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Top Document: [sci.astro] Astrophysics (Astronomy Frequently Asked Questions) (4/9) Previous Document: D.05 What are gravitational waves? Next Document: D.07 Do gravitational waves travel at the speed of light? See reader questions & answers on this topic! - Help others by sharing your knowledge Steve Willner <swillner@cfa.harvard.edu> The effects of gravitational waves are ridiculously weak, and direct evidence for their existence has (probably) not been found with the detectors built to date. However, no known type of source would emit gravitational waves strong enough for detection, so no one is worried. In the 60's and early 70's, Joe Weber at the University of Maryland attempted to detect gravitational waves using large aluminum bars, which would vibrate if a gravitational wave came by. Because local causes also created vibrations, the technique was to look for coincidences between two or more detectors some distance apart. Weber claimed to see more coincidences than expected statistically and even to see a correlation with sidereal time. Unfortunately, other groups have used far more sensitive detectors operating on the same principles and found nothing. Two new experiments, far more sensitive than those using metal bars, are being built now. These are LIGO in the US and Virgo in Italy. They will work by detecting displacements between two elements separated by several kilometers. An indirect measurement of gravitational waves has been made, however. Gravitational waves are formed when a mass undergoes change of acceleration. They are stronger if the mass is dense and the acceleration changes rapidly. One place where this might happen would be two pulsars circling each other. A couple of systems like this exist, and one has been studied actively over the past 20 years or so. Pulsars make good clocks so you can time the orbital period of the pulsars quite easily. As the pulsars circle, they emit gravitational waves, and these waves remove energy (and angular momentum) from the system. The energy released has to come from somewhere, and that somewhere is the orbital energy of the pulsars themselves. This leads to the pulsars becoming closer and closer over time. A formula was worked out for this effect, and the observed pulsars match it amazingly well. So well, in fact, that if you plot the data on top of the prediction, there is no apparent deviation. (It's actually rather disgusting, none of my results ever come out that well.) Anyway, Joe Taylor of Princeton and a student of his, Russell Hulse, shared the Nobel Prize in Physics for, in part, this work. Useful references are given in section D.03. V. M. Kaspi discusses pulsar timing in 1995 April Sky & Telescope, p. 18. The conference proceedings volume _General Relativity and Gravitation 1989_, eds. Ashby, Bartlett, & Wyss, (Cambridge U. Press 1990) contains a summary of the aluminum bar results. _General Relativity and Gravitation 1992_, eds. Gleiser, Kozameh, & Moreschi (IOP Publishing 1993) contains an article by Joe Taylor summarizing the pulsar results. An example of recent pulsar research is the article by Kaspi, Taylor, and Ryba, 1994 ApJ 428, 713, who give instructions for obtaining their archival timing data via Internet. Some references to Weber's work are: 1969 Phys. Rev. Lett. 22, 1320. 1970 Phys. Rev. Lett. 24, 276. 1971 Nuovo Cimento 4B, 199. Information on gravitational wave detection experiments can be found on the Web for LIGO <URL:http://www.ligo.caltech.edu/>, VIRGO <URL:http://www.virgo.infn.it/>, GEO 600 <URL:http://www.geo600.uni-hannover.de/>, and TAMA <URL:http://tamago.mtk.nao.ac.jp/>. User Contributions:Top Document: [sci.astro] Astrophysics (Astronomy Frequently Asked Questions) (4/9) Previous Document: D.05 What are gravitational waves? Next Document: D.07 Do gravitational waves travel at the speed of light? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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