| Revision as of 12:53, 27 April 2004 editJheise (talk | contribs)354 edits refining "sticking your hand through an event horizon"← Previous edit | Revision as of 12:59, 27 April 2004 edit undoJheise (talk | contribs)354 editsm typosNext edit → | ||
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| An '''event horizon''' is a boundary in ] for a given observer beyond which no information, including light, can reach the observer. The most famous example is a ], which for a distant and stationary observer (such as someone at Earth) is surrounded by an event horizon |
An '''event horizon''' is a boundary in ] for a given observer beyond which no information, including light, can reach the observer. The most famous example is a ], which for a distant and stationary observer (such as someone at Earth) is surrounded by an event horizon. It is a spherical surface located at the ] (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. | Light emitted from inside the event horizon will never reach a stationary observer outside the horizon, hence the name black hole. | ||
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| The event horizon for an outside observer really acts as a horizon. He sees an object falling toward the horizon approaching it, but (in his own proper time) never reaching it. In his observations the object goes slower and slower toward the horizon and at the same time the redshift increases byond bounds to infinity. Also the intensity of the falling object quicky becomes zero. In a finite time the outside observer will receive the last photon from the falling object. He will never see the falling object passing through the event horizon. | The event horizon for an outside observer really acts as a horizon. He sees an object falling toward the horizon approaching it, but (in his own proper time) never reaching it. In his observations the object goes slower and slower toward the horizon and at the same time the redshift increases byond bounds to infinity. Also the intensity of the falling object quicky becomes zero. In a finite time the outside observer will receive the last photon from the falling object. He will never see the falling object passing through the event horizon. | ||
| One can hypothetically ask what happens, when a stationary observer is in orbit just outside the event horizon and (against all advice) sticks his hand through the horizon? The answer is: he won't succeed in doing so. Free orbits are only possible at a certain distance (for a non-rotating black hole at at least three times the Schwarzschild radius). Near the event horizon, an observer can only remain at a constant radius when he uses a force (e.g. from a rocket) to keep him there. The force needed, grows to infinity when the observer wants to maintain a steady constant orbit approaching the event horizon. When he sticks out his hand, the tidal force (the difference |
One can hypothetically ask what happens, when a stationary observer is in orbit just outside the event horizon and (against all advice) sticks his hand through the horizon? The answer is: he won't succeed in doing so. Free orbits are only possible at a certain distance (for a non-rotating black hole at at least three times the Schwarzschild radius). Near the event horizon, an observer can only remain at a constant radius when he uses a force (e.g. from a rocket) to keep him there. The force needed, grows to infinity when the observer wants to maintain a steady constant orbit approaching the event horizon. When he sticks out his hand, the tidal force (the difference between body and hand along his arm) also becomes infinitly high, so his hand will chopped off before he manages to do so. | ||
| The physical consequences of the previous paragraph are drawn by ]. Everywhere in the vacuum of space ] are created and ] quickly. Near an event horizon they can be separated. Effectively, a particle or photon will be emitted from the horizon, the so called ]. | The physical consequences of the previous paragraph are drawn by ]. Everywhere in the vacuum of space ] are created and ] quickly. Near an event horizon they can be separated. Effectively, a particle or photon will be emitted from the horizon, the so called ]. | ||
| Hypothetically, an event horizon can also exist in a ], for an observer at a given location in ], who remains at the same ] spatial position. When a universe expands quickly enough, for example a ], it can be possible for an event horizon to exist. | Hypothetically, an event horizon can also exist in a ], for an observer at a given location in ], who remains at the same ] spatial position. When a universe expands quickly enough, for example a ], it can be possible for an event horizon to exist. | ||
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| The '''event horizon''' is distinct from the ]. | The '''event horizon''' is distinct from the ]. | ||
| See also ], ], ], ], ], ] and ]. | See also ], ], ], ], ], ], ] and ]. | ||
| ==External link== | ==External link== | ||
Revision as of 12:59, 27 April 2004
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. It is 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 uniform accelerated particle (and whose speed will thus approach the speed of light but will always be smaller). 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.
The event horizon for an outside observer really acts as a horizon. He sees an object falling toward the horizon approaching it, but (in his own proper time) never reaching it. In his observations the object goes slower and slower toward the horizon and at the same time the redshift increases byond bounds to infinity. Also the intensity of the falling object quicky becomes zero. In a finite time the outside observer will receive the last photon from the falling object. He will never see the falling object passing through the event horizon.
One can hypothetically ask what happens, when a stationary observer is in orbit just outside the event horizon and (against all advice) sticks his hand through the horizon? The answer is: he won't succeed in doing so. Free orbits are only possible at a certain distance (for a non-rotating black hole at at least three times the Schwarzschild radius). Near the event horizon, an observer can only remain at a constant radius when he uses a force (e.g. from a rocket) to keep him there. The force needed, grows to infinity when the observer wants to maintain a steady constant orbit approaching the event horizon. When he sticks out his hand, the tidal force (the difference between body and hand along his arm) also becomes infinitly high, so his hand will chopped off before he manages to do so.
The physical consequences of the previous paragraph are drawn by Stephen Hawking. Everywhere in the vacuum of space virtual particle pairs are created and annihilate quickly. Near an event horizon they can be separated. Effectively, a particle or photon will be emitted from the horizon, the so called Hawking radiation.
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, Schwarzschild metric, gravitational singularity, naked singularity, particle horizon and quantum physics.
External link
Event Horizon is also a science fiction film.