Gravitational radiation

(relativity) A propagating gravitational field predicted by general relativity, which is produced by some change in the distribution of matter; it travels at the speed of light, exerting forces on masses in its path. Also known as gravitational radiation.

A type of wave generated by accelerated masses that propagates through vacuum with the speed of light. The existence of gravitational radiation is an inescapable consequence of A. Einstein's general theory of relativity.

Properties

The general theory of relativity posits that matter (and energy) introduces curvature into four-dimensional spacetime and that matter moves in response to this curvature. The degree of curvature generated by a distribution of matter can be calculated by using the Einstein field equations, which are analogous to the Maxwell field equations of electromagnetism. The Einstein equations admit solutions corresponding to weak, transverse waves that propagate in vacuum with the speed of light just like the electromagnetic waves predicted by Maxwell's equations and demonstrated experimentally by H. Hertz in 1884. However, while electromagnetic waves are fluctuations in the electric field vector which can be measured by the acceleration of a single charged particle, gravitational waves are fluctuations in the tidal gravitational force which is measured by a tensor and can be detected by the relative acceleration induced between two free test masses (see illustration). See also Electromagnetic radiation; Maxwell's equations.

Generation and detection of gravitational waves. A powerful source of these waves, suchas a neutron star binary with a circular orbit, radiates with a fundamental period equal to half the orbital period. The presence of the wave can be demonstrated conceptually by measuring the relative acceleration between two free masses.

Sources

In 1974, R. Hulse and J. Taylor discovered the first binary pulsar, now known as PSR 1913+16. This is believed to comprise two neutron stars, one of which spins with a period of 59 milliseconds and emits regular radio pulses with an equal period. Radio astronomers on Earth observe a pulse period that varies slightly as the pulsar source traces an elliptical orbit aroundits companion due to the Doppler effect. This system emits gravitational waves with a fundamental period equal to the orbital period of 7.8 h, and these waves carry energy away from the binary, causing it to become more tightly bound. The orbital period decreases as the two stars approach one another. The fractional change in the measured orbital period agrees with the relativistic prediction to approximately 0.5%, a striking verification of general relativity. See also Doppler effect; Neutron star; Pulsar.

When such binary stars shrink so that their orbital periods are only a few milliseconds, therate of radiation increases dramatically. The waves emitted by such coalescing stars may be strong enough to measure at Earth even when the stars reside in quite distant galaxies. Other proposed sources of detectable gravitational radiation include supernovae, binary black holes in active galactic nuclei, and cosmic strings. See also Black hole; Cosmic string; Supernova.

Detectors

The direct detection of gravitational waves has not yet been achieved, though there is an active research program directed toward the construction of appropriate ultrasensitive detectors.The two classes of detectors are resonant-mass and free-mass antennas. Both types comprise a mechanical element and a transducer that converts mechanical motion to an electronic signal. See also Transducer.

The resonant-mass antennas are commonly cylindrical bars of a few tons mass, although a massin the shape of a icosahedron (a body with 60 faces, like a soccer ball) would be superior to acylinder. The material most commonly used is aluminum. To reduce thermal noise, the bar is brought to cryogenic temperatures in a vacuum vessel cooled by liquid helium and is isolated against vibrations from the terrestrial environment by a series of mechanical filters. The lowest-frequency longitudinal mode of the antenna, in which the two end faces of the cylinder are displaced in opposition, is most strongly excited by a passing gravitational wave. An electromechanical transducer is attached to one end face of the bar to detect the bar's vibration.

The free-mass antennas are composed of almost inertial masses which are actually very low frequency pendulums. A common design is the arrangement of three such masses at the vertices of aright triangle with equal legs lying on the surface of the Earth. The passage of a gravitational wave in a direction perpendicular to the plane of the free-mass antenna lengthens one leg of the triangle relative to the perpendicular leg. This change in length can be detected by a laser interferometer which is composed of mirrors mounted on the masses and a high-power visible laser light source. Each mass is rigorously isolated from vibrations in the environment, and the entire system is placed in a high-vacuum chamber to eliminate light scattering by gas and dust particles. See also Interferometry.