White Holes | Space Time

White holes are the time-reversed mirror image of black holes: instead of trapping everything behind an event horizon, they eject everything and forbid entry. In the simplest mathematical setting of Einstein’s general relativity, a white hole has an event horizon that acts like a one-way gate in the opposite direction—nothing from outside can cross in, while light and matter inside must stream outward. That makes white holes look like extreme, “radiate-like-crazy” objects in principle, but the physics that would allow them to exist in the real universe is highly constrained.

The cleanest origin of the idea comes from the Schwarzschild solution, the earliest exact result for a non-spinning, uncharged mass in otherwise empty spacetime. That solution can be interpreted as an “eternal” black hole—one that exists for all time and doesn’t form from collapse. When the Schwarzschild spacetime is extended as far as the mathematics allows, it contains both an infinite future and an infinite past version of the black hole. Tracing the eternal black hole backward in time reveals a region where the roles of past and future swap: the singularity becomes a past event, the event horizon becomes a barrier to entry, and the interior flows outward rather than inward. In Penrose diagrams—tools that compactify spacetime so infinite past and future fit on a finite plot—this past region behaves like a white hole: light rays inside must move toward the outside universe, meaning anything “inside” gets expelled.

Even so, this particular white hole is effectively unobservable from our perspective. The past event horizon and past singularity sit infinitely far in the past relative to outside observers, so light emitted from the would-be white hole cannot reach us. There’s also a more fundamental issue: the “eternal” black hole used to generate the white-hole region is not how black holes arise in realistic cosmology. Real black holes form from gravitational collapse of massive stellar cores, and the universe itself has not existed forever.

For white holes to form dynamically, general relativity’s time-reversal symmetry isn’t enough. The second law of thermodynamics pushes time forward by requiring entropy to increase. Creating a white hole would require an effective reversal of entropy—an enormous, statistically improbable entropy dip. Even if such a dip occurred, the moment normal entropy increase resumed, the white hole would likely erupt violently.

That tension is why some physicists look to the Big Bang. The Big Bang resembles a white-hole-like “expanding outpouring” that cannot be entered because it lies in the past, though it differs by lacking a singularity in the same way. A further speculative step—attributed here to Lee Smolin—is that black hole formation in one universe could spawn a white hole that becomes the Big Bang of a new “baby universe,” potentially linking black holes and cosmic origins.

Finally, the Schwarzschild spacetime’s maximal extension doesn’t just create a past white-hole region; it also introduces an additional, parallel region of spacetime on the opposite side of the black hole. That connection is described by an Einstein-Rosen bridge, commonly associated with wormholes, which is flagged for later exploration.