated test masses. The arms of the interferometer are 3 kilometres long and they are at right angles to each other (see figure). Changes in the interference pattern produced by two laser beams reflected off mirrors on the test masses will reflect changes in the length of the arms.

However, the changes caused by any gravitational wave will be extremely small - only about 10-21m - so the detector must be very sensitive. In particular, it must be seismically isolated from its environment, and it must use an ultrahigh vacuum cavity and the best possible mirrors and other optical components.

Virgo has passed preliminary tests and will begin to record data in a few months. It will join a global network of gravitational-wave detectors that already includes the two LIGO detectors (which are both 4 km long) in the US, GEO600 in Germany (600 m) and TAMA in Japan (300 m).