Pinning down gravity

June 11, 1999

Paul Bompard reports on several projects to detect and analyse gravitational waves.

Two stainless steel pipes, 3 kilometres long, will soon be striking their way through the Tuscan countryside 15 kilometres east of Pisa in Italy. The pipes, 1.2 metres in diameter, are being laid down on concrete foundations at 90 degrees to each other. Where the two pipes meet, and at each end, will be the complex system of mirrors and sensors with which Italian and French scientists hope to pick up and analyse gravitational waves from the cosmos.

It will be able to hunt gravity waves over a frequency range from 50 to 8,000Hz - a significant step forward, as the search has until now been limited to the 900Hz sensitivity of cryogenic detectors.

The Pisa project, called Virgo and costing Pounds 70 million, is among the most ambitious in the world and should begin operating in 2002. Two similar plants are being built in the United States, near Seattle and in Louisiana. These may start operating a little sooner but, according to the Italian physicists at least, will have less sophisticated technology that will need to be updated. The Italo-French plant should be more sensitive to low frequencies than the US ones. In Japan a much smaller "gravity antenna", Tama 300, with 300-metre tubes, is already operating, and in Germany an Anglo-German plant, Geo 600, is being built with 600-metre tubes.

Virgo is backed by Italy's National Institute of Nuclear Physics (INFN) and by the French National Research Council. Italy is footing 55 per cent of the cost, and France 45 per cent, as the project is in Italy and it is thought that the Italians will profit more, both financially and in terms of prestige.

"When we operate, the pipes and the entire system of sensors at the centre and ends will be pumped to a vacuum of one thousand billionth normal atmospheric pressure," explains Adalberto Giazotto, the INFN physicist who is directing the project. "To achieve this, the pipes must be heated to 150oC to remove the absorbed water in the metal, so we have to include bellows-type sections to compensate the expansion due to the heat." The theory is that gravitational waves will be detected as they interact or "interfere" at the junction of the pipes. The device is called an interferometer.

The greatest threat to accurate reception of gravitational waves will be vibrations. The basic concept being used is over a decade old, but the major obstacle until now has been the development of a system of "shock absorbers" capable of filtering out even the most minute vibration.

"This is where the most research has been concentrated," says Giazotto. "The apparatus of mirrors to pick up the waves is held by an extremely sophisticated system of suspensions called superattenuators. These isolate the interferometer by damping out the vast background of small vibrations that normally exist in the ground. Otherwise, tiny ground vibrations might be mistaken for gravitational waves. Tractors and other farm machinery are also being kept at a safe distance."

According to Giazotto, the first tangible results of this new area of research will come from the combined "readings" of the Italo-French and the two American interferometers. "The German and Japanese plants are large and sensitive enough to provide preliminary research and development, and may be upgraded in the future to gravitational wave-detection sensitivity. This type of research is extremely complicated. It is reasonable to expect that the first real reception of gravitational waves will come from us and the Americans, and that their detection will require correlation and triangulation from the three interferometers, but also from the cryogenic detectors now operating, which could have a key role in this first detection." Competition between the US and Italy/France promises to be fierce.

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