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An event horizon is a boundary such that no information, including light, from beyond the boundary can cross it.
An event horizon is not merely a mathematical construct, but has physical consequences. For example, a person passing through an event horizon will in a way be dissected slice by slice. If, hypothetically, you put your hand through the event horizon, and retract your arm, your hand will effectively be chopped off. The measurement of Hawking radiation suggests that even virtual particle pairs that regularly spring from the vacuum of space, and normally annihilate quickly, can be separated by the event horizon.
From an outside observer, an object nearing an event horizon will appear redder and dimmer and will disappear at the moment the object passes through it.
An event horizon can form around a gravitational singularity, such as a black hole, where its gravitational pull exceeds the ability of any form of mass or energy, including light, to escape. The size of the event horizon of a black hole is determined by its Schwarzschild radius. Outside the event horizon, there is a region where light is bent because of the gravitational pull. However it has been demonstrated that a naked singularity where a gravitational singularity exists without an event horizon is possible.
Hypothetically, an event horizon can also exist in a universe, for an observer at a given location in space-time, who remains at the same comoving spatial position. When a universe expands quickly enough, for example a de Sitter universe, it can be possible for an event horizon to exist.
The event horizon is distinct from the particle horizon.
See also gravity, general relativity, black hole, gravitational singularity, particle horizon and quantum physics.
External link
Event Horizon is also a science fiction film.