A white hole, in general relativity, is a hypothetical region of spacetime which cannot be entered from the outside, but from which matter and light may escape. The reverse of a black hole, in other words, from which nothing, including light, can escape. It is theoretically possible for a traveler to enter a rotating black hole, avoid the singularity at its core, and travel into a rotating white hole which allows the traveler to escape into another universe. That’s all well and good, but while there is evidence of the existence of black holes, which can form through gravitational collapse of a star, there is no known physical process that could form a white hole.
White holes appear as part of a solution to the Einstein field equations known as the maximally extended version of the Schwarzschild metric describing an eternal black hole with no charge and no rotation. In one sense a black hole is a white hole and like black holes, white holes have properties of mass, charge, and angular momentum. In the Schwarzschild solution, 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.
This post was inspired by a sudden flux of visitors passing the SciScoop event horizon with the search term: “white hole” and “artificial black hole”