Spacetime

Spacetime is the four-dimensional continuum combining three spatial dimensions and one time dimension, in which all physical events occur. Special relativity (Einstein, 1905) demonstrated that space and time are not independent: the measured distance and duration between two events depend on the observer’s state of motion, but the spacetime interval between them does not. This interval – roughly, the difference between squared time separation and squared spatial separation – is the invariant quantity that all observers agree on.

General relativity (Einstein, 1915) goes further: spacetime is not a fixed stage on which physics plays out. It is a dynamic entity that curves in response to mass and energy, and whose curvature directs the motion of matter. A universe described by general relativity is one in which spacetime itself can expand, contract, ripple, and warp.

In cosmology, the central object of study is the large-scale geometry and evolution of spacetime itself. The question “is the universe expanding?” is a question about spacetime: are the spatial distances between galaxies growing over time? The answer is yes, and the dynamics of that growth are governed by general relativity.