On July 23rd in Cascina, near Pisa (Italy), the new Virgo interferometer will be inaugurated. The innovative Virgo gravitational-wave-detector is the outcome of more than ten years of collaborative research and development between the National Institute of Nuclear Physics (Infn, Italy) and the National Scientific Research Centre (Cnrs, France). Letizia Moratti, Italy's Minister for Education and Research, and Claudie HaignerÈ, the French Minister for Research and New Technologies, will participate in the inauguration ceremony. Journalists are also being invited to tour the scientific infrastructure and interview researchers.

The existence of gravitational waves is one of the most fascinating puzzles of modern physics. They are predicted by Albert Einstein's general theory of relativity, and their existence has been demonstrated indirectly (Joseph. H. Taylor and Russell A. Hulse received the Nobel Prize for this discovery in 1993), but until now it has never been possible to observe them directly. "Gravitational waves are elusive perturbations of space-time curvature, produced by material bodies when accelerating, and can be considered similar to electromagnetic waves emitted by charged particles when they are accelerating. They are difficult to detect, however, because of the fact that they are extremely weak perturbations and, at the best, we can only hope to register those produced by huge phenomenona, like the explosion of a supernova, the interaction between a neutron star and a black hole, or the fusion of two neutron stars belonging to a binary system", says Enzo Iarocci, president of Infn.

"Virgo will reveal these gravitational waves using extremely sophisticated technological solutions. The measurement system is based on a laser beam that is split into two identical and perpendicular beams by a 'beam dividing' mirror. Each beam goes into an optical hollow (known as a Fabry-Perot cavity) that holds two mirrors, one close by and the other positioned three kilometres away. The beams always travel in a vacuum. Each photon of the beams undergoes an average of 50 reflections before it exits the hollow and returns to the 'beam dividing' mirror. This mirror then recombines the two beams and another device measures the interference between them. If a gravitational wave collides with the mirrors of the Fabry-Perot cavities, the distance between the mirrors changes and the interference of the two beams becomes disturbed. From the variation of the interference is possible to detect the signal produced by a gravitational wave", explains Adalberto Giazotto, Virgo's scientific coordinator. To make the system work, it is also necessary to have very advanced mechanical equipment that allows a perfect sealing from the external environment and that prevents perturbations that could mask the passage of the wave. In proportion, the accuracy required to observe the existence of gravitational waves is analogous to the precision needed to measure the distance between the Earth and the Sun with an error lower than the diameter of an atom, but on a scale of billions of times smaller!

"Virgo is the result of a project begun in the 1980s and inspired by the ideas and pioneering development of the Infn team in Pisa, with the collaboration of the Cnrs group, at that time directed by Alain Brillet. Afterwards, other teams from Cnrs, In2P3 and Infn joined the original group of people: in particular, Lal Orsay, Espci Paris, Lapp Annecy, Ipn Lyon, Infn Naples, Infn Perugia, Infn's National Laboratories of Frascati, Infn Roma 1 and Infn Florence-Urbino. The interferometer has already passed its initial running tests and within the next few months the working of all component systems will be verified. After that, it will begin recording data. The mirrors, made with nanometer precision, and its sophisticated mechanical systems make Virgo one of the most sensitive instruments in the global network, which also includes the American Ligo, the Anglo-German Geo and the Japanese Tama", says Adalberto Giazotto.

At the moment the Virgo project operates in the context of the Ego laboratory (European Gravitational Observatory), built on purpose by Infn and Cnrs. "The difficulty of intercepting the waves hypothesized by Einstein demonstrates that we still have much to understand about gravitational force, even though it has attracted mankind from time immemorial, since among all the forces it is the one that shows the most evident effects in everyday life", says Virgo director Filippo Menzinger.

Italy occupies a prominent position in the field of gravitational wave research and Infn has, among all the detectors in the world, those that permit the exploration of the largest frequency band of gravitational waves. Besides Virgo, two ultracryogenic bars are in active use: Nautilus (at the National Laboratories of Frascati, near Rome) and Auriga (at the National Laboratories of Legnaro, near Padua). These two detectors, which are kept at a temperature very close to absolute zero (-273 Celsius degrees) are thought to be the coldest large objects in the entire Universe. This peculiarity allows the bars to register weak signals from Space, minimizing the perturbations due to internal thermal agitation of molecules.

The Virgo online brochure is at: http://www.ego-gw.it/brochure/
The program of the inauguration is at: http://www.ego-gw.it/inauguration/
Some other images are available at: http://www.virgo.infn.it/