Detector

Virgo is a laser interferometer with two perpendicular, 3 km-long arms. Hosted at the European Gravitational Observatory, in the countryside near Pisa, in Italy. Its purpose is to detect gravitational waves from astrophysical sources.

Inside Virgo, the light beam produced by a laser is split in 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. As they pass back through the beam splitter, the two returning beams overlap and interfere with one another.

The dark fringe

This ‘interference pattern’ changes depending on the difference in the amount of time it takes for the laser beams to travel along the two arms. When there is no gravitational wave signal, the two beams should take the same amount of time to travel through the two equal arms. In this case, they recombine destructively at the output port of the detector. This means that the beams cancel one another out and so no light reaches the photodetector at the output port of the interferometer. This operating point is called the ‘dark fringe’ for the interferometer.

The effect of a gravitational wave

Instead, when a gravitational wave passes through the detector, it alternately stretches one arm while shrinking the other by a tiny amount (about one thousandth of the diameter of the proton). As a consequence, light takes different amounts of time to travel through the two arms. In this case, when the two beams recombine, they are no longer cancelling each other out and a small amount of light, proportional to the shift caused by the passage of the gravitational wave, is now detectable by the output-port photodetector.

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 the arms of Virgo are so long: three kilometers.Of course, actually putting this operating principle into practice with a real instrument is extremely complex and presents several scientific and technological challenges. Two of the most complex ones involve increasing the strength of the gravitational-wave signal and reducing the background noise in which it is hidden.