The Stalled Hunt for a Gravity Wave

Geoff Brumfiel in News at Nature:

Physicists rejoiced this week at the successful test of their massive new particle accelerator, the Large Hadron Collider, near Geneva, Switzerland. But some 450 kilometres south-east — and making a lot less of a hullabaloo about things — another major physics experiment is working to recover from a debilitating accident.

The Virgo gravity-wave interferometer, an €80 million (US$114 million) experiment located outside of Pisa, Italy, has been incapacitated by a vacuum failure for most of the summer, and is expected to stay out of commission for a month or two to come. Much of the lost time, though, would not have been used for observations anyway; downtime was already scheduled to allow an upgrade of the machine, and that work has gone on in parallel with the necessary repairs. "We were lucky because of the timing," says Francesco Fidecaro, Virgo's spokesperson and a physicist at the University of Pisa.

Virgo is one of a handful of detectors worldwide searching for gravity waves, vibrations in the fabric of space-time created by the motion of extremely massive objects such as black holes. The L-shaped detector splits a laser beam in two and sends the parts down each of its three-kilometre arms. At the end of the two arms, the beams are reflected back into the central tower. After several trips down the arms and back, the beams are recombined, creating an interference pattern of light and dark lines that is extremely sensitive to the lengths of the arms.