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For the science fiction film, see Event Horizon (film) An event horizon is a boundary in Spacetime at which the escape velocity for a given mass reaches and then exceeds the speed of light, making escape impossible. This makes the observation of events within (or any other type of communication) also impossible, hence "event horizon." Light emitted from inside an event horizon will never reach a stationary observer outside the horizon, hence the name Black Hole . An observer, free falling toward a black hole witnesses and experiences an event horizon differently from an outside observer (e.g. see catastrophic Gravitational Collapse ), but will inevitably experience the impo. While the appearance of the event horizon is relative and depends on the observer, its location around a particular black hole is the same for any observer. 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 beyond bounds to Infinity . Also the intensity of the falling object quickly 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 gets its name because information from events that occur within can never reach the outside observer. The event horizon is distinct from the Particle Horizon . STICKING YOUR HAND THROUGH AN EVENT HORIZON One can ask what happens, when a stationary observer is in ). 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 in gravity between body and hand along his arm) also becomes infinitely high, so his hand will be severed immediately. This assumes, of course, you wish to remain outside the black hole. If you are willing to fall into the black hole, things are much less dramatic. For such an observer the event horizon is simply the point at which escape to the outside is no longer possible. The physical consequences of the first paragraph are drawn by Stephen Hawking . Everywhere in the Vacuum of space Virtual Particle Pairs are created and Annihilated 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 . Recently, however, Stephen Hawking has reversed his position regarding black holes, having claimed that an event horizon never actually forms around a black hole. "The Euclidean path integral over all Topologically trivial metrics can be done by Time Slicing and so is unitary when analytically continued to the Lorentzian . On the other hand, the path integral over all topologically non-trivial metrics is Asymptotically independent of the initial state. Thus the total path Integral is unitary and information is not lost in the formation and Evaporation of Black Holes . The way the information gets out seems to be that a true event horizon never forms, just an apparent horizon." EVENT HORIZON IN THE ABSENCE OF GRAVITY Event horizons also exist in the absence of Gravity . A simple example is a ''uniform accelerated particle'' (whose Speed will thus eventually 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 Accelerator s. A part of spacetime forms an event horizon as observed from a constantly accelerated observer. The World Line of the observer is given as the solid Curve in a two dimensional spacetime representation with time ''x''0 in the vertical direction and a one dimensional space coordinate ''x''3 to the right. An angle of 45° indicates the speed of light, such as the world line of a photon traveling to the right and starting in ''a''. The world line of the observer is described by a Hyperbola . The parameter along his path is τ, his proper time. In 0 his speed is zero and eventually he will reach a speed close to the velocity of light, inclined at an angle of 45 degrees. This asymptotic line is his ''future event horizon''. A photon emitted at any event to the left of it (such as the emission of a photon from event '''a''') will never reach him (as long as the observer maintains a constant acceleration). If someone at constant zero Velocity (a static observer with a vertical line as worldline) would emit photons to the right, then the accelerated observer would see all photons below the event horizon, but in his proper time it would take longer and longer when these photons are emitted closer to the horizon. Also they are more and more redshifted. The accelerated observer would never see the static observer pass the event horizon. OTHER EXAMPLES OF AN EVENT HORIZON 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. EQUATION OF EVENT HORIZON FOR UNIVERSE MODELS where ''a'' is the Scale Factor , ''c'' is the Speed Of Light , and ''t0'' is the Present Age Of The Universe . If , there is an event horizon, but if it goes to infinite then there is no event horizon for that model of the universe. For example, a Matter Dominated Universe ( Einstein-de Sitter Universe ) or a Radiation Dominated Universe would have no event horizon, but a Cosmological Constant dominated universe ( De Sitter Universe ), possibly what we're living in now, would have an event horizon at ''ct0'', i.e. the distance light has traveled from the beginning of the universe. SEE ALSO
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