What is Spacetime?
[[File:GPB circling earth.jpg|thumb|280px|An illustration of the spacetime curvature caused by Earth.]] '''Spacetime''' or '''space–time''' is a [[mathematical model]] that joins [[Space (physics)|space]] and [[time]] into a single idea called a '''continuum'''. This four-dimensional continuum is known as [[Minkowski spacetime|Minkowski space]]. Combining these two ideas helped [[cosmology]] to understand how the universe works on the big level (e.g. [[galaxies]]) and small level (e.g. [[atom]]s). In non-relativistic [[classical mechanics]], the use of [[Euclidean space]] instead of space–time is good, because time is treated as universal with a constant rate of passage which is independent of the state of motion of an observer. But in a [[Theory of relativity|relativistic]] universe, time cannot be separated from the three dimensions of space. This is because the observed rate at which time passes depends on an object's [[velocity]] relative to the observer. Also, the strength of any [[gravitational field]] slows the passage of time for an object ''as seen by an observer outside the field''. == Further aspects == [[File:spacetime curvature.png|thumb|right|250px|Two-dimensional analogy of spacetime distortion]] Wherever [[matter]] exists, it bends the [[geometry]] of spacetime. This results in a curved shape of spacetime which can be understood as [[gravity]]. The white lines on the picture on the right represent the effect of [[mass]] on spacetime. In [[classical mechanics]], the use of spacetime is optional, as time is independent of motion in the three dimensions of Euclidean space. However, when a body is moving at speeds close to the [[speed of light]] ([[theory of relativity|relativistic speeds]]), time cannot be separated from the three dimensions of space. Time, from the point of view of a stationary observer,<ref>Einstein, Albert and Infeld, Leopold 1938. ''The evolution of physics: from early concept to relativity and quanta''. Cambridge University Press.</ref> depends on how close to the spe