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.

Virgo is hosted by the European Gravitational Observatory (EGO), a consortium founded by the French Centre National de la Recherche Scientifique (CNRS) and the Italian Istituto Nazionale di Fisica Nucleare (INFN).

[3] The broader Virgo Collaboration, gathering 940 members in 20 countries,[4] operates the detector, and defines the strategy and policy for its use and upgrades.

[5]The Virgo interferometer is managed by the European Gravitational Observatory (EGO) consortium, which was created in December 2000 by the French National Centre for Scientific Research (CNRS) and the Istituto Nazionale di Fisica Nucleare (INFN).

[8][9] This includes institutions in France, Italy, the Netherlands, Poland, Spain, Belgium, Germany, Hungary, Portugal, Greece, Czechia, Denmark, Ireland, Monaco, Switzerland, Brazil, Burkina Faso, China, Israel, Japan and South Korea.

After several observation runs in which no gravitational waves were detected, the interferometer was shut down in 2011 for upgrading as part of the Advanced Virgo project.

This validated its design choices, and demonstrated that giant interferometers were promising devices for detecting gravitational waves in a broad frequency band.

[32][33] Construction of the initial Virgo detector was completed in June 2003,[34] and several data collection periods ("science runs") followed between 2007 and 2011, after 4 years of commissioning.

[38] Even after several months of data collection with the upgraded suspension system, no gravitational waves were observed, and the detector was shut down in September 2011 for the installation of Advanced Virgo.

[48] On 11 May 2023, Virgo announced that it would not join the beginning of O4; the interferometer was not stable enough to reach the expected sensitivity and one mirror needed replacement, requiring several weeks of work.

The schedule was further revised in January 2025, with an additional two-month break starting in April 2025, and an extension of the run until 7 October 2025 to accommodate for the missing time.

This can be detected with a Michelson interferometer, in which a laser is divided into two beams travelling in orthogonal directions, bouncing on a mirror at the end of each arm.

As the gravitational wave passes, it alters the path of the two beams differently; they are then recombined, and the resulting interferometric pattern is measured with a photodiode.

Since the induced deformation is extremely small, precision in mirror position, laser stability, measurements, and isolation from outside noise are essential.

The injection system includes the input mode cleaner, which is a 140-metre-long (460 ft) cavity designed to improve beam quality by stabilising the frequency, removing unwanted light propagation and reducing the effect of laser misalignment.

It also features a Faraday isolator preventing light from returning to the laser, and a mode-matching telescope which adapts the size and position of the beam before it enters the interferometer.

In the Advanced Virgo configuration, the instrumentation used to detect gravitational-wave signals and steer the interferometer (photodiodes, cameras, and associated electronics) is installed on several benches suspended in a vacuum.

[68] Due to the addition of the squeezed vacuum injection, quantum noise was reduced by 3.2 dB at high frequencies and the detector's range was increased by five to eight per cent.

At the intersection of the two arms, the central building is found, containing most of Virgo's key components including the laser, the beamsplitter and the input mirrors.

South of the west arm, additional buildings contains offices, workshops, as well as the site computing center and the instrument control room.

[73] The two 3-km (1.9 mi) arms are made of a long steel pipe 1.2 m (3.9 ft) in diameter, in which the target residual pressure is about one-thousandth of a billionth of an atmosphere (100 times thinner than in the original Virgo).

[41]: 526 Due to the interferometer's high power, its mirrors are susceptible to the effects of heating induced by the laser (despite extremely low absorption).

These effects can cause deformation of the surface due to dilation or a change in refractive index of the substrate, resulting in power escaping from the interferometer and perturbations of the signal.

Recombination of stray light with the interferometer's main beam can be a significant noise source, often difficult to track and model.

[82][83] A method known as Newtonian calibration (NCal) was introduced at the end of O2 to validate the PCal results; it relies on gravity to move the mirror, placing a rotating mass at a specific distance from it.

[87][95] An important part of Virgo collaboration resources is dedicated to the development and deployment of data-analysis software designed to process the detector's output.

[99] Other efforts are carried out after the data-acquisition period (offline), including searches for continuous sources,[100] a stochastic background,[101] or deeper analysis of detected events.

[2] This event, involving over 4,000 astronomers,[102] improved the understanding of neutron-star mergers[103] and put tight constraints on the speed of gravity.

[110] Although none of the searches identified a signal, this enabled upper limits to be set on some parameters; in particular, it was found that the deviation from perfect spinning spheres for close known pulsars is at most 1 mm (0.039 in).

[119] It includes activities promoting gender equality in science, highlighting women working in Virgo in communications to the general public.