Space and time are not separate. They are woven together into a four-dimensional continuum called spacetime. Einstein's special relativity showed that measurements of space and time depend on the observer's motion. General relativity showed that mass and energy curve spacetime, and that curvature is what we call gravity. Spacetime is not just a stage where events happen. It is a dynamic entity that responds to the matter and energy within it. That insight is one of the deepest in modern physics.

Spacetime has three spatial dimensions and one time dimension. An event is a point in spacetime, with coordinates for position and time. The distance between two events in spacetime is called the spacetime interval. It is invariant, meaning all observers agree on it, even if they disagree on the spatial and temporal components separately. That invariance is the mathematical heart of relativity. Spacetime can be flat or curved. Flat spacetime is described by special relativity. Curved spacetime is described by general relativity. The curvature is produced by mass and energy. A black hole is a region where spacetime curvature becomes infinite. Gravitational waves are ripples in spacetime, produced by accelerating masses. They were detected directly in 2015, a century after Einstein predicted them.

Spacetime facts

• Definition. Four-dimensional continuum of space and time.
• Dimensions. Three spatial, one temporal.
• Curvature. Produced by mass and energy.
• Gravity. Curvature of spacetime.
• Gravitational waves. Ripples in spacetime, detected 2015.

Spacetime is not the same as space. Space is three-dimensional. Spacetime includes time as a fourth dimension.