The Gravitational radiation reference article from the English Wikipedia on 24-Jul-2004
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Gravitational radiation

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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, which describe electromagnetism imply, that any accelerated charge radiates electromagnetic energy.

Table of contents
1 Theoretical description
2 Experimental evidence
3 Sources
4 Detectors
5 Experiments
6 See also
7 External links
8 References

Theoretical description

But the gravitational interaction's coupling strength is less than tiny in comparison to electromagnetism: It is 10 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.

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.

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:


Experimental evidence

Quite some experiments have been conducted in the last four decades to directly detect gravitational wave, but so far, none have succeeded.

However, 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 extemely 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.

Now, general relativity predicts that the pulsars, being so massive, emit a significant amount of energy as gravitational radiation, when they orbit each other, i.e. pull each other on circular orbits by means of gravitation. This loss of kinetic energy let them slowly ``spiral'' towards each other, changing the rate of their pulsing. As the measured rate of slowing down corresponds precisely with the value calculated by assumin the emission of gravitational waves, and this is taken as a good confirmation of the theory.

Physicists Russell Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for this observation and interpretation.

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. The confirmed observation of gravitational waves would be important further evidence for the validity of general relativity.


Sources

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.

More promising is the hope to detect waves emitted by sources on astronomic size scales, such as:

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.

Detectors

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.

Experiments

As already mentioned, all attempt for direct detection did not yield confirmed successes. However, many researches are confident, that the next generation of experiments, those designed and built up now, might finally see a true signal above the noise.

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:

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.

See also

External links

References