White hole

White Holes
In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it.wikipedia
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Wormhole

wormholesEinstein–Rosen bridgeEinstein-Rosen bridge
White holes appear in the theory of eternal black holes.
In order to satisfy this requirement, it turns out that in addition to the black hole interior region that particles enter when they fall through the event horizon from the outside, there must be a separate white hole interior region that allows us to extrapolate the trajectories of particles that an outside observer sees rising up away from the event horizon.

Kruskal–Szekeres coordinates

Kruskal coordinatesKruskal-Szekeres coordinatescomplete extension
They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole's event horizon (though in the case of the maximally extended Schwarzschild solution, discussed below, the white hole event horizon in the past becomes a black hole event horizon in the future, so any object falling towards it will eventually reach the black hole horizon).
The negative T singularity is the time-reversed black hole, sometimes dubbed a "white hole".

Igor Dmitriyevich Novikov

Igor NovikovIgor Dmitrievich NovikovIgor D. Novikov
The possibility of the existence of white holes was put forward by Russian cosmologist Igor Novikov in 1964.
Novikov put forward the idea of white holes in 1964.

Hawking radiation

Hawking temperatureblack hole radiationBekenstein-Hawking radiation
In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation (not compulsory).
The same effect occurs for regular matter falling onto a white hole solution.

Black hole

black holesblack-holeblack hole physics
In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which matter and light cannot escape.

T-symmetry

time reversaltime reversal symmetrytime-reversal symmetry
But because the equations of general relativity are time-reversible – they exhibit Time reversal symmetry – general relativity must also allow the time-reverse of this type of "realistic" black hole that forms from collapsing matter.
The time reversal of a black hole would be a hypothetical object known as a white hole.

GRB 060614

Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented occurrence of a white hole.
GRB 060614 was subsequently classified as a "hybrid gamma-ray burst", defined as a long burst without accompanying supernova, and was hypothesized to have been an observation of a white hole.

White Hole (Red Dwarf)

White HoleWhite Hole" (''Red Dwarf'')
The idea of white holes was explored in an episode of the British science fiction sitcom Red Dwarf entitled "White Hole".
Written by Rob Grant and Doug Naylor, and directed by Ed Bye and Paul Jackson, the episode features the crew's attempt to escape the influence of a white hole.

Gravitational singularity

singularitysingularitiesgravitational singularities
Here, "maximally extended" refers to the idea that the spacetime should not have any "edges": for any possible trajectory of a free-falling particle (following a geodesic) in the spacetime, it should be possible to continue this path arbitrarily far into the particle's future, unless the trajectory hits a gravitational singularity like the one at the center of the black hole's interior.

Planck star

Recent work demonstrates that Planck stars may exist inside of black holes as part of a cycle between black hole to white hole.

General relativity

general theory of relativitygeneral relativity theoryrelativity
In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it.

Spacetime

space-timespace-time continuumspace and time
Here, "maximally extended" refers to the idea that the spacetime should not have any "edges": for any possible trajectory of a free-falling particle (following a geodesic) in the spacetime, it should be possible to continue this path arbitrarily far into the particle's future, unless the trajectory hits a gravitational singularity like the one at the center of the black hole's interior. In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it.

Matter

corporealsubstancematerial
In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it.

Light

visible lightvisiblelight source
In general relativity, a white hole is a hypothetical region of spacetime which cannot be entered from the outside, although matter and light can escape from it.

Einstein field equations

Einstein field equationEinstein's field equationsEinstein's field equation
In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past.

Gravitational collapse

collapsecollapsedcollapsed star
However, this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed.

Mass

inertial massgravitational massweight
Like black holes, white holes have properties like mass, charge, and angular momentum.

Electric charge

chargeelectrical chargecharged
Like black holes, white holes have properties like mass, charge, and angular momentum.

Angular momentum

conservation of angular momentumangular momentamomentum
Like black holes, white holes have properties like mass, charge, and angular momentum.

Event horizon

event horizonshorizoncosmic event horizon
They attract matter like any other mass, but objects falling towards a white hole would never actually reach the white hole's event horizon (though in the case of the maximally extended Schwarzschild solution, discussed below, the white hole event horizon in the past becomes a black hole event horizon in the future, so any object falling towards it will eventually reach the black hole horizon).

Quantum mechanics

quantum physicsquantum mechanicalquantum theory
In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation (not compulsory).

Thermal equilibrium

equilibriumequilibrium temperaturethermal
In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation (not compulsory).

Stephen Hawking

HawkingProfessor Stephen HawkingStephen William Hawking
Because 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.