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On the Lookout for Gravitational Waves

VIRGO, the Franco-Italian Detector is Soon Operational

Assembly of VIRGO's detection mechanism used in the three-kilometer-long interferometer.

Producing direct proof of gravitational waves generated by violent cosmic events is the raison-d'etre of the large interferometer VIRGO. Finishing touches are being put to this high-technology facility in preparation for a first data-gathering trial.

One of the predictions made by Einstein's General Relativity Theory is the existence of gravitational waves, which are disturbances of the space-time geometry and which propagate at the speed of light. These waves have never been directly observed, and since they  are expected to come with a very low amplitude, their detection is a towering scientific and technical challenge. Taking up this challenge, the ambitious VIRGO interferometer project, a cooperative French and Italian effort, is nearing completion at Cascina, Italy, near Pisa.

Once operational, this instrument will represent the leading edge of the quest for gravitational waves generated by cosmic cataclysms like supernovas or the gravitational collapse of a pair of stars. Similarly to its American counterpart, LIGO, the VIRGO interferometer puts into play a laser beam split into two beams sent along perpendicular three kilometer long light paths which form an interference pattern when brought back together. A specific difference in path length spells a specific light intensity at the interferometer output. A gravitational wave produces an oscillation of this difference which will be recorded as a change in the output-light intensity if its amplitude is sufficient.

The sensitivity of the recording instrument must be extremely acute in order to detect a typical relative variation of the arm lengths of 10-21 (which is equivalent to the size of an atom compared to the distance from the sun to the earth!) and to do so in a millisecond, while being completely isolated  from all sources of noise. All of which requires some technological feats, such as the ultra-high reflective mirrors built by the Nuclear Physics Institute in Lyons and boasting the best performance in the world, the system of seismic insulation (by the Italian National Institute of Physics (INFN) in Pisa), the stabilised laser radiation (by OCA-Nice), or the total vacuum required inside the two arms of the interferometer (by LAL-Orsay, LAPP-Annecy and INFN-Pisa).

A test of the whole system on a reduced scale (light lines travelling 6 meters) was completed in July 2002. A second, year-long test begun in the first half of 2003 using the full distance is nearly terminated, and VIRGO will soon have its sharp eye out for wrinkles in the cosmos. What is at stake is not only another confirmation of General Relativity but also the beginnings of a whole new type of astronomy.




Élisabeth Giacobino
Laboratoire Kastler-Brossel

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