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Spacetime is a model in physics that joins the three dimensional space and one dimensional time into the idea of space-time continuum. Combining these two ideas helped physicists to make many laws of physics easier to understand, and to explain how the universe works on the big level (e.g., stars) and small level (e.g., atoms).
The actual number of dimensions in spacetime is not fixed, but usually spacetime means a four dimensional (three dimensions of space and one dimension of time) spacetime. Some other theories claim that there are more than four dimensions.
Historical origin [change]
Many people link spacetime with Albert Einstein who found special relativity. However, it was Einstein's teacher, Hermann Minkowski, who suggested this idea. He hoped this new idea would clarify the theory of special relativity. It was Einstein, though, who discovered the curvature of spacetime (gravity) in general relativity.
Spacetime in Special Relativity [change]
The idea of spacetime originated from the concept of Minkowski spacetime. Minkowski spacetime is a 4-dimensional manifold that is the arena of every event in the universe. To the right, a 3-dimensional analogy of 4-dimensional Minkowski spacetime is given. Time is taken to be the vertical axis, and the two spacial dimensions are the forward-backwards and left-right axes. In reality, there are four dimensions, but this concept is virtually impossible to visualize.
Minkowski spacetime is only accurate at describing constant velocity; this means the velocity cannot change at any period in time (acceleration is dealt with in general relativity). Minkowski spacetime also allows for visualizations of the Lorentz transformations, time dilation, and Lorentz contraction without the algebraic equations.
Spacetime in general Relativity [change]
In general relativity, Einstein generalized Minkowski spacetime to include the effects of acceleration. Einstein discovered that the curvature in his 4-dimensional spacetime representation was actually the cause of gravity. If you graph accelerated motion on a typical x-and-time coordinate system, you will end up with some form of curve; never a straight line. Einstein felt that this was a deep connection between the nature of spacetime and acceleration. Via his equivalence principle, he showed that a curve in generalized spacetime would result in the effects of gravity.