Gravitational Waves

When a gravitational source undergoes a change of motion, this change will result in a disturbance in its gravitational field that is propagated through space. The effect will normally be a very small one, not detectible by even the most sensitive instruments. One of the discoveries of modern astrophysics has been the occurrence of extreme events: a supernova explosion, the rapid implosion of matter into a black hole, binary stars spiraling into each other with huge rotational velocities, quasars involving the collision of galactic nuclei, and so on. It is thought that such events should be powerful enough to emit sizeable gravitational waves, and there is currently a concerted effort to detect them using specially built instruments.

Gravitational waves are analyzed today from the viewpoint of the general theory of relativity, the dominant theory of gravity employed in astrophysics. During the 1960s an American physicist named Joseph Weber (1919—2000) constructed instruments to detect gravitational waves. Despite determined efforts over many years, he was unable to convince the scientific community that he had found anything. The first actual empirical evidence for the existence of gravitational waves emerged by accident within astrophysics in the course of an investigation of a new type of body called a pulsar. In 1967 S. Jocelyn Bell (1943-), a graduate student working under Anthony Hewish (1924—) at Cambridge University, detected a celestial source emitting a rapid series of radio pulses at extremely regular intervals. Other such pulsating sources were soon found. Although pulsars were initially seen as an enigma, Thomas Gold arrived in 1968 at the explanation that is now generally accepted. Massive stars are believed to end their lives as very compact and dense objects, so dense that the protons and electrons are fused together as neutrons. Neutron stars possess extremely powerful magnetic fields, which result in the emission of radio waves from the ends of a magnetic axis through the star that is inclined to the star's axis of rotation. The star rotates very rapidly, and as it does so, the beam of radiation periodically crosses the observer's line of sight, resulting in the detection of a regular sequence of radio pulses.

During the 1970s University of Massachusetts astronomer Joseph Taylor (1941-) and his graduate assistant Russell Hulse (1950—) embarked on a search for pulsars at the Arecibo radio astronomical facility in Puerto Rico. The search involved the use of a minicomputer that was programmed to pick out pulsar signals as the large dish at Arecibo scanned the sky. In 1975 Hulse found a source, designated PSR 1913+16, which proved to be a pulsar paired with a companion star at a distance of 14,000 light-years. The pulsar and its companion revolve about each other with a velocity of over 300 kilometers per second. From observation over a period of several years it was found that the binary system was losing rotational energy and that the two stars were spiral-ing toward each other. A calculation by Taylor based on the general theory of relativity revealed that the energy lost was exactly equal to the energy that would arise from the dissipation of gravitational waves from the system. PSR 1913+16 was the first example ever in which there was real evidence of gravitational waves, and the work of Taylor and Hulse was heralded as a major breakthrough. In 1993 the two researchers were awarded the Nobel Prize for Physics for their research. In his acceptance speech Hulse called attention to the serendipity of their discovery: it occurred not as a result of a search for gravitational waves but as a result of a program within radio astronomy to identify pulsars.

Following the discovery of the Hulse-Taylor pulsar, there were renewed efforts to design and build instruments that could detect gravitational waves directly. The largest facility built for this purpose was LIGO run from Caltech and MIT. LIGO, which stands for Laser Interferometer Gravitational-Wave Observatory, came on-line in 2003 when it joined several other large American and international observatories already in operation. The direct discovery of gravitational waves, should it occur, would be a milestone in the history of astrophysics and theories of gravity and would have fundamental implications for cosmology. The general theory of relativity would be verified and subject to analysis by the procedures and methods of the traditional astronomical observatory. Unlike electromagnetic radiation, gravitational waves are not impeded by intermediate dust and stars that may lie along the line of sight from the observer to the emitting object. The reception of these waves would provide an unprecedented view of distant objects and could yield fundamental information about the gravitational interaction of the early universe.

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