VIRGO is a Michelson laser interferometer with two orthogonal arms each 3 kilometers long. A beam splitter divides the incident laser beam into two equal components sent into the two arms of the interferometer. In each arm, a two mirrors Fabry-Perot resonant cavity extends the optical length from 3 to about 100 kilometers because of multiple reflections and therefore amplifies the tiny distance variation caused by a gravitational wave. The two beams of laser light coming from the two arms, are recombined out of phase on a detector so that, in principle, no light reaches the detector. The variation of the optical path’s length, caused by the changing distance between the mirrors, produces a very small phase shift between the beams and, thus, a variation of the luminous intensity, which is proportional to the wave’s amplitude.

However, in this scheme, a large fraction of the light is sent back toward the laser. In order to further increase the power, this light is sent back to the interferometer by a recycling mirror, in phase with the incident beam thus increasing the light power that can reach several tens of kilowatts in the Fabry-Perot resonant cavities. A high light power is important because it allows to improve the sensitivity of the interferometer. With these resonant cavities coupled together, the interferometer can be seen as a giant light trap. If the optics would be perfect and the mirrors perfectly stable, no light should normally reach the detector except when the interferometer plane is crossed by a gravitational wave. The quality and stability of the optics represent therefore one of the major challenges of the interferometer.

VIRGO is sensitive to gravitational waves in a wide frequency range, from 10 to 10,000 Hz. This should allow the detection of gravitational radiation caused by the coalescence of binary systems (stars or black holes), pulsars and those produced by supernovae in the milky way and in outer galaxies, for instance from the Virgo cluster, hence the name of the project.

VIRGO runs day and night, listening to all signals that arrive at any time from any part of the universe. The data cominig from the interferometer as well as the ancillary data necessary to its control (4Mbytes/s) are subjected to a preliminary analysis for quality check and a quick detection of potentially interesting anomalous signals. The data is then put at the disposal of the scientific collaboration, for a deeper analysis.