ARG%SU-AI@sri-unix.UUCP (11/04/83)
From: Ron Goldman <ARG@SU-AI> n054 1329 31 Oct 83 BC-SCIENCE-WATCH (UNDATED) c. 1983 N.Y. Times News Service In 1976 the Soviet Union's Crimean Astrophysical Observatory found that the surface of the sun is heaving up and down every 160 minutes. ''The interpretation of this phenomenon,'' it reported, ''seems to cause much theoretical difficulty.'' More recently, astronomers have been puzzled by the enigmatic nature of an extremely powerful celestial source of gamma rays called Geminga, which also have a 160-minute periodicity. Now, George Isaak of the University of Birmingham in England has proposed that Geminga causes the solar oscillations. Geminga is believed to be the closest neutron star to the solar system. Isaak argues that if, like many other stars, Geminga is in a tight orbit around some companion body, the pair might radiate gravity waves sufficiently powerful to jostle the core of the sun. Einstein's general theory of relativity predicts the existence of gravity waves, but they have never yet been convincingly detected. The core of the sun, with a density of 30 tons per cubic foot, should respond to such waves far more efficiently than the metal cylinders used in earth-based detection efforts. According to the October 20 issue of Nature, Philippe Delache of France's Nice Observatory and his colleagues have examined five months' worth of gamma ray emissions from Geminga, recorded by the satellite COS-B over a seven-year period. They report a 160-minute variation and also note that tiny earth tremors reach a maximum every 160 minutes, as though the earth were also responding to the gravity waves. As noted by Nature, however, there are several difficulties with such proposed links. When two massive bodies are circling one another every 160 minutes, as indicated by the gamma ray variations, gravity waves should be emitted by each object. The pair would therefore radiate one every 80 minutes. A research group at the University of Rome, led by Eduardo Amaldi, has used suspended bars of metal to record oscillations that could be coming from the core of the Milky Way Galaxy, but they are not yet persuaded the cause is gravitational. Evidence for gravity waves from the galactic core was reported a number of years ago by Joseph Weber of the University of Maryland, a pioneer in such observations, but was never generally accepted.
gjphw@ihuxm.UUCP (11/09/83)
This brief note is an attempt to clarify a minor error in the NY Times article quoted by R. Goldman. It is a reflection on the intended audience for the original article. > Einstein's general theory of relativity predicts the existence of > gravity waves, but.... The instructor for my general relativity course claimed that Einstein's unadulterated theory does not permit gravity waves. As originally formulated, general relativity is a nonlinear theory which does not permit waves to travel far from the source. A linearized version of general relativity, which is also easier to work with, does allow gravity waves. These comments were made in light of the claims by J. Weber that he had detected gravity waves. Einstein's full theory does not permit the generation of gravity waves, so Weber's observations were considered very interesting. Since then (1978), other experimenters have constructed equipment similar to Webers in an attempt to repeat his findings, but none of the other trials have detected gravity waves. -- Patrick Wyant AT&T Bell Laboratories (Naperville, IL) *!ihuxm!gjphw
mwe@astrovax.UUCP (Web Ewell) (11/10/83)
RELATIVISTIC JET ON! I don't know who your instructor was, but hopefully it was you who misunderstood him, not him making an unforgivable error. But because we seem to be quoting authorities here... My general relativity instructor was Kip Thorne. He said that Einstein's field equations could describe the propogation of gravity waves. In fact, any change in the curvature of space time is caused by such waves. He also said that the waves are non-linear (because gravity waves carry energy => mass; a similar case would be if the photon carried electric charge). This means that the equations can't be easily solved, but it doesn't mean that Einstein's theory doesn't predict the waves. The linearized equations you saw are the weak-field limit, which make the approximation that the energy carried in the waves is negligible. These can be solved, and they represent very closely what happens in Einstein's theory in this limit. CRAWLING BACK INTO MY BLACK HOLE web ewell astrovax!mwe