Virgo interferometer
Encyclopedia
The Virgo is a gravitational wave
Gravitational wave
In physics, gravitational waves are theoretical ripples in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, gravitational waves theoretically transport energy as...

 detector in Italy, which commenced operations in 2007. It is one of a handful of the world's major experiments working towards the observation of gravitational waves.

Virgo is a massive Michelson
Michelson
Michelson may refer to:* Albert Abraham Michelson American physicist* Robert C. Michelson American roboticist* Michelson discovered in 1991* Michelson on the moon* Michelson-Gale-Pearson experiment, science...

 laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...

 interferometer made of two orthogonal arms, each three kilometers long.

Multiple reflections between mirrors located at the extremities of each arm extend the effective optical length of each arm up to 120 kilometers. Virgo is located within the site of EGO (European Gravitational Observatory) at Cascina
Cascina
Cascina is a comune in the Province of Pisa in the Italian region Tuscany, located about 60 km west of Florence and about 13 km southeast of Pisa....

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

.

The frequency range of Virgo extends from ten hertz to ten kilohertz. This range as well as the very high sensitivity should allow detection of gravitational radiation produced by supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

e and coalescence of binary system
Binary system
Binary system may refer to:* Binary numeral system, the base-2 internal "machine language" of computers* Binary opposition, a bipolar distinction in philosophy, structuralism and critical theory...

s in the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 and in outer galaxies, for instance from the Virgo cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...

.

In order to reach the extreme sensitivity required, the whole interferometer attains optical perfection and is extremely well isolated, in order to be only sensitive to the gravitational waves.

To achieve this, Italian and French project scientists have developed many of the current leading techniques in the fields of high power ultrastable lasers, high reflectivity mirrors, seismic isolation and position and alignment control.

Regarding optics, Virgo uses a new generation of ultrastable lasers, and the most stable oscillator ever built.

A specific optical coating
Optical coating
An optical coating is one or more thin layers of material deposited on an optical component such as a lens or mirror, which alters the way in which the optic reflects and transmits light. One type of optical coating is an antireflection coating, which reduces unwanted reflections from surfaces, and...

 facility was built to produce extremely high quality mirrors combining reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...

 over 99.999 %, with nanometer surface control.

To avoid spurious motions of the optical components due to seismic noise; each component is isolated by a 10 m high, very elaborate system of compound pendulums.

Because the presence of a residual gas would slightly perturb the measurements the light beam must propagate under ultra high vacuum
Ultra high vacuum
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

. Indeed, the two tubes, 3 km long and 1.2 m diameter are the largest ultra high vacuum vessels in Europe and the second largest in the world.

The environment of the Virgo interferometer is quieter than that of a spacecraft orbiting the earth.

Virgo, the construction of which was completed in June 2003, started its first science run in May 2007 and it is currently running day and night listening for any and all gravitational signals which may arrive at any time from any part of the Universe. The signals are detected, recorded and pre-analysed through an on-line computing system. These data will then be made available to the scientific community for further analysis.

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