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The Laser Interferometer Gravitational-Wave Observatory (LIGO) has begun its initial search phase, scientists announced here over the weekend at a meeting of the American Physical Society.
Einstein's general theory of relativity predicts that concentrations of mass or energy warp space-time, what he considered to be the fabric of the universe. Significant, rapid changes in the shape and concentration of mass or energy should unleash distortions that move outward at the speed of light much like ripples on a pond.
Gravitational waves are not electromagnetic energy, the radiation ranging from radio to light to X-rays and gamma rays that are the targets for conventional telescopes.
Gravitational waves are said to be similar to light waves in that they, too, propagate through space at different frequencies, but all much weaker than electromagnetic waves. They are weaker because the fundamental force of gravity is weaker than the fundamental electromagnetic force.
The LIGO team reported this weekend that it has not ramped up to full sensitivity and so has not detected gravitational waves. But early observations, conducted in September 2002, put new constraints on the phenomena and show that the instruments are working as designed.
"We are eager to see what [LIGO's] future detections will reveal, as the instrument attains its full design sensitivity over the next couple of years," said Penn State physicist and astronomer Lee Samuel Finn, one of the project leaders.
Among the quarry of LIGO will be pairs of dense, neutron stars bound in orbit around one another. Neutron stars are the collapsed remnants of regular stars that can pack the mass of the Sun into a ball just a few miles wide. Einstein's theory holds that two neutron stars in close orbit should disrupt space-time significantly. One pair of neutron stars was found in the 1970s to lose energy at a rate consistent to dissipation by gravitational waves.
LIGO will sense gravitational waves by shining a laser light to a detector. As ripples pass by, the light beam should be slightly perturbed. By noting different frequencies in the waves, astronomers expect to learn about the source of a disturbance. LIGO uses the distance between its detectors -- situated in Louisiana and Washington state -- to confirm that any recorded events indeed involve the passage of gravitational waves and not some local event.
A similar instrument, called GEO, is coming on line in Germany. The two projects are being integrated in what scientists hope will become a worldwide detection effort. A
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