(ed.). Gravitational waves (IOP, 2001)(422s).

104 The Earth-based large interferometer Virgowhere TT denotes the transverse-traceless gauge. The maximum change in thedistance AB is then:δL = h 2 L (9.2)where h is the dimensionless gravitational-wave amplitude and L is the distanceAB at rest. One of the most important gravitational-wave sources is theSuperNovae explosion. In order to detect several SuperNovae events per year, it isnecessary to have a sensitivity of h close to 10 −21 for a millisecond signal; in factthis is the expected amplitude for gravitational waves from SuperNovae comingfrom the closest cluster of galaxies: Virgo, distant 10–20 Mpc. The idea of usingan interferometer to detect gravitational waves has been proposed independently,about 30 years ago, by Weber and two Russian physicists, Gerstenstein andPustovoit [1] and a lot of work has been done to develop the optical design ofthe large interferometers presently under construction [2, 3]. Let me mention(see, for example, [4]) the work done in Garching, Glasgow, Caltech and MIT,where interferometers of 30–40 m arms have been developed and are still runningwith displacement sensitivity equal to the shot noise limit. The power spectrumof the displacement sensitivity obtained so far is of the order of 10 −19 m/ √ Hz.Following the above equation it is straightforward to see that a few kilometrearmlength is necessary in order to reach the sensitivity of h = 10 −21 for themillisecond signal. Several antennae are under construction [5]: two antennae4 km arms in the USA (LIGO), one 0.6 km in Germany (GEO600), one inJapan 0.3 km (TAMA), in Italy 3 km (Virgo) and one is planned with 3 kmarms in Australia (ACIGA). Most of them (TAMA, LIGO and GEO) have beenconstructed and are making working together parts of the interferometer. TAMAis already operational, and work is concentrated in order to reach the plannedsensitivity; moreover Japan already plans to build a longer one. The aboveinterferometers, all Earth based, will cover the 10–10 000 Hz window, whereevents from SuperNovae explosions, coalescing binaries and pulsars events areexpected. Using the signals of the resonant bars, and the other kind of detectorsalready operational, in the near future gravitational-wave astronomy is expectedto become a reality. Furthermore, a big space antenna is planned to be launchedin this decade, it is called LISA [5], and will cover the frequency spectrum 10 −4 –10 −1 Hz. In the following the essential characteristics of the Virgo antenna [6,7],the Virgo SuperAttenuator suspension, the Low Frequency Facility [9] and the Rand D experiment of the Virgo project, are described.9.1.1 Interferometer principles and Virgo parametersFigure 9.1 shows a simple Michelson interferometer: the passage of agravitational wave changes the phase shift between the two outgoing beamsinterfering on the beam splitter. For the sake of simplicity it is assumed thatthe wave is optimally polarized and travels perpendicularly to the interferometer