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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
References in periodicals archive ?
Tipler, "Singularities in conformally flat spacetimes," Physics Letters A, vol.
These solutions (spacetimes) have been classified by using different spacetime symmetries.
Physically, for example, Minkowski, de-Sitter or anti-de-Sitter spacetimes do not admit an ACV.
In the coordinate free method of differential geometry the spacetime of general relativity is regarded as a connected four dimensional semi-Riemannian manifold ([M.sup.4], g) with Lorentz metric g with signature (-, +, +, +).
Keywords Embedding of Riemannian spacetimes, local and isometric embedding.
In 1949, Kurt Godel (1949) found a solution to the field equations of general relativity that described a spacetime with some unusual properties.
348.) However, the theorems of Hawking and Hawking and Penrose highlighted here are of far greater generality, applicable to a much wider range of spacetimes, and hence of greater significance.
In recent years, a sizeable literature has been devoted to exploring the physical consequences of assuming nontrivial commutation relations among spacetime coordinates.
General plane symmetric, cylindrically symmetric and spherically symmetric static spacetimes are consider for calculating the vacuum solutions of EFEs [1].
Closed timelike curves seem to be unavoidable within rotating black hole spacetimes, with potentially disturbing connotations for causality and hence physics at its most fundamental level.
Radinschi et al., along with various collaborators, have determined the energy-momentum distribution for Horava-Lifshitz black holes [14], stringy black holes [15, 16], charged black holes in generalized dilatonaxion gravity [17], asymptotic de Sitter spacetimes [18], and a Schwarzschild-quintessence spacetime [19].
After a brief review of general relativity, material is in sections on the 3+1 formalism, initial data, gauge conditions, hyperbolic reductions of the field equations, evolving black hole spacetimes, relativistic hydrodynamics, graviational wave extraction, numerical methods, and examples of numerical spacetimes.