Gravitational effects propagate at the speed of light, according the General Relativity. Accelerating masses radiate gravitational waves. When these waves reach other masses, they exert forces on these masses.
This wave is quadrupole radiation, meaning that it causes masses to stretch in one direction while compressing in the perpendicular direction. This is illustrated below. The figure shows the motion of a ring of mass under the action of a gravitational wave traveling perpendicular to the plane of the page.
There is indirect evidence of gravitational waves. Pulsars provide an accurate "clock" with which to measure their orbital periods. Imagine a pulsar in orbit around another object. The pulsar spirals in as it loses energy. This energy loss is attributed to gravitational radiation. In 1993, the Nobel Prize in Physics was awarded to Taylor and Hulse for discovering a binary pulsar system (PSR 1913+16) in 1974. In 1983, Taylor and his collaborators reported a decrease in the orbital period of 76 µs per year. The detailed mechanism and results are described here.
There is an effort underway to detect gravitational radiation directly with laser interferometers. Two such systems are under construction in the United States: the Laser Interferometer Gravitational-Wave Observatory (LIGO). For more details, visit the LIGO web page.
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This page is copyright ©1999 by G. G. Lombardi. All
The author acknowledges the assistance of F.G. Kinley with the development of this page.