# Is black hole formation reversible if physics law holds even in time reverse?

As we know many situations still fulfill physics law if time is reversed, such as particle collision. But how about black hole formation?

Suppose a star is turning to a black hole and starts to have event horizons. If now I can reverse all travel directions of all matter in the universe, can the 'black hole' turn back to a star again? If not, which part makes this situation asymmetry?

• The time reversed metric of a black hole is a white hole, not another black hole. – Slereah Dec 2 '15 at 7:58

## 2 Answers

The entropy of a black hole is the maximum possible for a given volume of spacetime. Spontaneous un-formation of a black hole in this view would be like un-smashing an egg: theoretically permitted, but so improbable as to be ridiculous.

• What about the information paradox? – innisfree Dec 2 '15 at 8:15
• @innisfree All possible scenarios leading to a black hole with mass $M$ are equally (un)likely to occur in reverse. In the improbable event that a black hole un-forms, it'd be improbable that you would get your own information back out. – rob Dec 3 '15 at 0:29
• ah ok, so "unformation" doesn't mean that black hole formation is reversible, just that a black hole could cease to be a black hole. gotcha, i think – innisfree Dec 3 '15 at 7:15

You are asking the dynamics of General Relativity without considering thermodynamics. Most processes are not reversible from the perspective of thermodynamics because of the second law.

However, if you persist, as a non-thermodynamical process, is the black hole formation reversible? Yes, the dynamics of General Relativity is reversible. You can simply reverse the time direction in the Black hole formation, or in the Kruskal coordinates. Then you will have a white hole explosion instead of black hole formation. But since we need to consider the initial condition for our universe, we don't believe such process indeed exists.

• I really like this answer. According to physics.stackexchange.com/questions/76842/…, the second law of thermodynamics hasn't been proven to be an absolute law and unlike rob's answer, this answer does not use the second law of thermodynamics to explain why white holes don't exist. – Timothy Mar 13 '18 at 3:08