Gravitational Radiation *

Gravity is one of the fundamental forces of Nature, and it is the dominant force in most astronomical systems. In common with all other phenomena, gravity must obey the principles of Special Relativity. In particular, gravitational forces must not be transmitted or communicated faster than light. This means that when the gravitational field of an object changes, the changes ripple outwards through space and take a finite time to reach other objects. These ripples are called gravitational radiation or gravitational waves. In Einstein’s theory of gravitation (see General Relativity and Gravitation), as in many other modern theories of gravity (see Non-general Relativity Theories of Gravity), gravitational waves travel at exactly the speed of light. Different theories make different predictions, however, about details, such as their strength and polarization. There is strong indirect observational evidence (see Binary Stars as a Probe of General Relativity, Hulse-Taylor Pulsar) that gravitational waves follow the predictions of general relativity, and instruments now under construction are expected to make the first direct detections of them in the first years of the 21st century. These instruments and plans for future instruments in space are described in the article Gravitational Radiation Detection on Earth and in Space. Detectors must look for gravitational radiation from astronomical systems, because it is not possible to generate detectable levels of radiation in the laboratory. It follows that gravitational wave detection is also a branch of observational astronomy. The most striking aspect of gravitational waves is their weakness. A comparison with the energy in light