White hole

A white hole is a hypothetical spacetime region from which matter and light can escape but into which nothing can enter. It appears in certain solutions to Einstein’s field equations.

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Event horizon: A boundary in spacetime beyond which events cannot affect a distant observer. A white hole horizon permits outward passage while forbidding inward passage.
Black hole: A spacetime region from which nothing, including light, can escape beyond its event horizon. Its horizon allows entry but blocks escape, opposite to a white hole’s defining behavior.
Karl Schwarzschild: A German physicist who found the first exact solution to Einstein’s field equations for a spherical mass. His solution became the starting point for the spacetime later extended to include a white hole.
General relativity: Einstein’s theory in which gravity is described as the curvature of spacetime caused by matter and energy. White holes are permitted by some exact solutions, but the theory alone does not establish their formation.
Kruskal–Szekeres coordinates: A coordinate system that extends the Schwarzschild solution across its event horizons. Its maximally extended diagram contains a white-hole region alongside a black hole.
Wormhole: A hypothetical spacetime geometry connecting otherwise separated regions or universes. The maximally extended Schwarzschild solution also contains a bridge, distinct from its white-hole region.
Einstein–Rosen bridge: A non-traversable bridge between two asymptotically flat regions in the maximally extended Schwarzschild solution. Its 1935 analysis helped expose the richer structure of the extended solution containing a white-hole region.
Gravitational collapse: The contraction of an object under its own gravity, potentially forming a compact remnant or black hole. Known collapse processes form black holes rather than the time-reversed white holes of idealized solutions.
Schwarzschild metric: An exact solution of Einstein’s field equations describing the vacuum exterior of a spherical, nonrotating mass. Its extended form provides the classic mathematical setting for a white hole.
Black hole thermodynamics: The framework relating black-hole mechanics to temperature, entropy, and laws resembling thermodynamics. Its time-reversal puzzles highlight why a white hole is not simply an ordinary black hole run backward.