relativity physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference. One consequence of the theory is that space and time are no longer viewed as separate, independent entities but rather are seen to form a four-dimensional continuum called space-time . Full comprehension of the mathematical formulation of the theory can be attained only through a study of certain branches of mathematics, e.g., tensor calculus. Both the special and general theories have been established and accepted into the structure of physics. Einstein also sought unsuccessfully for many years to incorporate the theory into a unified field theory valid also for subatomic and electromagnetic phenomena.

Evolution from Classical Theory

The modern theory is an extension of the simpler Galilean or Newtonian concept of relativity, which holds that the laws of mechanics are the same in one system as in another system in uniform motion relative to it. Thus, it is impossible to detect the motion of a system by measurements made within the system, and such motion can be observed only in relation to other systems in uniform motion. The older concept of relativity assumes that space and time are correctly measured separately and regards them as absolute and independent realities. The system of relativity and mechanics of Galileo and Newton is perfectly self-consistent, but the addition of Maxwell's theory of electricity and magnetism to the system leads to fundamental theoretical difficulties related to the problem of absolute motion.

It seemed for a time that the ether , an elastic medium thought to be present throughout space, would provide a method for the measurement of absolute motion, but certain experiments in the late 19th cent. gave results unexplained by or contradicting Newtonian physics. Notable among these were the attempts of A. A. Michelson and E. W. Morley (1887) to measure the velocity of the earth through the supposed ether as one might measure the speed of a ship through the sea. The null result of this measurement caused great confusion among physicists, who made various unsuccessful attempts to explain the result within the context of classical theory.

The Special Theory of Relativity

The validity of the classical concepts of absolute and independent time and space was challenged by H. A. Lorentz and others. Since absolute motion cannot be confirmed by objective measurement, Einstein suggested that it be discarded from physical reasoning; he explained the results of the Michelson-Morley experiment by means of the special relativity theory, which he enunciated in 1905. This theory accepts the hypothesis that the laws of nature are the same in different moving systems applies also to the propagation of light, so that the measured speed of light is constant for all observers regardless of the motion of the observer or of the source of the light. Einstein deduced from these hypotheses the full logical consequences and reformulated the mathematical equations of physics, basing them in part on equations of H. A. Lorentz (see Lorentz contraction ) by which measurements made in one uniformly moving system can be correlated with measurements in another system if the velocity of one relative to the other is known.

The theory resolves the conflict between Newton's mechanics and Maxwell's electrodynamics by introducing fundamental changes in Newton's theory. In most phenomena of ordinary experience the results obtained from the application of the special theory approximate those based on Newtonian dynamics, but the results deviate greatly for phenomena occurring at velocities approaching the speed of light. In innumerable cases where the results predicted by these theories are incompatible, experimental evidence supports the Einstein theory. Among its assertions and consequences are the propositions that the maximum velocity attainable in the universe is that of light; that mass and energy are equivalent and interchangeable properties (this is spectacularly confirmed by nuclear fission, on which the atomic bomb is based); that objects appear to contract in the direction of motion; that the rate of a moving clock seems to decrease as its velocity increases; that events that appear simultaneous to an observer in one system may not appear simultaneous to an observer in another system; and that, since absolute time is excluded from physical reasoning because it cannot be measured, the results of observers in different systems are equally correct.

The General Theory of Relativity

Einstein expanded the special theory of relativity into a general theory (completed c.1916) that applies to systems in nonuniform (accelerated) motion as well as to systems in uniform motion. The general theory is principally concerned with the large-scale effects of gravitation and therefore is an essential ingredient in theories of the universe as a whole, or cosmology . The theory recognizes the equivalence of gravitational and inertial mass . It asserts that material bodies produce curvatures in space-time that form a gravitational field and that the path of a body in the field is determined by this curvature. The geometry of a given region of space and the motion in the field can be predicted from the equations of the general theory.

Details of the motions of the planet Mercury had long puzzled astronomers; Einstein's computations explained them. He stated that the path of a ray of light is deflected by a gravitational field; observations of starlight passing near the sun, first made by A. S. Eddington during an eclipse of the sun in 1919, confirmed this. He predicted that in a gravitational field spectral lines of substances would be shifted toward the red end of the spectrum. This has been confirmed by observation of light from white dwarf stars. Further confirmation has been obtained in more recent years from precision measurements using artificial satellites, the Viking lander on Mars, and Gravity Probe B (designed specifically to test the theory) as well as from detailed observations of pulsars.

Bibliography

See A. Einstein, The Meaning of Relativity (6th ed. 1956) and, with others, The Principle of Relativity (1923, repr. 1958; a collection of original papers on the theory); M. Gardner, Relativity for the Million (1962); D. Bohm, The Special Theory of Relativity (1965); J. Schwinger, Einstein's Legacy (1986).

relativity Theory, proposed by German-born US physicist Albert Einstein, based on the postulate that the motion of one body can be defined only with respect to that of a second body. Mass, space and time are interdependent. This theory led to the concept of a four-dimensional space-time continuum in which the three space dimensions and time are treated on an equal footing. Einstein devised two theories concerning relativity. The special theory, put forward in 1905, is limited to the description of events as they appear to observers in a state of uniform relative motion. The more important consequences of the theory are: (1) that the velocity of light is absolute, that is, not relative to the velocity of the observer; (2) that the mass of a body increases with its velocity, although appreciably only at velocities approaching that of light; (3) that mass (m) and energy (E) are equivalent, that is, E = mc², where c is the velocity of light (this shows that when mass converts to energy, a small mass gives rise to large energy); (4) the Lorentz-Fitzgerald contraction, that is, bodies contract as their velocity increases, again only appreciably near the velocity of light; and (5) an object's sense of elapsed time expands – ‘time dilation’. The general theory of relativity, completed in 1915, is applicable to observers not in uniform relative motion. This showed the relation of space and gravitation. The presence of matter in space causes space to ‘curve’, forming gravitational fields; thus gravitation becomes a property of space itself. The existence of black holes, where not even light can escape extreme gravity, is postulated as a consequence of this.

relativity A collective term for two theories, special relativity and general relativity, developed by A.Einstein. The special theory of relativity, published in 1905, is concerned with the laws of physics as viewed by observers moving relative to each other at constant speed (i.e. not subject to acceleration). It describes how the motion of one observer relative to another affects measurements made by these observers. At low speeds special relativity reduces to situations describable by classical physics, embodied in Newton's laws of motion. Differences between Newton's and Einstein's physics become apparent only at velocities towards that of light. The general theory of relativity (1915) describes how the relationship between space and time is affected by the gravitational effects of matter (see gravitation) and how space and time change as seen by an observer on an accelerating object. The theory concludes that gravitational fields created by the presence of matter cause spacetime to become curved. This curvature controls the motion of bodies in space.

rel·a·tiv·i·ty / ˌreləˈtivətē/ • n. 1. the absence of standards of absolute and universal application: moral relativity. 2. Physics the dependence of various physical phenomena on relative motion of the observer and the observed objects, esp. regarding the nature and behavior of light, space, time, and gravity.

Relativity

See Gravitation; Relativity, General Theory of; Relativity, Special Theory of