A team of physicists spent most of 1997 and 1998 listening to five supercooled, 2-metric-ton metal bars scattered across the globe. They were listening for the faint ping of a gravitational wave striking the bars like a tiny mallet hitting a huge xylophone. No waves have arrived yet, but the long listening did show that the instruments are able to detect them.
Gravity waves begin when two black holes collide. That immense burst of energy should snap the fabric of space-time like a practiced maid whipping a bedsheet into place. As the resulting ripple of gravity spreads through space, it slightly stretches every object it crosses. This phenomenon is still just a theory, but physicists hope to detect it by measuring tiny vibrations in 3-meter-long metal bars. When the gravity wave passes by, it should expand and contract the aluminum or niobium bars. The detectors must be extremely accurate, because the bar's length is predicted to change by less than one part in one billion trillions.
That doesn't leave much room for error. Tiny temperature fluctuations, for example, occasionally trigger false alarms. To cross-check detections and see how often such false alarms happen--as well as listen for gravity waves--the International Gravitational Event Collaboration (IGEC) coordinates five independent detectors. During 1997 and 1998, IGEC kept at least two detectors operating simultaneously for 260 days. The detector is only sensitive enough to detect gravity waves from nearby black hole collisions, so no one was surprised that they didn't find anything. But they report in the 11 December issue of Physical Review Letters that running three detectors together reduced the false alarm rate to one every 10,000 years.
"This is a beautiful development that gives us much higher confidence that we will soon detect gravitational waves," says physicist Rainier Weiss of the Massachusetts Institute of Technology in Cambridge. IGEC team member Giovanni Prodi, a physicist at the University of Trento, Italy, estimates that the detectors should be sensitive enough to detect gravitational waves originating outside the Milky Way within about a year.