What is the 'apparent horizon' of a black hole?

Question

The Wikipedia article is full of general relativistic and differential geometry jargon, and its accuracy is questionable. So, without such (or explaining said jargon without more jargon), what is the 'apparent horizon' of a black hole?

Answer 1

The apparent horizon is a technical term, so you're going to have to have SOME jargon to explain it--the distinctions between trapping, apparent, dynamical and event horizons does just require some knowledge of differential geometry and general relativity, even if you can explain them using common words, as below.

Freeze a moment in time in some particular coordinate system. Then, if that coordinate system contains an apparent horizon, it will be closed two-dimensional surface (think a ball, but not necessarily undistorted) where all light rays but one are forced to move into the surface when evolved in time, and the last light ray will be 'frozen' on the surface at that moment of time (think of the outgoing light ray at $r=2M$ in the Schwarzschild solution--it tries to get away, but is just stuck on the surface).

It is called apparent because it:

1. depends on the notion of constant time on that one surface
2. is therefore coordinate dependent, and in some cases, can be eliminated entirely by choice of coordinate
3. can have strange, unpredictable behaviour
4. is generally inside the proper event horizon, but
5. if the ''stack'' of apparent horizons forms a timelike surface, the surface will be two-way transversible, and therefore, doesn't correspond with what we normally think of as a 'horizon'.

Answer 2

Simply, The apparent horizon is the outermost trapped surface surrounding a black hole.The name Apparant is given because it separates light rays that are trapped inside a black hole from those that can move away from it.