White+hole

White Holes

In [|astrophysics], a **white hole** is the hypothetical [|time reversal] of a [|black hole]. While a black hole acts as an attractor, drawing in any [|matter] that crosses the [|event horizon], a white hole acts as a source that ejects matter from its event horizon. The sign of the acceleration is invariant under time reversal, so both black and white holes attract matter. The only potential difference between them is in the behavior at the horizon. Black hole event horizons can only "suck up" matter, while white hole horizons ostensibly recede from any incoming matter at the local speed of light, so that the infalling matter never crosses. The infalling matter is then scattered and re-emitted at the death of the white hole, receding to infinity after having come close to the final singular point where the white hole is destroyed. The total [|proper time] until an infalling object encounters the singular endpoint is the same as the proper time to be swallowed by a black hole, so the white hole picture does not say what happens to the infalling matter. Ignoring the classically unpredictable emissions of the white hole, the white hole and black hole are indistinguishable for external observers. In quantum mechanics, the black hole emits [|Hawking radiation], and so can come to thermal equilibrium with a gas of radiation. Since a thermal equilibrium state is time reversal invariant, [|Stephen Hawking] argued that the time reverse of a black hole in thermal equilibrium is again a black hole in thermal equilibrium.[|[1]] This implies that black holes and white holes are the same object. The Hawking radiation from an ordinary black hole is then identified with the white hole emission. Hawking's semi-classical argument is reproduced in a quantum mechanical [|AdS/CFT] treatment,[|[2]] where a black hole in [|anti-de Sitter space] is described by a thermal gas in a gauge theory, whose time reversal is the same as itself.

Origin
White holes appear as part of the [|vacuum solution] to the [|Einstein field equations] describing a [|Schwarzschild wormhole]. One end of this type of [|wormhole] is a black hole, drawing in matter, and the other is a white hole, emitting matter. While this gives the impression that black holes in our [|universe] may connect to white holes elsewhere, in reality, this is untrue, for two reasons. First, Schwarzschild wormholes are unstable, disconnecting as soon as they form. Second, Schwarzschild wormholes are only a solution to the [|Einstein][|stars]. When the infalling stellar matter is added to a diagram of a black hole's history, it removes the part of the diagram corresponding to the white hole.[|[3]] field equations in vacuum (when no matter interacts with the hole). Real black holes are formed by the collapse of The entropy of a black hole is the horizon area in Planck units, and this is the most entropy which a given region can contain. When an object is emitted out of a white hole, the area of the horizon always decreases by more than the maximum possible entropy that can be squeezed into the object, which is a time-reversed statement of the [|Bekenstein bound]. So the existence of white holes that are not part of a wormhole is doubtful, as they appear to violate the [|second law of thermodynamics]. However, a White Hole can exhibit Hawking Radiation and emit higher levels of this radiation in shorter periods of time, thus allowing White Holes to barely exist for periods of time.

Recent speculations
A more recently proposed view of black holes might be interpreted as shedding some light on the nature of classical white holes. Some researchers proposed that when a black hole forms, a big bang occurs at the core, which creates a new universe that expands into extra dimensions outside of the parent universe.[|[4]] See also [|Fecund universes]. The initial feeding of matter from the parent universe's black hole and the expansion that follows in the new universe might be thought of as a cosmological type of white hole. Unlike traditional white holes, this type of white hole would not be localized in space in the new universe, and its horizon would have to be identified with the [|cosmological horizon].