Spacetime

In <a href="http://en.wikipedia.org/wiki/Physics" title="Physics" >physics</a>, spacetime (or space-time, space time, space-time continuum) is any <a href="http://en.wikipedia.org/wiki/Mathematical_model" title="Mathematical model" >mathematical model</a> that combines <a href="http://en.wikipedia.org/wiki/Space" title="Space" >space</a> and <a href="http://en.wikipedia.org/wiki/Time_in_physics" title="Time in physics" >time</a> into a single <a href="http://en.wikipedia.org/wiki/Continuum_(theory)" title="Continuum (theory)" >continuum</a>.

Spacetime is usually interpreted with space as being <a href="http://en.wikipedia.org/wiki/Three-dimensional_space" title="Three-dimensional space" >three-dimensional</a> and time playing the role of a <a href="http://en.wikipedia.org/wiki/Minkowski_space" title="Minkowski space" >fourth dimension</a> that is of a different sort from the spatial dimensions. From a <a href="http://en.wikipedia.org/wiki/Euclidean_space" title="Euclidean space" >Euclidean space</a> perspective, the <a href="http://en.wikipedia.org/wiki/Universe" title="Universe" >universe</a> has three <a href="http://en.wikipedia.org/wiki/Dimension" title="Dimension" >dimensions</a> of space and one dimension of time. By combining space and time into a single <a href="http://en.wikipedia.org/wiki/Manifold" title="Manifold" >manifold</a>, physicists have significantly simplified a large number of <a href="http://en.wikipedia.org/wiki/Theoretical_physics" title="Theoretical physics" >physical theories</a>, as well as described in a more uniform way the workings of the universe at both the <a href="http://en.wikipedia.org/wiki/Physical_cosmology" title="Physical cosmology" >supergalactic</a>and <a href="http://en.wikipedia.org/wiki/Quantum_mechanics" title="Quantum mechanics" >subatomic</a> levels.

In non-relativistic <a href="http://en.wikipedia.org/wiki/Classical_mechanics" title="Classical mechanics" >classical mechanics</a>, the use of Euclidean space instead of spacetime is appropriate, as time is treated as universal and constant, being independent of the state of motion of an observer.

In <a href="http://en.wikipedia.org/wiki/Theory_of_relativity" title="Theory of relativity" >relativistic</a> contexts, time cannot be separated from the three dimensions of space, because the observed rate at which time passes for an object depends on the object's <a href="http://en.wikipedia.org/wiki/Velocity" title="Velocity" >velocity</a> relative to the observer and also on the strength of <a href="http://en.wikipedia.org/wiki/Gravitational_fields" title="Gravitational fields" class="mw-redirect" >gravitational fields</a>, which can slow the passage of time.

In <a href="http://en.wikipedia.org/wiki/Cosmology" title="Cosmology" >cosmology</a>, the concept of spacetime combines space and time to a single abstract<a href="http://en.wikipedia.org/wiki/Universe" title="Universe" >universe</a>. Mathematically it is a <a href="http://en.wikipedia.org/wiki/Manifold" title="Manifold" >manifold</a> consisting of "events" which are described by some type of <a href="http://en.wikipedia.org/wiki/Coordinate_system" title="Coordinate system" >coordinate system</a>. Typically three spatial dimensions (length, width, height), and one temporal dimension (<a href="http://en.wikipedia.org/wiki/Time" title="Time" >time</a>) are required.

Dimensions are independent components of a coordinate grid needed to locate a point in a certain defined "space". For example, on the globe the <a href="http://en.wikipedia.org/wiki/Latitude" title="Latitude" >latitude</a> and <a href="http://en.wikipedia.org/wiki/Longitude" title="Longitude" >longitude</a> are two independent coordinates which together uniquely determine a location. In spacetime, a coordinate grid that spans the 3+1 dimensions locates <a href="http://en.wikipedia.org/wiki/Event_(relativity)" title="Event (relativity)" >events</a> (rather than just points in space), i.e. time is added as another dimension to the coordinate grid. This way the coordinates specify where and when events occur. However, the unified nature of spacetime and the freedom of coordinate choice it allows imply that to express the temporal coordinate in one coordinate system requires both temporal and spatial coordinates in another coordinate system. Unlike in normal spatial coordinates, there are still restrictions for how measurements can be made spatially and temporally (see <a href="http://en.wikipedia.org/wiki/Spacetime#Spacetime_intervals" >Spacetime intervals</a>). These restrictions correspond roughly to a <a href="http://en.wikipedia.org/wiki/Lorentzian_manifold#Lorentzian_manifold" title="Lorentzian manifold" class="mw-redirect" >particular mathematical model</a> which differs from Euclidean space in its manifest <a href="http://en.wikipedia.org/wiki/Symmetry" title="Symmetry" >symmetry</a>.

Until the beginning of the 20th century, time was believed to be independent of motion, progressing at a fixed rate in all <a href="http://en.wikipedia.org/wiki/Frame_of_reference" title="Frame of reference" >reference frames</a>; however, later experiments revealed that time slowed down at higher speeds of the reference frame relative to another reference frame (with such slowing called "<a href="http://en.wikipedia.org/wiki/Time_dilation" title="Time dilation" >time dilation</a>" explained in the theory of "<a href="http://en.wikipedia.org/wiki/Special_relativity" title="Special relativity" >special relativity</a>"). Many experiments have confirmed time dilation, such as <a href="http://en.wikipedia.org/wiki/Atomic_clock" title="Atomic clock" >atomic clocks</a> onboard a <a href="http://en.wikipedia.org/wiki/Space_Shuttle" title="Space Shuttle" >Space Shuttle</a> running slower than synchronized Earth-bound inertial clocks and the relativistic <a href="http://en.wikipedia.org/wiki/Particle_decay" title="Particle decay" >decay</a> of <a href="http://en.wikipedia.org/wiki/Muon" title="Muon" >muons</a> from <a href="http://en.wikipedia.org/wiki/Cosmic_ray" title="Cosmic ray" >cosmic ray</a> showers. The duration of time can therefore vary for various events and various <a href="http://en.wikipedia.org/wiki/Frame_of_reference" title="Frame of reference" >reference frames</a>.

When dimensions are understood as mere components of the grid system, rather than physical attributes of space, it is easier to understand the alternate dimensional views as being simply the result of <a href="http://en.wikipedia.org/wiki/Coordinate_transformation" title="Coordinate transformation" class="mw-redirect" >coordinate transformations</a>.

The term spacetime has taken on a generalized meaning beyond treating spacetime events with the normal 3+1 dimensions. It is really the combination of space and time. Other proposed spacetime theories include additional dimensions—normally spatial but there exist some speculative theories that include additional temporal dimensions and even some that include dimensions that are neither temporal nor spatial. How many dimensions are needed to describe the universe is still an open question. Speculative theories such as <a href="http://en.wikipedia.org/wiki/String_theory" title="String theory" >string theory</a> predict 10 or 26 dimensions (with <a href="http://en.wikipedia.org/wiki/M-theory" title="M-theory" >M-theory</a> predicting 11 dimensions: 10 spatial and 1 temporal), but the existence of more than four dimensions would only appear to make a difference at the <a href="http://en.wikipedia.org/wiki/Subatomic" title="Subatomic" class="mw-redirect" >subatomic</a> level.