event horizon: a standalone construct

Karl Schwarzschild's 1916 solution of Einstein's field equations for a spherical mass first exhibited the surface, at what is now called the Schwarzschild radius, that would later be understood as the boundary of a black hole, though Schwarzschild himself did not describe it as a horizon. Wolfgang Rindler coined the term event horizon in 1956 while classifying the limiting surfaces of cosmological world models, naming the boundary beyond which events can never affect a given observer. In the theory of gravitational collapse the event horizon is the one-way causal boundary of a black hole: light and matter can cross it inward, but nothing inside, not even light, can cross back out to reach an outside observer, so events within it are permanently hidden from anyone remaining outside. Roger Penrose's 1965 proof that gravitational collapse forces a spacetime singularity, a place where the curvature of spacetime grows without bound and the known physical laws no longer apply, together with the global geometric framework set out by Stephen Hawking and George Ellis in 1973, established the horizon as a feature of the spacetime geometry itself rather than an artefact of a particular coordinate choice. The same name is shared, as a cross-reference only, by a 1997 science-fiction film, which is not treated as a construct here.[1][2][3][4]

Unit

A boundary surface in a spacetime model, the null surface enclosing the region from which no signal can reach a distant observer

Recovery means

Not applicable

Measured by

Located in a spacetime model as the boundary of the causal past of future null infinity, and studied observationally through the imaged shadow of a black hole, gravitational-wave ringdown, and the orbital dynamics of matter near compact objects