en.wikipedia.org/wiki/Spac..._continuum

In physics, spacetime is a mathematical model that combines three-dimensional space and one-dimensional time into a single construct called the space-time continuum, in which time plays the role of the 4th dimension. According to Euclidean space perception, our universe has three dimensions of space, and one dimension of time. By combining space and time into a single manifold, physicists have significantly simplified a good deal of physical theory, as well as described in a more uniform way the workings of the universe at both the supergalactic and subatomic levels.

In classical mechanics, spacetime is a mere formal option, but in special relativity, space and time are inseparable. The notion of space depends on the observer, as instantaneous events depend on a reference frame. Spacetime is also vital to general relativity, an extension of special relativity that takes gravitation into account.

How many dimensions are needed to describe the universe is still an open question. Speculative theories (such as string theory) predict from 10 to 26 dimensions, but all dimensions other than the usual 4 (3 spatial and 1 temporal) matter only at subatomic scales.

Basic concepts

Spacetimes are the arenas in which all physical events take place — for example, the motion of planets around the Sun may be described in a particular type of spacetime, or the motion of light around a rotating star may be described in another type of spacetime. The basic elements of spacetime are events. In any given spacetime, an event is a unique position at a unique time. Examples of events include the explosion of a star or the single beat of a drum.

A space-time is independent of any observer. However, in describing physical phenomena (which occur at certain moments of time in a given region of space), each observer chooses a convenient coordinate system. Events are specified by four real numbers in any coordinate system. The worldline of a particle or light beam is the path that this particle or beam takes in the spacetime and represents the history of the particle or beam. The world line of the orbit of the Earth is depicted in two spatial dimensions x and y (the plane of the Earth orbit) and a time dimension orthogonal to x and y. The orbit of the Earth is an ellipse in space alone, but its worldline is a Helix in spacetime.

The unification of space and time is exemplified by the common practice of expressing distance in units of time, by dividing time measurements by the speed of light.
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Space-time intervals

Spacetime entails a new concept of distance. Whereas distances are always positive in Euclidean spaces, the distance between any two events in spacetime (called an "interval") may be real, zero, or even imaginary. The spacetime interval quantifies this new distance (in Cartesian coordinates x,y,z,t):

s^2 = \, r^2 - c^2t^2

where c is the speed of light, differences of the space and time coordinates of the two events are denoted by r and t, respectively and r2 = x2 + y2 + z2.

Pairs of events in spacetime may be classified into 3 distinct types based on 'how far' apart they are:

* time-like (more than enough time passes for there to be a cause-effect relationship between the two events; s2 < 0).
* light-like (the space between the two events is exactly balanced by the time between the two events; s2 = 0).
* space-like (not enough time passes for there to be a cause-effect relationship between the two events; s2 > 0).

Events with a negative space-time interval are in each other's future or past, and the value of the interval defines the proper time measured by an observer traveling between them. Events with a spacetime interval of zero are separated by the propagation of a light signal.

Certain types of worldlines (called geodesics of the space-time), are the shortest paths between any two events, with distance being defined in terms of space-time intervals. The concept of geodesics becomes critical in general relativity, since geodesic motion may be thought of as "pure motion" (inertial motion) in space-time, that is, free from any external influences.
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