# What would happen if a Black Hole and White Hole Collided?

Given our understanding of both black holes and white holes, what would be the outcome if they were both to suddenly collide?

Black Holes:

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon.

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.

## 1 Answer

From Wikipedia:

In quantum mechanics, the black hole emits Hawking radiation and so can come to thermal equilibrium with a gas of radiation. 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. This implies that black holes and white holes are the same object

The excerpt above was an argument made by Stephen Hawking, explaining that a white hole and a black hole are essentially the same object. With this in mind, we can assume that the collision will be between two black holes. The result will then be the radius of the black hole added to the radius of the white hole.

• The metric of the white hole (as a theoretical construction in the GR) has nothing to do to the metric of the black hole. They are different things. Hawking tried probably to explain the Hawking-radiation on an enthusiadt and not on the professional level. Commented Nov 1, 2014 at 15:28