gravitational radiation

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.

Gravity wave

A 'Gravity wave' is, in the present context, a wave in the gravitational field. Caution should be used with this term, because in hydrodynamics it refers to a surface wave, for example on the ocean, whose main restoring force is gravity (short wavelength water waves on the surface are largely capillary waves, not gravity waves). To disambiguate the two usages it is advisable to refer to general-relativistic waves in spacetime as Gravitational waves, and those on oceans, lakes, etc. as Gravity waves. Gravitational radiation is the overall result of gravitational 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. In 2005 it was announced that observations of the binary pulsar PSR J0737-3039 appeared to confirm predictions of general relativity with respect to energy emitted by gravitational waves, with the system's orbit observed to shrink 7 mm per day. 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 changing quadrupole moment.

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. 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 fully understood what difference the presence of gravitational radiation would make for the workings of the universe. A sufficiently strong sea of primordial gravitational radiation (energy density exceeding that of the big bang electromagnetic radiation by a few orders of magnitude) would shorten the life of the universe, violating existing data that show it is at least 13 billion years old. More promising is the hope to detect waves emitted by sources on astronomic size scales, such as:

Supernovas or gamma ray bursts
Inspiraling coalescing binary star "chirps"
Spherically asymmetric periodic quark stars', neutron stars', and pulsars' signals
Stochastic gravitational wave background sources

What is more, a detection of gravitational waves of these objects might also give information about the objects themselves. Although the idea is far-fetched, some astronomers already dream of "gravitational telescopes," to see in gravity, as opposed to light, and gravity phones to communicate via gravity waves across p-branes.

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. 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 detectors which use large masses at very low temperatures, such as EXPLORER and NAUTILUS.

In November 2002, a team of Italian researchers at the Instituto 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. MiniGRAIL is a spherical gravitational wave antenna based at Leiden University.

Einstein@Home

Bruce Allen of UWM's LIGO Scientific Collaboration, LSC group is leading the development of the Einstein@Home project. "Einstein@Home" is a project developed to search data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) at the California Institute of Technology, in the US and from the GEO 600 gravitational wave observatory in Germany for signals coming from selected extremely dense, rapidly rotating stars. Such sources are believed to be either quark stars or neutron stars, and a subclass of these are already observed by conventional means, and are known as pulsars, electromagnetic wave emitting celestial bodies. If some of these stars are not quite near-perfectly spherical, they should emit gravitational waves, which LIGO and GEO 600 may begin to detect. Einstein@Home is one, small part of the LSC scientific program. It has been set up and released as a distributed computing project, like SETI@home. Meaning, it relies on computer time donated by private computer users to process data generated by LIGO's and GEO 600's search for gravity waves. For more information, or to sign up, see the external links. Join einstein@home: http://einstein.phys.uwm.edu/

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.