Definition
An event horizon is the boundary surface of a black hole: once a particle or light ray crosses inward, no causal signal it emits can ever reach the outside universe.
For a non-rotating Schwarzschild black hole, the event horizon is a sphere of radius r_s = 2GM/c² — the Schwarzschild radius. For one solar mass that is about 3 km; for a billion solar masses it is the size of the inner solar system.
Why it matters
How it works
In the Schwarzschild geometry, the event horizon is the locus where the escape velocity — computed naively — equals the speed of light. The deeper relativistic statement is that beyond this surface, the timelike and spacelike roles of coordinates flip: the singularity at r = 0 lies in the future of every interior observer, the way next Tuesday lies in your future. There is no spatial path that avoids it.
Light cones tip as gravity strengthens. Far from a black hole, light cones open symmetrically into past and future. Approaching the horizon, they tilt inward; at the horizon they tilt enough that no future-directed null ray points outward. Inside, every future-directed ray points toward the singularity.
For a Kerr (rotating) black hole there are two horizons — the outer event horizon and an inner Cauchy horizon — plus an ergosphere outside the event horizon where rotational frame-dragging forces co-rotation. Energy can be extracted from the rotation (the Penrose process) without crossing the event horizon itself.
The 2019 Event Horizon Telescope image of M87* and the 2022 image of Sagittarius A* are not photographs of the horizon itself — light cannot leave it — but of the "shadow" cast on the accretion disk behind: a dark region of radius about 2.6 Schwarzschild radii produced by the horizon's gravitational lensing.