Spacetime

A grid of blue squares on a white background, like a sheet of fabric. Yellow spheres sit on the grid, causing curving of the material.
The 4-D space-time continuum is distorted in the vicinity of any mass, depending on the mass and the distance from the mass. Encyclopaedia Britannica/UIG

Spacetime is a four-dimensional coordinate system containing three spatial dimensions and one time dimension. The behavior of this four-dimensional spacetime represents the physical basis for the force of gravity in modern physics, as articulated in Einstein's theory of relativity

Origins of Spacetime

The concept of a spacetime coordinate system was introduced in 1907, two years after Albert Einstein originally proposed the theory of special relativity.

Hermann Minkowski, a former professor of Einstein, presented the idea of this spacetime coordinate system. The ideas were inherent in Einstein's version of the theory, but he hadn't thought of it that way. In a 1908 talk called "Space and Time," Minkowski elaborated on these concepts and they began to gain popularity.

The key insight that Minkowski offered was to think of Einstein's approach as being geometric. Einstein was already trying to articulate his theory of special relativity into a more general theory of relativity that would account for gravity, having realized in 1907 that someone in free fall didn't actually feel gravity. (The story goes that this insight came when he saw a painter fall off of a roof, but this may be apocryphal.)

Using these concepts that Minkowski contributed, Einstein began to think of the universe in terms of masses embedded within the fluid fabric of spacetime.

He realized that these massive objects from stars to planets to everything else would not be passive, but would interact with and cause distortions in the physical structure of the spacetime. This physical structure, though, could not be explained in terms of Euclidean geometry, but fortunately the non-Euclidean geometric approaches that had been developed in the previous centuries (for pure mathematics, without any belief they would have any physical application) turned out to be exactly what Einstein needed.

Physical objects move through spacetime along the most energy-efficient path, just as an object being thrown under classical physics will always move along the most energy-efficient path. Because of the curves in spacetime around massive objects, like the sun, the most energy-efficient path is the elliptical path that matches what is described by Newton's law of universal gravitation. 

Spacetime and the Expanding Universe

In modern cosmology, it's possible for spacetime to expand. Einstein noticed this in his original equations for general relativity, but thought that expanding space made no sense, so he inserted a correction factor called the cosmological constant into his equations, which balanced the expansion to result in a static and eternal universe. Years later, Edwin Hubble discovered that space was actually expanding. The modern theory of inflation actually indicates that space was once accelerating faster than the speed of light ... nothing in space can move faster than the speed of light, but space itself can move faster.

Also Known As: spacetime coordinates, Minkowski space

Alternate Spellings: space-time