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# Gravity wave

Gravity waves refer to different (but basically similar) concepts in fluid dynamics and electrodynamics.

## Fluid dynamics

Gravity waves are generated in a fluid medium (e.g. the atmosphere), when a fluid parcel is displaced [e.g. by convection] to a region with a different density. If the fluid stratification is stable, the parcel will start to oscillate around the point where there is no net force. Since the fluid is a continuous medium, a traveling disturbance will result.

In physics, 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. The proposed quantum of gravitational radiation is the graviton. According to general relativity, gravity can cause oscillations (or waves) in spacetime which can transmit energy. If gavity waves are in the space-time, and they are produced by media, it is proposed that the "waves" will start to oscillate around center points. Since the space is continuous, a traveling disturbance will result.

### General

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.

The theory must account for the essential difference between gravitational and electric forces:

[1] Electric forces act statically locally interacting with nearby opposite charges.
[2] Gravitational forces have infinite range. ("spooky action at a distance")

Proposed sources of Gravity waves include all bodies on space-time, but are currently only detectable on the galactic scale. These include:

1. Supernovas or gamma radiation bursts
2. Inspiraling coalescing binary star "chirps"
3. Spherically asymmetric periodic pulsars signals
4. Chaotic cosmic microwave background radiation (CMBR) sources

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.

### Experiments

Physicists Russell Hulse and Joseph Taylor explained their observations of a binary neutron 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 1993 Nobel 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 certian 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.