In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outwards but moving inwards.
Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from absolute horizons.
See, however, the articles on ergosphere, Cauchy horizon, the Reissner-Nordström solution, photon sphere, Killing horizon and naked singularity; the notion of a horizon in general relativity is subtle, and depends on fine distinctions.
Read more about Apparent Horizon: Definition, Differences From The (Absolute) Event Horizon
Famous quotes containing the words apparent and/or horizon:
“The apparent rulers of the English nation are like the imposing personages of a splendid procession: it is by them the mob are influenced; it is they whom the spectators cheer. The real rulers are secreted in second-rate carriages; no one cares for them or asks after them, but they are obeyed implicitly and unconsciously by reason of the splendour of those who eclipsed and preceded them.”
—Walter Bagehot (1826–1877)
“The thought of you will constantly elevate my life; it will be something always above the horizon to behold, as when I look up at the evening star. I think I know your thoughts without seeing you, and as well here as in Concord. You are not at all strange to me.”
—Henry David Thoreau (1817–1862)