In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from the source. Predicted to exist by Albert Einstein in 1916 on the basis of his theory of general relativity, the waves transport energy known as gravitational radiation. Sources of gravitational waves include binary star systems composed of white dwarfs, neutron stars, or black holes.

Although gravitational radiation has not yet been directly detected, it has been indirectly suggested to exist. This is related to the 1993 Nobel Prize in Physics, awarded for measurements of the Hulse-Taylor binary system which suggests that gravitational waves may be more than a mathematical anomaly. Various gravitational wave detectors exist. However, they have been unsuccessful in detecting such phenomena.

In Einstein's theory of general relativity, gravity is treated as a phenomenon resulting from the curvature of spacetime. This curvature is caused by the presence of massive objects. Roughly speaking, the more massive the object is, the greater the curvature it produces and hence the more intense the gravity. As massive objects move around in spacetime, the curvature changes to reflect the changed locations of those objects.

In certain circumstances, objects that are accelerated generate a disturbance in spacetime which spreads, as the metaphor goes, "like ripples on the surface of a pond", although perhaps a better analogy would be electromagnetic waves. This disturbance is known as gravitational radiation. Gravitational radiation is thought to travel through the Universe at the speed of light, diminishing in strength but never stopping or slowing down.

As waves of gravitational radiation pass a distant observer, that observer will find spacetime distorted by the effects of strain. Distances between free objects will increase and decrease rhythmically as the wave passes. The magnitude of this effect will decrease the further the observer is from the source. Binary neutron stars are predicted to be a strong source of such waves owing to the acceleration of their enormous masses as they orbit each other and yet even those waves are expected to be very weak by the time they reach the Earth, resulting in strains of less than 1 part in 1020. Scientists are attempting to prove the existence of these waves with ever more sensitive detectors; the current best upper limit thus found (as of September 2009), provided by the LIGO detector, is a wave amplitude of 2.3 x 10−26.. Another attempt, still under development, is Laser Interferometer Space Antenna, a joint effort of NASA and ESA.

Gravitational waves should penetrate regions of space that electromagnetic waves cannot. It is hypothesized that they will be able to provide observers on Earth with information about black holes and other mysterious objects in the distant Universe. Such systems cannot be observed with more traditional means such as optical telescopes and radio telescopes. Precise measurements of gravitational waves will also allow scientists to test the general theory of relativity more thoroughly.

In principle, gravitational waves could exist at any frequency. However, very low frequency waves would be impossible to detect and there is no credible source for detectable waves of very high frequency. Stephen W. Hawking and Werner Israel list different frequency bands for gravitational waves that could be plausibly detected, ranging from 10−7 Hz up to 1011 Hz.

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The music of gravitational waves   PhysOrg - November 24, 2010

This artist’s concept shows the proposed LISA mission, which would consist of three distinct spacecraft, each connected by laser beams. It would be the first space-based mission to attempt the detection of gravitational waves -- ripples in space-time that are emitted by exotic objects such as black holes.

A team of scientists and engineers at NASA's Jet Propulsion Laboratory has brought the world one step closer to "hearing" gravitational waves -- ripples in space and time predicted by Albert Einstein in the early 20th century. Studies of these cosmic waves began in earnest decades ago when, in 1974, researchers discovered a pair of orbiting dead stars -- a type called pulsars -- that were spiraling closer and closer together due to an unexplainable loss of energy. That energy was later shown to be in the form of gravitational waves. This was the first indirect proof of the waves, and ultimately earned the 1993 Nobel Prize in Physics.