According to Einstein’s General Theory of Relativity, gravity as a phenomenon is strikingly different from electromagnetic radiation. Gravity can be completely described as a distortion of time/space in the region of a body that has mass. The greater the mass, the more distortion. Gravity is a comparatively weak force, far more feeble than magnetic or electrostatic interactions. Another prominent difference is that in common experience gravity acts only as a force of attraction, not repulsion.
The foregoing notwithstanding, gravity does make waves. To clarify, it does not consist of waves like sound in air or light in space, but it does produce waves.
How is this possible?
With reference to gravity, distortion in one region sets the stage for an opposite and opposing distortion in an adjacent area. The end result is that there is a change in pressure that travels through space at the speed of light.
Gravitational waves, unlike electromagnetic radiation, consist of vibrations that take place in a plane perpendicular to the direction in which the waves travel. Gravitational waves are generated when an object having mass is accelerated. A body at rest with respect to the observer or one whose rate and/or direction of motion does not change, produces gravity but not gravitational waves. Gravitational waves are perturbations in gravity.
Gravitational waves that radiate from a source grow weaker in proportion to the distance they travel. Any gravitational waves that reach us are quite weak indeed and close to impossible to detect. They have never been detected directly but there is indirect evidence for their existence. The source must be quite strong—a neutron star orbiting a black hole for instance.
Neutron stars form when massive old stars collapse due to their own gravity. A neutron star has not shrunk and accreted additional matter to the extent of a black hole, but it is not far removed from the extreme case of a body with such high mass contracted into such a small volume that even close to massless photons cannot escape. This is the definition of a black hole, and if two such bodies, black holes or neutron stars, are in orbit, sufficient energy is present so gravitational waves will be emitted that are strong enough to be detected by existing instrumentation.
As two of these orbiting objects draw ever closer to one another, their total angular momentum soars. A vast amount of gravitational waves are generated and soon with increasingly sensitive instrumentation it is highly probable that we on earth or orbiting nearby will be capable of detecting them.
One class of gravitational wave detector uses laser interferometry to measure gravitational-wave induced motion between two’free’ masses. This lets the masses be separated by large distances, increasing the signal size. This type of detector is also sensitive to a wide range of frequencies, not just those near a resonance, as is the case for other kinds of detectors.
Gravitational wave interferometers are now operational. As of this writing, the most sensitive is the Laser Interferometer Gravitational Wave Observatory. LIGO has three widely separated detectors. One is in Livingston, Louisiana, one resides at the Hanford nuclear site in Richland, Wash., and a third scheduled to move to India. Each observatory has two light storage arms that are four kilometers long. These are at 90° to each other, with the light passing through 1-m-diameter vacuum tubes running the entire four kilometers. A passing gravitational wave will slightly stretch one arm as it shortens the other. This is precisely the motion to which an interferometer is most sensitive.
Even with such long arms, the strongest gravitational waves will only change the distance between the ends of the arms by at most roughly 10−18 m.