Virgo

The Virgo interferometer is a large-scale scientific instrument near Pisa, Italy, for detecting gravitational waves. The detector is a Michelson interferometer, which can detect the minuscule length variations in its two 3 km (1.9 mi) arms induced by the passage of gravitational waves. The required precision is achieved using many systems to isolate it from the outside world, including keeping its mirrors and instrumentation in an ultra-high vacuum and suspending them using complex systems of pendula. Between its periodic observations, the detector is upgraded to increase its sensitivity. The observation runs are performed in collaboration with other similar detectors, including the two Laser Interferometer Gravitational-Wave Observatories (LIGO) in the United States and the Japanese Kamioka Gravitational Wave Detector (KAGRA), because cooperation between several detectors is crucial for detecting gravitational waves and pinpointing their origin. Virgo was conceived and built when gravitational waves were only a prediction of general relativity. The project, named after the Virgo galaxy cluster, was approved in 1992 and construction was completed in 2003. After several years without detection, Virgo was shut down in 2011 for the "Advanced Virgo" upgrades. In 2015, the first observation of gravitational waves was made by the two LIGO detectors, while Virgo was still being upgrad

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