GRAVITATIONAL WAVES INTERFEROMETERS AND THE VIRGO PROJECT

Radio, optical and X-rays telescopes are improving our knowledge of deep space. All these telescopes detect electromagnetic radiation at various frequencies. But a different kind of radiation is generated in the deeper space; it is the gravitational one. Gravitational waves change the space-time metric. As a consequence, GW telescopes should detect an extremely small strain (h < 10^-21) of the geometry of a reference frame; if the frame has a reference dimension (L) of some kilometers, the deformation amplitude (ΔL = h × L) is limited to 10^-16 meters. Laser interferometers are the most suitable devices to make precise measurements of distances. Their resolution is limited by the laser wavelength (λ = 10^-6 meters) and by the light wave-shift detection capability (Δ Φ= 1 ppb). These theoretical limits are strongly degraded by different noise sources, which reduce the actual resolution by several orders of magnitude. Applied physicists and engineers are working together to overcome the technical problems that still keep the distance between theoretical and actual detectors' performances. Three large GW telescopes, based on the laser interferometric technology, are under commissioning in the USA (2) and Europe (1). They will become operatives in the next years, with sensitivity of the order of h = 10^-21, in the range between 10 Hz and a few kHz. Among the others, two characteristics are peculiar of the VIRGO interferometer: the high performance of the mirrors' seismic isolation system and the huge ultra high vacuum volume, that will result in the biggest UHV apparatus ever built all over Europe.