The Virgo Instrument
Virgo is a gravitational interferometer, i.e. a special kind of telescope designed to detect gravitational waves from astrophysical phenomena driven by gravity, such as supernova explosions or stellar mergers. However, gravitational waves are very different from any kind of light, therefore Virgo does not look at all like a telescope.
Virgo is an extremely sophisticated machine based on advanced, cutting-edge technologies, fundamental to detect these extremely faint gravitational signals. In fact, the passage of a gravitational wave will induce a deformation of a billionth of a billionth of a meter in the 3-km arms of Virgo. How small is it? Less than a thousandth of the diameter of a proton!
The design of Virgo is that of a giant laser interferometer, i.e. an instrument that exploits the interference of two laser beams to measure very tiny displacements and deformations. The optical scheme is based on the interferometer built by the American physicists Albert A. Michelson and Edward W. Morley at the end of the 19th century to detect the motion of the Earth through the aether. The famous Michelson-Morley experiment proved that the aether does not exist, and that electromagnetic waves can propagate in vacuum. This result was an important step forward toward the formulation of the theory of Special Relativity published in 1905 by Albert Einstein.
As a bigger “cousin” of the Michelson-Morley interferometer, Virgo is made of two 3-km long arms perpendicular to each other. One arm is aligned in the South-North direction, and the other in the East-West direction.
Simplified optical scheme of the Virgo interferometer (Credits: The Virgo Collaboration)
At the entrance of the Virgo detector, a laser beam impinges on a semi-reflecting mirror, called Beam Splitter (BS) tilted at 45° and is split into two beams which are shot in two straight vacuum tubes housed in the 3-km arms.
The laser used in Virgo is very powerful. The first version of Virgo used a 20 Watt laser that provided a 10 Watt beam at the interferometer input, while in Advanced Virgo (LINK) the laser will have a power of 200 Watts and provide a 125 Watt beam at the interferometer input. This laser is of the order of 100 thousands times the power of a common laser pointer. However, it emits mainly infrared light with a wavelength of 1064 nanometers and is extremely stable in frequency and power. The laser beams go back and forth hundreds of times along the high-vacuum tubes housed in the arms.
The Virgo north vacuum tube, 1.2 m in diameter, inside its 3 km long tunnel (Credits: The Virgo Collaboration).
At the West End (WE) and North end (NE) of the tubes there is a mirror on which the light beam is reflected back. Each 40-Kg mirror is perfectly polished so that it can reflect 99.999% of the incoming light. The reflected beams recombine gently on the tilted mirror, producing an interference pattern that is observed by a photodetector, a device that converts the incoming light into electrical current, that can be further amplified, recorded, and then analysed.
When a gravitational wave passes by the detector, the space and therefore the distances to the end mirrors is stretched and squeezed alternatively: the fringes start very slightly blinking at the gravitational wave frequency. This blinking, recorded by the photodetector, is carefully analysed by dedicated computer algorithms running on the computing farm hosted on the EGO site. The main challenge is to disentangle the genuine and extremely weak gravitational wave signals from signals produced by spurious effects, that physicists call “noises”.
There are many kinds of noises in the Virgo detector, from seismic noise induced by local ground movements to thermal noise produced by the motion of the air molecules impinging on the mirrors. All these effects make mirrors vibrate or simulate their motion, therefore mimicking the signal induced by a gravitational wave.
For instance, to reduce these instrumental noises, the Virgo interferometer has been carefully designed from the beginning. In order to prevent the propagation of seismic and acoustic vibrations, all the mirrors are suspended in vacuum by up to eightfold pendulum chains. With an attenuation factor of 100 times per pendulum stage, the seismic noise is reduced by up to 1016 times.
The pendulum chain supporting a Virgo mirror (Credits: The Virgo Collaboration)