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space-time, central concept in the theory of relativity that replaces the earlier concepts of space and time as separate absolute entities. In relativity one cannot uniquely distinguish space and time as elements in descriptions of events. Space and time are joined together in an intimate combination in which time becomes the "fourth dimension." The mathematical formulation of the theory by H. Lorentz (see Lorentz contraction) preceded the interpretation by A. Einstein that space and time are not absolute. The abstract description of space-time was made by H. Minkowski. In space-time, events in the universe are described in terms of a four-dimensional continuum in which each observer locates an event by three spacelike coordinates (position) and one timelike coordinate. The choice of the timelike coordinate in space-time is not unique; hence, time is not absolute but is relative to the observer. A striking consequence is that simultaneity is no longer an intrinsic relation between two events; it exists only as a relation between two events and a particular observer. In general, events at different locations that are simultaneous for one observer will not be simultaneous for another observer. Other relativistic effects, such as the Lorentz contraction and time dilation, are due to the structure of space-time.

See E. F. Taylor and J. A. Wheeler, Spacetime Physics (1966); N. D. Mermin, Space and Time in Special Relativity (1968).

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space-time In relativity theory, a central concept that unifies the three space dimensions (length, breadth, and height) with time to form a four-dimensional frame of reference. Durations and rates of processes depend on the relative state of motion of the observer and the system observed. In 1907, Hermann Minkowski clarified relativity theory by describing space-time in terms of a four-dimensional geometry. Three co-ordinates of space and a time co-ordinate specify an event in space-time. A line drawn in this space represents a particle's path both in space and time. Albert Einstein incorporated this viewpoint into his theory of relativity: in the General Theory, gravity is a distortion of space-time by matter.

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