Published online 12 September 2008 | Nature | doi:10.1038/news.2008.1105


Gravity-wave hunt stalled

Accident sidelines Virgo detector throughout the summer.

The Virgo interferometer is currently off-line.Virgo

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.

The mirror, crack'd

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.

“The impact of the accident could have been disastrous, but turned out to be far from that.”

Barry Barish

Because the arms are at right angles, a passing gravity wave will cause one to shrink more than the other, thus changing the interference pattern in a detectable way. The change would be tiny, only about 10-19 metres, and to have even a chance of seeing it Virgo's instrumentation needs to be of the highest precision. Each of the mirrors must be flat to within around 10 nanometres, and the entire system must be kept at a vacuum of 10-8 millibar so that air molecules do not disrupt the laser beam's journey.

It was while pumping the system down to that low vacuum on 9 May in order to test a new subsystem that the accident occurred. A glass viewport in the north arm shattered, sending shards into one of the mirrors and some of its ancilliary instrumentation. The mirror was damaged beyond repair and is being replaced with a spare, according to Fidecaro.

The viewports, fix'd

After an investigation, the team has decided to replace all the instrument's 100 viewports to avoid a second failure; Fidecaro says the original viewports may not have been "appropriate" for operating conditions at the machine. Physicists believe that it was not the vacuum itself, but the vibrations created by the pumps that caused the panel to fail. They are confident that the new viewports will not fail in the same way.

This vacuum tube was compromised.Virgo

Virgo is one of four major gravity-wave experiments worldwide. Its main rival in Europe is Geo600, a smaller German-British collaboration located in Schäferberg, Germany. Japan is also home to a small detector, but by far the most ambitious experiment is the US Laser Interferometer Gravitational-Wave Observatory (LIGO) — a US$300 million machine with four-kilometre L-shaped interferometers in both Louisiana and Washington State.

As part of its operations, Virgo sometimes partners with other gravity-wave inteferometers, most notably LIGO; Virgo and the two US interferometers can in effect be combined into a single global antenna. Because of the accident's coincidental proximity to the planned shutdown, those combined experiments will not be seriously affected by the mishap in Italy, according to Barry Barish, a physicist at the California Institute of Technology at Pasadena, and collaborator on LIGO. LIGO is currently in shutdown as well, and so the global search will be able to continue unimpeded when Virgo's repairs are complete. "The impact of the accident could have been disastrous, but turned out to be far from that," he says.

Fidecaro agrees. In the end, the only long-term impact will be on the detector's physicists, he says: "We've got more white hairs." 

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