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Top view of Virgo © Nicola Baldocchi/EGO

Virgo is a laser interferometer with two perpendicular, 3km-long arms: its purpose is detecting gravitational waves from astrophysical sources It is hosted at the European Gravitational Observatory, in the countryside near Pisa, in Italy.

Inside Virgo the light beam produced by a laser source is split into two by a semi-reflective mirror known as the ‘beam splitter’. The two resulting beams travel along separate perpendicular paths: the ‘arms’.
A mirror placed at the end of each of the 3-km long arms reflects the light back so that the two beams return to the beam splitter. By passing again through the beam-splitter, the two returning beams are overlapped and interfere. The interference pattern depends on the difference in the travel time of the laser beams along the two arms.When there is no gravitational-wave signal, the two beams take the same amount of time to travel through the two equal arms. They recombine in what is known as ‘anti-phase’ at the output port of the detector, i.e. the beams cancel each other and the output port is ‘dark’: a photodetector, placed there, measures no light.

Instead, when a gravitational wave passes through the detector, it alternately stretches one arm while shrinking the other by a tiny amount. As a consequence,  light takes different amounts of time to travel the two arms. Thus, when the two beams recombine, they are no longer in perfect anti-phase: the output port is slightly  bright and the gravitational-wave signal is now detectable by the photodiode. The longer the arms of the detector, the bigger the change in their length caused by the passage of a gravitational wave. This is the reason why Virgo’s arms are three kilometer long.
Of course, realizing this operating principle with a real instrument is extremely complex and presents several unprecedented scientific and technological challenges, especially for increasing the signal and for reducing the noise.