What spacetime is the flat, gravity-free model used in special relativity?

Minkowski spacetime is the flat, gravity-free model used in special relativity. It combines three spatial dimensions with one time dimension and provides the geometric setting in which inertial observers describe motion, light, and events.

Its defining feature is the Minkowski metric, which gives the invariant spacetime interval between two events. Observers in different inertial frames may disagree about distances, durations, and simultaneity, but they agree on this interval. Lorentz transformations connect those frames while preserving the interval and the measured speed of light.

“Gravity-free” means that the model has zero spacetime curvature, not that no objects or fields can exist in it. Special relativity uses this flat geometry; general relativity extends the framework to curved spacetime when gravity is represented geometrically. Locally, however, curved spacetime resembles Minkowski spacetime at sufficiently small scales.

Hermann Minkowski reformulated special relativity as a four-dimensional spacetime theory in 1908. Schwarzschild and Kerr spacetimes are curved solutions associated with gravitating objects, while de Sitter spacetime is a curved cosmological solution, so they are not the standard flat background of special relativity.