A white hole is not a type of black hole; it is a black hole run in reverse. Aim a rocket straight at its heart, and no amount of thrust will let you in.
Where a black hole is an inescapable gravitational sink whose event horizon permits matter and light to enter but never leave, a theoretical white hole is an emitter whose boundary permits energy and matter to escape but never enter.
The concept emerged directly from Karl Schwarzschild's 1916 solution to Albert Einstein's field equations. When theorists later mapped the full mathematical geometry of an eternal, non-rotating vacuum spacetime, they discovered that the equations contained two complementary halves. In the future lay the familiar black hole, drawing everything inward toward a singularity. In the past lay a time-reversed twin—a white hole expelling matter that originated from a past singularity. Connecting these two theoretical regions produced the first mathematical model of a wormhole, known as an Einstein-Rosen bridge.
Astronomers classify black holes by tangible physical properties like mass, electric charge, and spin, and observations have confirmed their presence throughout the cosmos. White holes share none of that empirical reality. Real black holes form when massive stars collapse under their own gravity, an asymmetrical process that produces an event horizon in the future without generating the past horizon required for a white hole.
Thermodynamics and basic stability present severe barriers to their existence. The second law of thermodynamics dictates that entropy in an isolated system must increase over time, yet a white hole spitting organized matter out of a singularity would cause entropy to plunge. Furthermore, general relativity indicates that a white hole would be violently unstable. Any ambient radiation or stray particle attracted toward its perimeter would collide with outgoing matter, generating an extreme energy density that forces the object to collapse into an ordinary black hole. Some loop quantum gravity models propose that dying black holes might bounce into microscopic white holes at the end of their lives, but for now, they remain purely mathematical curiosities.