General Relativity

A theory of gravitation, published in its final form in 1916. It was developed by Albert Einstein (1878-1955) from his earlier (1905) Special Relativity theory. One of the fundamental postulates of the general theory is that, over a limited region of spacetime, it is impossible for observers to tell whether they are undergoing uniformly accelerated motion or are in a gravitational field. This is known as the principle of equivalence. Einstein showed that it was not necessary to think of gravity as a force acting at a distance. Instead, he described gravity in terms of its local effects on space and time, i.e. as the curved geometry of spacetime, which is determined by the distribution of matter and energy. A good three-dimensional analogy to help understand the meaning of curved, four-dimensional spacetime is geometry on the surface of a sphere. For example, two travellers who set out from different places on the equator and travel due north will eventually find that their paths cross, even though they started out travelling parallel to each other. This contrasts with what happens on a flat surface, where parallel lines never cross. The two travellers might say they had been pulled together by some force (gravity, for example), but their experience is more effectively explained in terms of geometry. In regions where the gravitational field is weak, General Relativity approximates to the theory set out by Isaac Newton. For a strong gravitational field, General Relativity gives the best description yet devised. There are other theories of gravity but none has met with the total success enjoyed by General Relativity. Several areas of astronomy have proved to be testing grounds for the theory. The perihelion of Mercury's elliptical orbit around the Sun advances by 43 arc seconds per century more than is predicted by Newton's gravitational theory, but General Relativity explains it exactly. The light from stars deviates from a straight line if it passes very close to the Sun, and this has been observed at solar eclipses. The motion of pulsars in binary systems is readily accounted for by General Relativity. The most frequent application of General Relativity is in cosmology, since gravity is the dominant factor in all attempts to make mathematical models of the universe.