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The event horizon is a null surface in spacetime separating events that are causally connected to an observer from those that are not. Does anyone have a good, simple argument on why it must be a null surface?
The event horizon is the point at which the gravitational pull of a black hole becomes so strong that even light cannot escape. This means that anything, including light, that crosses the event horizon will be pulled into the black hole and cannot escape, hence the term "point of no return".
The size of a black hole is directly related to the size of its event horizon. The larger the event horizon, the larger the black hole and the stronger its gravitational pull. The event horizon is essentially the boundary of a black hole's gravitational influence.
Once an object or particle crosses the event horizon, it cannot escape the gravitational pull of the black hole. This includes light, which is the fastest thing in the universe. Therefore, nothing can pass through the event horizon of a black hole and escape its pull.
The event horizon is referred to as a "null surface" because it is defined by the boundary where the escape velocity is equal to the speed of light. This means that anything that enters the event horizon, including light, cannot escape and is essentially "trapped" within the black hole.
The event horizon is directly linked to the curvature of space-time caused by the extreme gravitational pull of a black hole. The stronger the gravitational pull, the more curved space-time becomes and the larger the event horizon. This is why larger black holes have larger event horizons compared to smaller black holes.