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The single physical entity into which the concepts of space and time can be unified such that an event may be specified mathematically by four coordinates, three giving the position in space and one the time. The path of a particle in spacetime is called its world line. The world line links events in the history of the particle.

The concept of spacetime was used by Einstein in both the special and the general theories of relativity. In special theory, where only inertial frames are considered, spacetime is flat. In the presence of gravitational fields, treated by general relativity, the geometry of spacetime changes: it becomes curved. The rules of geometry in curved space are not those of three-dimensional Euclidean geometry.

Matter tells spacetime how to curve: massive objects produce distortions and ripples in the local spacetime. The question of whether the spacetime of the real Universe is curved, and in what sense, has yet to be resolved. If the Universe contains sufficient matter, i.e. if the mean density of the Universe is high enough, then the spacetime of the Universe must be bent round on itself and closed.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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