spacetime

Spacetime is the four-dimensional continuum that combines the three spatial dimensions with time into a single manifold on which physical events take place. Each event is specified by coordinates (t, x, y, z), and the separation between events is measured by a spacetime interval that remains invariant under coordinate changes.

Special relativity treats spacetime as a flat four-dimensional stage where the speed of light is constant and the laws of physics are the same for all observers. The spacetime interval between events is invariant, leading to phenomena such as time dilation, length contraction, and the relativity of simultaneity.

In general relativity, matter and energy curve spacetime, and gravity is the effect of this curvature. Free-falling matter follows geodesics, and gravitational effects include time dilation in strong fields, gravitational redshift, and light deflection (lensing).

In cosmology, spacetime is dynamic and can expand. The large-scale structure is described by the Friedmann-Lemaitre-Robertson-Walker metric with a time-dependent scale factor, capturing the expansion of space and the observed redshift of distant galaxies.

Spacetime underpins modern physics, from GPS timing corrections to gravitational wave detection and models of the early universe. Ongoing work seeks to unify spacetime with quantum theory in a quantum gravity framework.