# What is Event Horizon of a Black Hole? [closed]

The boundary of a black hole is said to be surrounded by event horizon - the point of no return! What is its significance in terms of general relativity?

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## closed as too broad by called2voyage♦Mar 10 '14 at 18:38

There are either too many possible answers, or good answers would be too long for this format. Please add details to narrow the answer set or to isolate an issue that can be answered in a few paragraphs. If this question can be reworded to fit the rules in the help center, please edit the question.

The question "What is its significance in terms of general relativity?" is both too broad and out of scope for this site (belongs on Physics.SE). Could you clarify exactly what information you are looking for? –  called2voyage Feb 25 '14 at 17:28
What I mean to say is, how are the characteristics of event horizon related to Space-Time? –  Spacetrekker Feb 26 '14 at 13:45

## 2 Answers

It is exactly the point of no return for light. In other words, it is the point (actually more like a sphere) in which the escape velocity from the Black Hole gravity reaches $c$.

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What I mean to say is, how are the characteristics of event horizon related to Space-Time?

Formally, an event horizon is the boundary of a region of spacetime that's not in the causal past of future null infinity. In other words, the boundary of a region from which even idealized light rays cannot escape to infinity. Whenever the event horizon is smooth, it is also null hypersurface--i.e., the direction perpendicular to it is a light ray.

Because the event horizon is defined in terms of the infinite future, the definition is very non-local, and one would need to know the entire future history to be sure where the event horizon is. As such, it is only one of a half-dozen different types of horizons used to study black holes.

Though the event-horizon itself is a three-dimensional hypersurface in spacetime, it can also be viewed as an evolving two-dimensional membrane made of a viscuous electrically conducting fluid with finite temperature and entropy but zero thermal conductivity.

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