space-time

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.

Bibliography

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

Single entity that relates space and time in a four-dimensional structure, postulated by Albert Einstein in his theories of relativity. In the Newtonian universe it was supposed that there was no connection between space and time. Space was thought to be a flat, three-dimensional arrangement of all possible point locations, which could be expressed by Cartesian coordinates; time was viewed as an independent one-dimensional concept. Einstein showed that a complete description of relative motion requires equations that include time as well as the three spatial dimensions. He also showed that space-time is curved, which allowed him to account for gravitation in his general theory of relativity.