- gravitational radiation
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▪ physicsthe transmission of variations in the gravitational field as waves. According to general relativity (relativity), the curvature of space-time is determined by the distribution of masses, while the motion of masses is determined by the curvature. In consequence, variations of the gravitational field should be transmitted from place to place as waves, just as variations of an electromagnetic field travel as waves. If the masses that are the source of a field change with time, they should radiate energy as waves of curvature of the field.There are strong grounds for believing that such radiation exists. One particular double- star system, PSR 1913+16, has a pulsar as one of its components, and, from measurements of the shift of the pulsar frequency due to the Doppler effect, precise estimates of the period of the orbit show that the period is changing, corresponding to a decrease in the energy of the orbital motion. Gravitational radiation is the only known means by which that could happen. (American physicists Russell Hulse (Hulse, Russell Alan) and Joseph H. Taylor, Jr. (Taylor, Joseph H., Jr.), won the Nobel Prize for Physics in 1993 for their discovery of PSR 1913+16.)Double stars in their regular motions (such as that for which a change in period has been detected) and massive stars collapsing as supernovas (supernova) have been suggested as sources of gravitational radiation, and considerable theoretical effort has gone into calculating the signals to be expected from those and other sources.Three types of detectors have been designed to look for gravitational radiation, which is expected to be very weak. The changes of curvature of space-time would correspond to a dilation in one direction and a contraction at right angles to that direction. One scheme, first tried out about 1960, employs a massive cylinder that might be set in mechanical oscillation by a gravitational signal. The authors of this apparatus argued that signals had been detected, but their claim has not been substantiated. In later developments the cylinder has been cooled by liquid helium, and great attention has been paid to possible disturbances. In a second scheme an optical interferometer is set up with freely suspended reflectors at the ends of long paths that are at right angles to each other. Shifts of interference fringes corresponding to an increase in length of one arm and a decrease in the other would indicate the passage of gravitational waves. One such interferometer is the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two interferometers with arm lengths of 4 km (2 miles), one in Hanford, Wash., and the other in Livingston, La. LIGO and other interferometers have not yet directly observed gravitational radiation. A third scheme, the Laser Interferometer Space Antenna (LISA), is planned that uses three separate, but not independent, interferometers installed in three spacecraft located at the corners of a triangle with sides of some 5 million km (3 million miles).Sir Alan H. Cook
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Universalium. 2010.