In physics, gravitational radiation is energy that is transmitted through waves in the gravitational field of space-time, according to Albert Einstein's theory of general relativity: The Einstein field equations imply that any accelerated mass radiates energy this way, in the same way as the Maxwell equations that any accelerated charge radiates electromagnetic energy.
A 'Gravity wave' is a wave in the gravitational field. Gravitational radiation is the overall result of gravity waves in bulk and refers to the concept for the phenomenon known as gravity. According to general relativity, gravity can cause oscillations (or waves) in spacetime which can transmit energy.
Roughly speaking, the strength of gravity will vary as a gravitational wave passes, much as the depth of a body of water will vary as a water wave passes. More precisely, it is the strength and direction of tidal forces (measured by the Weyl tensor) that oscillate, which should cause objects in the path of the wave to change shape (but not size) in a pulsating fashion. Similarly, gravitational waves will be emitted by physical objects with a pulsating shape, specifically objects with a nonzero quadrupole moment .
The Einstein field equations imply that any accelerated mass radiates energy, but the gravitational interaction's coupling strength is small in comparison to electromagnetism: It is 1038 times weaker. This means (a) that only oscillating masses of astronomical sizes will radiate significant amount of energies and (b) that even so powerful waves are hardly noticeable because their coupling with matter is so small.
Gravitational radiation differs from electromagnetic radiation (such as light) in that electromagnetism contains both positive and negative charges and hence can radiate in a dipole mode. Gravity is only attractive, and hence can only radiate in a weaker quadrupole mode. As with electromagnetic radiation and the photon, gravitational radiation is expected to be quantized with the quantum being the graviton. However, unlike electromagnetic radiation, there is no general accepted theory of quantum gravity.
The existence of gravitational radiation with the features described above is predicted by the physical theory of general relativity, which describes gravitation in general. The equations of this theory are nonlinear, so that;  The solutions to the equations cannot be superimposed (added together) to produce new solutions. This makes solving the equations much harder than in linear analogues, such as the theory of electromagnetic radiation, and;  Gravitational waves interact with each other (not just with other physical objects). This is unlike, for instance, the interaction of two wave pulses travelling down a string, which can pass through each other without interference. However, weak gravitational waves can be described to a good approximation by linearised general relativity , which is linear.
Experiments have been conducted in the last four decades to directly detect gravitational waves, but so far, none have succeeded. Indirect effects have been observed. Observations of orbiting binary pulsars give strong evidence for the existance of gravitational radiation: These very massive neutron stars rotate around each other at a very close orbit and emit radio signals which are extremely regular pulses of a few seconds repetition period. Comparison of this repetition period with atomic clocks show that they are nearly as accurate as the latter in keeping their pace, but do get slower with time. This loss of speed is so minute that only comparison with atomic clocks can reveal it, but its amount is precisely consistent with the loss of kinetic energy which should be emitted as gravitational waves according to general relativity calculations.
However, gravitational radiation has never been directly observed -- that is, no one has yet witnessed a physical object actually changing shape as a gravitational wave passes through it -- although there have been a number of unconfirmed reports. Many researchers are confident, though, that the next generation of experiments (those designed and built up as of 2004) might finally see a true signal above the noise.
One reason for the lack of direct detection so far is that the gravitational waves that we expect to be produced in nature are very weak, so that the signals for gravitational waves, if they exist, are buried under noise generated from other sources. Reportedly, ordinary terrestrial sources would be undetectable, despite their closeness, because of the great relative weakness of the gravitational force. It has been proposed that certain conductors, especially superconductors, could be made to emit gravitational waves in the laboratory, but this work is still considered speculative.
Scientists are eager to find a way to detect these gravitational waves, since they could help reveal information about the very structure of the universe. In contrast to electromagnetic radiation, it is not known what difference the presence of gravitational radiation would make for the workings of the universe. More promising is the hope to detect waves emitted by sources on astronomic size scales, such as:
- Supernovas or gamma ray bursters
- Inspiraling coalescing binary star "chirps"
- Spherically asymmetric periodic pulsars' signals
- Chaotic cosmic microwave background radiation sources
What is more, a detection of gravitational waves of these objects might also give information about the objects themselves. So, some astronomers already dream of "gravitational telescopes" - but this is far-fetched.
Gravitational radiation has not been directly observed, although there are a number of proposed experiments such as LIGO that intend to do so. Scientists are eager to implement experiments which propose to detect gravitational waves, not so much because of the expected observations, but because unexpected and surprising results are believed to be likely to be found. A number of teams are working on making more sensitive and selective gravitational wave detectors and analysing their results. A commonly used technique to reduce the effects of noise is to use coincidence detection to filter out events that do not register on both detectors. There are two common types of detectors used in these experiments:
- laser interferometers, which use long light paths, such as GEO , LIGO, TAMA , VIRGO, ACIGA and the space-based LISA;
- resonant mass gravitational wave detector s which use large masses at very low temperatures, such as EXPLORER and NAUTILUS .
In November 2002, a team of Italian researchers at the Istituto Nazionale di Fisica Nucleare and the University of Rome produced an analysis of their experimental results that may be further indirect evidence of the existence of gravitational waves. Their paper, entitled "Study of the coincidences between the gravitational wave detectors EXPLORER and NAUTILUS in 2001", is based on a statistical analysis of the results from their detectors which shows that the number of coincident detections is greatest when both of their detectors are pointing into the center of our galaxy, the Milky Way.
Energy, momentum and angular momentum
Do gravitational waves carry energy, momentum and angular momentum? Well, in a vacuum, their stress-energy tensor would be zero. However, it's possible to define a noncovariant pseudo stress-energy tensor which is "conserved" such that they do carry them.
- Discussion of gravitational radiation on the USENET physics FAQ
- Table of gravitational wave detectors
- Laser Interferometer Gravitational Wave Observatory. LIGO Laboratory, California Institute of Technology.
- The Italian researchers' paper analysing data from EXPLORER and NAUTILUS
- Center for Gravitational Wave Physics. National Science Foundation [PHY 01- 14375].
- Australian International Gravitational Research Center. University of Western Australia.
- TAMA project. Developing advanced techniques for km-sized interferometer.
- Could superconductors transmute electromagnetic radiation into gravitational waves? -- Scientific American article
- Sica, R. J., A Short Primer on Gravity Waves. Department of Physics and Astronomy, The University of Western Ontario. 1999.
- Gravity waves. Physics central.
- B. Allen, et al., Observational Limit on Gravitational Waves from Binary Neutron Stars in the Galaxy. The American Physical Society, March 31, 1999.
- Gravitational Radiation. Davis Associates, Inc.
- Amos, Jonathan, Gravity wave detector all set. BBC, February 28, 2003.
- Rickyjames, Doing the (Gravity) Wave. SciScoop, December 8, 2003.
- Will, Clifford M., The Confrontation between General Relativity and Experiment. McDonnell Center for the Space Sciences, Department of Physics, Washington University, St. Louis MO.