for: Event Horizon (film) for: Event Horizon (sculpture) In general relativity, an event horizon is a boundary in spacetime, an area surrounding a black hole or a wormhole, inside which events cannot affect an outside observer. Light emitted from inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side disappears.

More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Reissner-Nordström solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.

Event horizon of a black hole

main: Black hole

The most commonly known example of an event horizon is defined around general relativity's description of a black hole, a celestial object so dense that no matter or radiation can escape its gravitational field. This is sometimes described as the boundary within which the black hole's escape velocity is greater than the speed of light. An alternate description is that within this horizon, all lightlike paths (paths that light could take), and hence all paths in the forward light cones of particles within the horizon, are warped so as to fall further into the hole. Once a particle is inside the horizon, moving into the hole is as inevitable as moving forward in time (and can actually be thought of as equivalent to doing so, depending on the spacetime coordinate system used).

The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body that fits inside this radius. (A rotating black hole operates slightly differently.) The Schwarzschild radius of an object is proportional to the mass. For the mass of the Sun it is approximately 3 km, and for that of the Earth about 9 mm. For a black hole created by the collapse of a star (which has a mass above the Chandrasekhar limit) the lower limit is about 4 km.

Black hole event horizons are especially noteworthy for three reasons. First, there are many examples near enough to study. Second, black holes tend to pull in matter from their environment, which provides examples where matter passing through an event horizon is expected to be observable. Third, the description of black holes given by general relativity is known to be an approximation, and it is expected that quantum gravity effects become significant near the vicinity of the event horizon. This allows observations of matter in the vicinity of a black hole's event horizon to be used to indirectly study general relativity and proposed extensions to it.

The definition of "event horizon" given by Hawking & Ellis, Misner, Thorne & Wheeler, and Wald differs from the one presented here. Their definition rules out the cosmological and particle horizons presented below (as well as the apparent horizon). However, modern usage has brought those ideas under the umbrella of the term "event horizon". (See, e.g.,.) To make the distinction clearer, some authors refer to their more specific notion of a horizon as an "absolute horizon". In the context of black holes, event horizon almost always refers to the absolute horizon, as distinct from the apparent horizon.