Gravitational waves are predicted by the theory of general relativity. By analogy to electromagnetic waves, which are produced when a charged particle is accelerated, gravitational waves should occur when mass is accelerated. The predicted effect is quite small. Gravitational waves have been observed indirectly from several projects since the 1960s using what is referred to as the Weber bar. Weber was the first scientist to make one of these in the 1960s. The Weber bar is essentially a large cylinder of aluminum that is super-cooled under vacuum. When a gravitational wave hit it, it makes the bar vibrate or "ring" very slightly at its fundamental frequency. By having two bars separated thousands of miles away, you can subtract out the Earth-based vibrations to record only the waves coming from space. The experiment only gives the total energy of the waves and the time they occurred; there is no way to tell from this type of instrument the energy of each graviton and the number of quanta, or the direction they are coming from.
The binary pulsar PSR 1913+16 (a pair of neutron stars) exhibits a small but steady loss of orbital energy, which may be the first demonstration of gravitational waves. Several new types of gravitational wave detectors are being developed. One is a high-frequency clock which is mounted in a satellite and launched far from Earth. When a gravitational wave hits the satellite, the clock signal is Doppler shifted by the gravitational wave and the shift can be picked up back at Earth. If three or more satellites are used, the direction of the gravitation wave could be known. Another type of detector currently under construction is LIGO.
Gravitational waves can be described by tensor spherical harmonics.
References
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Blair, D. G (Ed.). The Detection of Gravitational Waves. Cambridge, England: Cambridge University Press, 1991.
Blair, D. G. and McNamara, G. Ripples on a Cosmic Sea: The Search for Gravitational Waves. New York: Perseus, 1999.
Sibgatullin, N. R. Oscillations and Waves in Strong Gravitational and Electromagnetic Fields. New York: Springer-Verlag, 1991.
Smarr, L. L. (Ed.). Sources of Gravitational Radiation: Proceedings of the Battelle Seattle Workshop, July 24-August 4, 1978. Cambridge, England: Cambridge University Press, 1979.
Thorne, K. S. and Kenan, W. R. Gravitational Radiation: A New Window onto the Universe. Cambridge, England: Cambridge University Press, 1988.
Weisstein, E. W. "Books about Gravitational Waves." http://www.ericweisstein.com/encyclopedias/books/GravitationalWaves.html.
