# space-time

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## space-time,

central concept in the theory of relativity**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.

**.....**Click the link for more information. that replaces the earlier concepts of space and time

**time,**

sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.

**.....**Click the link for more information. 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

**Lorentz contraction**

, in physics, contraction or foreshortening of a moving body in the direction of its motion, proposed by H. A. Lorentz on theoretical grounds and based on an earlier suggestion by G. F.

**.....**Click the link for more information. ) 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).

## Space-time

A term used to denote the geometry of the physical universe as suggested by the theory of relativity. It is also called space-time continuum. Whereas in Newtonian physics space and time had been considered quite separate entities, A. Einstein and H. Minkowski showed that they are actually intimately intertwined.

Einstein showed that in general two observers, each using the same techniques of observation but being in motion relative to each other, will disagree concerning the simultaneity of distant events. But if they do disagree, they are also unable to compare unequivocally the rates of clocks moving in different ways, or the lengths of scales and measuring rods. Instead, clock rates and scale lengths of different observers and different frames of reference must be established so as to assure the principal observed fact. Each observer, using his or her own clocks and scales, must measure the same speed of propagation of light. This requirement leads to a set of relationships known as the Lorentz transformations.

In accordance with the Lorentz transformations, both the time interval and the spatial distance between two events are relative quantities, depending on the state of motion of the observer who carries out the measurements. There is, however, a new absolute quantity that takes the place of the two former quantities. It is known as the invariant, or proper, space-time interval τ and is defined by Eq. (1), where *T* is the ordi

*R*the distance between the two events, and

*c*the speed of light in empty space. Whereas

*T*and

*R*are different for different observers, τ has the same value. In the event that Eq. (1) would render τ imaginary, its place may be taken by σ, defined by Eq. (2). If both τ and σ are zero,

The existence of a single invariant interval led the mathematician Minkowski to conceive of the totality of space and time as a single four-dimensional continuum, which is often referred to as the Minkowski universe. In this universe, the history of a single space point in the course of time must be considered as a curve (or line), whereas an event, limited both in space and time, represents a point. So that these geometric concepts in the Minkowski universe may be distinguished from their analogs in ordinary three-dimensional space, they are referred to as world curves (world lines) and world points, respectively. *See* Gravitation, Relativity

## space-time

[′spās ′tīm]## space-time

,**space-time continuum**

*Physics*the four-dimensional continuum having three spatial coordinates and one time coordinate that together completely specify the location of a particle or an event