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The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument's two arms are three kilometres long and located in Santo Stefano a Macerata, near the city of Pisa, Italy.

Virgo is part of a scientific collaboration of laboratories from six countries: Italy and France, the Netherlands, Poland, Hungary and Spain. Other interferometers similar to Virgo have the same goal of detecting gravitational waves, including the two LIGO interferometers in the United States (at the Hanford Site and in Livingston, Louisiana). Since 2007, Virgo and LIGO have agreed to share and jointly analyze the data recorded by their detectors and to jointly publish their results. Because the interferometric detectors are not directional (they survey the whole sky) and they are looking for signals which are weak, infrequent, one-time events, simultaneous detection of a gravitational wave in multiple instruments is necessary to confirm the signal validity and to deduce the angular direction of its source.

The interferometer is named for the Virgo Cluster of about 1,500 galaxies in the Virgo constellation, about 50 million light-years from Earth. As no terrestrial source of gravitational wave is powerful enough to produce a detectable signal, Virgo must observe the Universe. The more sensitive the detector, the further it can see gravitational waves, which then increases the number of potential sources. This is relevant as the violent phenomena Virgo is potentially sensitive to (coalescence of a compact binary system, neutron stars or black holes; supernova explosion; etc.) are rare: the more galaxies Virgo is surveying, the larger the probability of a detection.

Seismic Noise
Seismic noise is the main limitation to the low frequency sensitivity of ground based interferometers. It propagates to the test masses through the suspension system causing perturbations in mirror displacement. The minimum coupling between the horizontal and vertical directions is due to the Earth curvature. The input and output mirrors of the Fabry-Perot cavities form an angle $$\alpha_{grav}=L/r_{\bigoplus}\simeq$$ 4.7 10-4 rad (where $$L$$ = 3 km is the cavity length and $$r_{\bigoplus}$$ is the Earth radius) with the global vertical direction. Therefore a vertical displacement $$\delta_z$$ has effect along the beam direction, producing a variation $$\alpha_{grav}\delta_z$$ of the optical path. The suspension system causes even larger mechanical couplings (1%), due to structural reasons.

Seismic noise has both natural and human origins and can vary by few orders of magnitude from site to site. However all ground motion displacement spectra observed worldwide share some common characteristics: they have essentially the same amplitude in all three orthogonal space directions and they exhibit a low pass behavior.

Suspensions
The Virgo superattenuator