# Can a black hole be turned into a white hole?

Following the Schwarzschild metric, we could consider that a white hole=black hole with no angular momentum (no spin?) or electric charge. Would be enough to slow down a black hole and somehow remove its electric charge to create a white hole?

To slow down and reduce the electric charge of a given black hole I can think of two methods, one by feeding the black hole other black holes with different charges and angular momentums or by creating a dyson-like object to slow the rotation little by little by using light beams and make it rebound near the event horizon. Which alternative seems more feasible?

I remember hearing that 'if' it was possible to turn a black hole into a white hole by somehow reversing entropy locally, it would implode back to a black hole as soon as you remove the energy source that is causing the "local entropy loss" (pretty sure I heard it on PBS youtube channel, I'm not sure if it's related to this paper studying the big bang as a fake white hole). Is this still up to date?

• Not...really...? A black hole with no angular momentum and no charge is just a static (Schwarzschild) black hole. Wikipedia makes this pretty clear. Feb 14, 2019 at 9:33
• I’m not sure where you got the idea that a chargeless, non-rotating black hole is equivalent to a white hole. The mathematical existence of white holes is plausible if you make certain assumptions about your black hole, but you can’t turn one into the other simply by altering charge/angular momentum, unless I’ve really misunderstood my general relativity. Feb 14, 2019 at 9:39
• Having no charge and no rotation isn’t actually important in the proof that white holes should exist: it’s a way to make the maths simpler. The important bit is the ‘maximally extended’ schwarzchild metric. Basically you can use the maths to show white holes should exist as the logical counterpoint to a black hole an infinite amount of time in the future, and therefore there’s no reason we shouldn’t see white holes appearing because of a black hole an infinite time ago. Feb 14, 2019 at 9:52
• (What is the correct VTC option for questions based on false or misunderstood assumptions?)
– rek
Feb 15, 2019 at 6:35
• @rek I have accepted the answer as it really answers the question, and even explains the misconceptions Feb 15, 2019 at 11:50

Following the Schwarzschild metric, we could consider that a white hole=black hole with no angular momentum (no spin?) or electric charge.

A Schwarzchild black hole is actually a black hole without charge or angular momentum.

A white hole is an abstract mathematical extension of a black hole, but none have been observed and mainstream physics basically considers them just a theoretical abstraction rather than expecting to see them. Note that while we have observed a lot of black holes, we have not chanced upon any white hole candidates that are accepted, suggesting (IMO) that they aren't out there to find.

So you cannot consider a chargeless, spinless black hole a white hole.

Real black holes are expected to have angular momentum and possibly a net charge. They are described by more elaborate metrics called the Kerr, Reissner–Nordström and Kerr–Newman metrics. These metrics reduce to the Schwarzchild metric when charge and spin are zero.

Would be enough to slow down a black hole and somehow remove its electric charge to create a white hole?

Clearly not.

To slow down and reduce the electric charge of a given black hole I can think of two methods, one by feeding the black hole other black holes with different charges and angular momentums or by creating a dyson-like object to slow the rotation little by little by using light beams and make it rebound near the event horizon. Which alternative seems more feasible?

Neither is "feasible" for human science in the foreseeable future. These would, however, require control of masses in range of stellar masses. It would probably be "just as easy" for people with this level of power to make their own black hole from scratch rather than trying to make one work the way they want. Either way they will not get a white hole.

• I understand it now, I mixed concepts :) Need to read more about what is known about white holes, because the concept of a "fake white hole" causing a new big bang is the one I'm after. Going from black to white by somehow reducing entropy locally (I'm thinking on "end of universe" civilization so not constraint by things we are capable of in the foreseable future) and then the subsequent "bang" as the universe "sorts" itself is what I'm looking for. Feb 14, 2019 at 11:03