What is the event horizon of a black hole?

Spacetime is made out of events. An event simply means a moment in time plus a point in space. Events can cause other events, e.g., if a spaceship flies from event A to event B, or a radio signal travels from A to B. It's also possible to have events that can't be causally linked, e.g., if B is 10 light-years away from A and 5 years in A's future, then A can't cause B.

When you have a black hole, an event A close to the center of the black hole can be causally disconnected from any distant event B, even if we place B very far in the future (technically, at null infinity, which is a kind of idealized place, infinitely far away and infinitely far in the future, that a light ray would normally be expected to be able to get to). The black hole has a nearby region which is the set of all events such as A. This region has a boundary, called a black hole event horizon, and a black hole is in fact defined as the kind of object that has this type of horizon. There are other types of event horizons, such as cosmological horizons or the horizon of an accelerated observer.

The event horizon is not an actual physical surface. If you fell through it, you would feel nothing special. Yes, it can basically be interpreted as the point of no return from which light can't escape -- but (a) it's not just light, it's all causal relationships, and (b) the relevant point is that it can't escape to infinity.

WikiPedia says:


This is not accurate. It makes it sound as though the event horizon was defined by the strength of a gravitational field, whereas in fact the field can have any strength at the event horizon, depending on the size of the black hole.

There are actually several types of black-hole horizons with different definitions, each of which are sometimes called the event horizon. But the one that scientists typically mean when they use this phrase is more precisely called the "absolute horizon". Wikipedia's page is accurate, but not very complete. There is also a nice overview given by the introduction to this paper.

In simple terms, the absolute horizon is defined as the boundary of the region from which light will never be able to escape. So, in particular, if you think of a black hole as being kind of like a ball, the horizon is just the surface of that ball. On and inside the surface, nothing can ever escape. Just outside the surface, a ray of light pointed perfectly outwards could just barely escape. So there isn't just one "point of no return", there are many. Those points of no return actually form a surface that completely surrounds the black hole, and define the black hole's boundary.

This is kind of a weird definition, because technically, you can't know if something will ever escape unless you know the entire future history of the universe. (Though you can sometimes know when something definitely won't escape, you can't know whether or not something that looks like it'll barely escape actually will or not. And with a few reasonable assumptions, there are some useful theorems.) Most ideas in physics are defined at a specific instant in time, or maybe a short period of time, so this type of definition is pretty unusual. Even more surprising, though, we can't even know if we're inside of an event horizon right now, because we don't know what happens in the future. Maybe something unavoidably comes crashing down on us and forms a black hole with us inside; we just don't know.

There are other types of horizons relevant for black holes, but the distinctions are a little more subtle, and it's probably best to wrap your head around the absolute horizon first. But if you get interested in those other ones, feel free to ask for more specific clarification -- maybe after looking at the other links on Wikipedia's page.