What Is A White Hole?

What would a white hole look like?

The opposite of a black hole. A white hole would have an event horizon, like a black hole, but the direction would be reversed. Anything inside the horizon would be forced outward, and nothing outside could fall in. From an outside observer's perspective, a white hole would appear to spontaneously eject matter, light, and information. The equations describing white holes are valid solutions to general relativity, just as the equations for black holes are.

Do white holes actually exist?

Probably not, according to most physicists. While white holes are mathematically possible, no observational evidence has ever been found for one. There are several reasons they may not exist. They would violate the second law of thermodynamics (entropy always increasing) in their normal form. They would also be highly unstable: matter falling onto a white hole's region would likely turn it back into a black hole. The conditions to form a white hole naturally aren't clear either.

Could black holes turn into white holes?

It's a long-shot theory. Some physicists have proposed that quantum effects might convert old black holes into white holes near the end of their lives, after Hawking radiation slowly evaporates them. The idea is speculative and depends on theories of quantum gravity that aren't well-established. If true, it might explain certain mysterious astrophysical phenomena like brief, intense bursts of radiation. The proposal is one of several speculative ideas in this area, but none has strong experimental support.

What about wormholes?

Wormholes are related but distinct. A wormhole would connect two points in spacetime through a kind of tunnel, with a black hole on one end and a white hole on the other. Wormhole solutions appear in Einstein's equations just as white holes do, but they're equally unproven in practice. Even theoretical wormholes would likely be unstable, collapsing before anything could pass through. They're a staple of science fiction (think Interstellar or Stargate) but currently have no observational support.