Roughly speaking, the strength of gravity will go up and down as a gravitational wave passes, much as the surface of a body of water will go up and down as a water wave passes. More precisely, it is the strength and direction of tidal forces (measured by the Weyl tensor) that oscillates, 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 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 do not form a vector space and 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.
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.
Graviational radiation has not been directly observed although there are a number of proposed experiments
such as LIGO that intend to do so. However, observations of orbiting binary pulsars indicate that their
orbits are decaying at a rate consistent with the emission of gravitational radiation. In fact that exact
rate of orbital decay places limits how far the actual theory of gravity can be different from general relativity.
Gravitational radiation differs from electromagnetic radiation 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 much weaker quadrapole mode. As with electromagnetic radiation,
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.
Proposed sources of Gravity waves include all bodies on space-time, but are currently only detectable on the galactic scale. These include:
Scientists are eager to implement the experiments which propose to detect gravitational waves, not so much
because of the expected observations, but because like every other new astronomical instruments, unexpected
and surprising results are believed to be likely to be found.
Physicists Russell Hulse and Joseph Taylor explained their observations of a binaryneutron star system as the result of the system's emitting gravitational waves in accordance with general relativity, an achievement for which they were awarded the 1993Nobel Prize in Physics. 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.
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. See the external link listed at the end of the article for more information.
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 and ACIGA;
resonant mass gravitational wave detectors 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.