gravitational waves

(gravitational radiation) Extremely weak wavelike disturbances that were predicted by Einstein's general theory of relativity. They represent the radiation associated with the gravitational force, and are produced when massive bodies are accelerated or otherwise disturbed. They are ripples in the fabric of spacetime that travel at the speed of light, with a wide range of frequencies, and carry energy away from the source. They should affect all matter: gravitational waves hitting a suspended body, for example, should make it vibrate slightly. The interactions are very small, however.

No conclusive direct evidence of the existence of gravitational waves has been forthcoming from the various highly sensitive experiments designed to detect them. Laser interferometers could be sufficiently sensitive. They consist of two identical extremely long tubes, set at right angles and with mirrors at both ends. A laser beam is split and sent down the tubes. A relative change in the two lengths would indicate a passing gravitational wave, and would be seen in the interference patterns produced when the reflected beams are recombined.

Gravitational waves should be emitted during supernova explosions or energetic events in the cores of active galaxies. They should also be emitted by two massive stars in close orbit. Recent observations of the binary pulsar PSR 1913+16 show that its orbital period is decreasing by 76 ± 2 microseconds per year. The observed value corresponds almost exactly with the decrease predicted to result from the emission of gravitational waves (75 μs per year) and is at present the best indirect evidence for their existence.

A quantum of gravitational radiation is known as a graviton, analogous to a photon.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

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