What is "apparent horizon"

Apparent \Ap*par"ent\, a. [F. apparent, L. apparens, -entis, p. pr. of apparere. See Appear.]

1. Capable of being seen, or easily seen; open to view; visible to the eye; within sight or view.

   The moon . . . apparent queen.
   --Milton.

2. Clear or manifest to the understanding; plain; evident; obvious; known; palpable; indubitable.

   It is apparent foul play.
   --Shak.

3. Appearing to the eye or mind (distinguished from, but not necessarily opposed to, true or real); seeming; as the apparent motion or diameter of the sun.

   To live on terms of civility, and even of apparent friendship.
   --Macaulay.

   What Berkeley calls visible magnitude was by astronomers called apparent magnitude.
   --Reid.

   Apparent horizon, the circle which in a level plain bounds our view, and is formed by the apparent meeting of the earth and heavens, as distinguished from the rational horizon.

   Apparent time. See Time.

   Heir apparent (Law), one whose to an estate is indefeasible if he survives the ancestor; -- in distinction from presumptive heir. See Presumptive.

   Syn: Visible; distinct; plain; obvious; clear; certain; evident; manifest; indubitable; notorious.

Horizon \Ho*ri"zon\, n. [F., fr. L. horizon, fr. Gr. ? (sc. ?) the bounding line, horizon, fr. ? to bound, fr. ? boundary, limit.]

1. The line which bounds that part of the earth's surface visible to a spectator from a given point; the apparent junction of the earth and sky.

   And when the morning sun shall raise his car Above the border of this horizon.
   --Shak.

   All the horizon round Invested with bright rays.
   --Milton.

2. (Astron.)

   1. A plane passing through the eye of the spectator and at right angles to the vertical at a given place; a plane tangent to the earth's surface at that place; called distinctively the sensible horizon.

   2. A plane parallel to the sensible horizon of a place, and passing through the earth's center; -- called also rational horizon or celestial horizon.

   3. (Naut.) The unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible.

3. (Geol.) The epoch or time during which a deposit was made.

   The strata all over the earth, which were formed at the same time, are said to belong to the same geological horizon.
   --Le Conte.

4. (Painting) The chief horizontal line in a picture of any sort, which determines in the picture the height of the eye of the spectator; in an extended landscape, the representation of the natural horizon corresponds with this line.

5. The limit of a person's range of perception, capabilities, or experience; as, children raised in the inner city have limited horizons.

6. [fig.] A boundary point or line, or a time point, beyond which new knowledge or experiences may be found; as, more powerful computers are just over the horizon.

   Apparent horizon. See under Apparent.

   Artificial horizon, a level mirror, as the surface of mercury in a shallow vessel, or a plane reflector adjusted to the true level artificially; -- used chiefly with the sextant for observing the double altitude of a celestial body.

   Celestial horizon. (Astron.) See def. 2, above.

   Dip of the horizon (Astron.), the vertical angle between the sensible horizon and a line to the visible horizon, the latter always being below the former.

   Rational horizon, and Sensible horizon. (Astron.) See def. 2, above.

   Visible horizon. See definitions 1 and 2, above.

n. the line at which the sky and Earth appear to meet [syn: horizon, visible horizon, sensible horizon, skyline]

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 outward but moving inward.

Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from event horizons. Within an apparent horizon, light is not moving away from the black hole, whereas in an event horizon, light cannot escape from the black hole. It is possible for light to be currently moving away from the black hole (and so outside the apparent horizon), but in the future will not be able to escape (e.g. because the mass of the black hole is growing) and therefore inside the event horizon. Thus the apparent horizon can be thought of as the boundary of black hole for light at this instant, whereas the event horizon is the boundary of the black hole for light in the future.

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.