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An event horizon is a boundary in spacetime for a given observer beyond which no information, including light, can reach the observer. The most famous example is a black hole, which for a distant and stationary observer (such as someone at Earth) is surrounded by an event horizon: a spherical surface located at the Schwarzschild radius (also called gravitational radius or radius of a black hole). Light emitted from inside the event horizon will never reach a stationary observer outside the horizon, hence the name black hole.

Event horizons also exist in the absence of gravity. A simple example is a particle undergoing uniform acceleration (and whose speed will thus approach the speed of light). Light emitted at a certain distance in the direction of that particle will never reach the accelerated particle. It is beyond the event horizon for that particle. Such event horizons occur in particle accelerators.

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. 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

• Metrics: distances in a relativistic Universe

Event Horizon is also a science fiction film.