General Relativity :

The general theory of relativity derives its origin from the need to extend the new space and time concepts of the special theory of relativity from the domain of electric and magnetic phenomena to all of physics and, particularly, to the theory of gravitation. As space and time relations underlie all physical phenomena, it is conceptually intolerable to have to use mutually contradictory notions of space and time in dealing with different kinds of interactions, particularly in view of the fact that the same particles may interact with each other in several different ways--electromagnetically, gravitationally, and by way of so-called nuclear forces.

Newton's explanation of gravitational interactions must be considered one of the most successful physical theories of all time. It accounts for the motions of all the constituents of the solar system with uncanny accuracy, permitting, for instance, the prediction of eclipses hundreds of years ahead. But Newton's theory visualizes the gravitational pull that the Sun exerts on the planets and the pull that the planets in turn exert on their moons and on each other as taking place instantaneously over the vast distances of interplanetary space, whereas according to relativistic notions of space and time any and all interactions cannot spread faster than the speed of light. The difference may be unimportant, for practical reasons, as all of the members of the solar system move at relative speeds far less than 1/1,000of the speed of light; nevertheless, relativistic space-time and Newton's instantaneous action at a distance are fundamentally incompatible. Hence Einstein set out to develop a theory of gravitation that would be consistent with relativity.

Proceeding on the basis of the experience gained from Maxwell's theory of the electric field, Einstein postulated the existence of a gravitational field that propagates at the speed of light, c, and that will mediate an attraction as closely as possible equal to the attraction obtained from Newton's theory. From the outset it was clear that mathematically a field theory of gravitation would be more involved than that of electricity and magnetism. Whereas the sources of the electric field, the electric charges of particles, have values independent of the state of motion of the instruments by which these charges are measured, the source of the gravitational field, the mass of a particle, varies with the speed of the particle relative to the frame of reference in which it is determined and hence will have different values in different frames of reference. This complicating factor introduces into the task of constructing a relativistic theory of the gravitational field a measure of ambiguity, which Einstein resolved eventually by invoking the principle of equivalence.

Though the general theory of relativity is universally accepted as the most satisfactory basis of the gravitational force now known, it has not been completely fused with quantum mechanics, of which the central concept is that energy and angular momentum exist only in finite and discrete lumps, called quanta. Since the 1920s quantum mechanics has been the sole rationale of the forces that act between subatomic particles; gravitation doubtless is one of these forces, but its effects are unobservably small in comparison to electromagnetic and nuclear forces. Relativistic phenomena in the subatomic realm have been adequately dealt with by merging quantum mechanics with the special, not the general, theory.

Many physicists, foremost among them Einstein himself, tried during the first half of the 20th century to enrich the geometric structure of space-time so as to encompass all known physical interactions. Their goal, a unified field theory, remained elusive but was brought nearer during the late 1960s by the successful unification of the electromagnetic and the so-called weak nuclear force.

Excerpt from the Encyclopedia Britannica without permission.