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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. 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. Note the dependency on the observer of the concept of event horizon. For example, a free falling observer toward a black hole does not experience an event horizon (see e.g. catastrophic gravitational collapse).

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 is distinct from the particle horizon.

1 Sticking your hand through an event horizon

One can 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, this figure is 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 infinitely high, so his hand will be chopped off before he manages to do so.

The physical consequences of the previous paragraph are drawn by Stephen Hawking. Everywhere in the vacuumThe article on the vacuum cleaner is located elsewhere. In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also of space virtual particle pairsIn the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. A virtual particle is never the end result of suc are created and annihilateAnnihilation occurs when a particle collides with an antiparticle. It transforms mass into energy of some form. The energy is carried by force carriers, particles that can decay into other particles. It means that, contrary to a popular belief, the annihi quickly. Near an event horizon, they can be separated. Effectively, a particle or photon will be emitted from the horizon, the so-called Hawking radiationIn physics, Hawking radiation is thermal radiation emitted by black holes due to quantum effects. It is named after British physicist Stephen Hawking who worked out the theoretical argument for its existence in 1976. Hawking's discovery became the first c.

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 asymptoticallySee also Asymptotic analysis but contrast asymptotic curve''. An asymptote to a curve is a straight line that the curve approaches in such a manner that it becomes as close as one might wish to the line by going far enough along the line. A "real-life" ex independent of the initial state. Thus the total path integralThis article deals with the concept of an integral in mathematical calculus. For other meanings of "integral" see integration. In calculus, the integral of a function is a generalization of area, mass, volume, sum, and total. Unlike the process of differe 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.