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In astrophysics, an event horizon is a boundary beyond which events cannot affect an outside observer. Wolfgang Rindler coined the term in the 1950s. [1]In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact objects that even light cannot escape. [2]
A horizon is a boundary in spacetime satisfying prescribed conditions. There are several types of horizons that play a role in Albert Einstein 's theory of general relativity : Absolute horizon , a boundary in spacetime in general relativity inside of which events cannot affect an external observer
In general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again by the observer.
Closed trapped surfaces are a concept used in black hole solutions of general relativity [1] which describe the inner region of an event horizon. Roger Penrose defined the notion of closed trapped surfaces in 1965. [2] A trapped surface is one where light is not moving away from the black hole.
It is the speed of light that arbitrarily defines the ergosphere surface. Such a surface would appear as an oblate that is coincident with the event horizon at the pole of rotation, but at a greater distance from the event horizon at the equator. Outside this surface, space is still dragged, but at a lesser rate. [citation needed]
The photon sphere is located farther from the center of a black hole than the event horizon. Within a photon sphere, it is possible to imagine a photon that is emitted (or reflected) from the back of one's head and, following an orbit of the black hole, is then intercepted by the person's eye, allowing one to see the back of the head, see e.g. [2]
The region beyond which not even light can escape is the event horizon: an observer outside it cannot observe, become aware of, or be affected by events within the event horizon. [5]: 25–36 Picture of space falling into a Schwarzschild black hole at the Newtonian escape speed. Outside the horizon (red), the infalling speed is less than the ...
Within an apparent horizon, light does not move outward; this is in contrast with the event horizon. In a dynamical spacetime, there can be outgoing light rays exterior to an apparent horizon (but still interior to the event horizon). An apparent horizon is a local notion of the boundary of a black hole, whereas an event horizon is a global notion.