GRAVITATIONAL RADIATION: TESTING RELATIVITY THEORY
One of Einstein's predictions lay idle for nearly half a century as too difficult to verify: Extremely weak gravitational waves are radiated into space with the velocity of light by rapidly accelerated or orbiting bodies. These waves might be detectable with sensitive apparatus, and large astronomical objects undergoing violent activity, such as supernova outbursts or the nucleus of an active galaxy, may be the best places from which to detect them. Any gravitational wave passing through an object momentarily deforms its space and causes
the object to vibrate slightly.
More than a decade ago experiments tried to pick up, inside large, suspended, aluminum cylinders, infinitesimal oscillations that would be produced by gravitational waves striking the cylinders. At first it seemed that they had succeeded in detecting gravity waves simultaneously in a Maryland laboratory and at the Argonne National Laboratory near Chicago. Most of these were reported to be coming from the Galaxy's center in Sagittarius. It now appears that the observed oscillations were much too large to be consistent with current physical theory. So far, other and far more sensitive gravitywave detectors, which can detect deformations as small as 10.,7 centimeter, have failed to find any evidence of gravitational radiation coming from the center of our Galaxy or anywhere else. However, indirect evidence of gravitational radiation has been found in the radio observations of a binary pulsar (see page 377). The
gravitational interaction between the pulsar and its close companion, perhaps a neutron star or white dwarf, results in part of the orbital kinetic energy's being radiated away in the form of gravity waves. The loss in energy decreases the orbital separation between the components. Radio monitoring during the period 1974 to 1979, covering some 1000 orbital revolutions, shows a decrease in the orbital period of about 101 microseconds per year. Allowing for the uncertainties in the mass of each component and the inclination of their orbital plane, the result is in reasonable agreement with general relativity's prediction of 76 microseconds per year.
