Can the black and white holes combine?

Main Question or Discussion Point

Good day
A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping
Black hole
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 have the ability to escape
White hole
The characteristics of these two kinds of wholes are opposite to each other. I would like to know can they combine when colliding to each other? The Black hole cannot enter the white one (since the latter does not accept anything) but the white hole can approach to the black hole and enter there. Is it possible? What kind of space object would be created at such collision? Maybe grey hole?

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PeterDonis
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The Black hole cannot enter the white one (since the latter does not accept anything) but the white hole can approach to the black hole and enter there.
No, it can't. Even assuming a white hole could exist (which it can't, really--see below), it couldn't approach a black hole, because the only way for a white hole to exist would entail it being in a completely separate region of spacetime from the black hole. (Basically, the black hole is in the future and the white hole is in the past; there's no way for the white hole to "move" from the past to the future.)

The reason a white hole can't actually exist is that there is no way for it to come into being. One way of describing why this is is to observe that the mathematical solution that describes a white hole is the time reverse of the one that describes a black hole; so just as nothing can get out of a black hole, nothing can get *into* a white hole. But that means there's no way for it to form; it would have had to magically be there in the past, with no physical way for it to have gotten there. (A black hole, on the other hand, can form from the collapse of a massive object; since the black hole is in the future, the object can be there before the hole is, so the hole can form from the object.)

Bill_K
But that means there's no way for it to form; it would have had to magically be there in the past, with no physical way for it to have gotten there.
In other words, a white hole would have to be primordial.

It is possible that the Big Bang was a white hole.

PeterDonis
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It is possible that the Big Bang was a white hole.
No, it isn't. A white hole doesn't occupy the entire universe; there is also a past infinity in the white hole spacetime representing a region outside the hole. The Big Bang occupied the entire universe, so it could not have been a white hole.

PeterDonis
Basically, the black hole is in the future
Excuse me…………but what about the black holes existing now in space? the ones that we observe by means of space telescopes?

Hawking radiation could create a white hole.

Nugatory
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Hawking radiation could create a white hole.
Have you a source for that statement?

Nugatory
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Excuse me…………but what about the black holes existing now in space? the ones that we observe by means of space telescopes?
Those exist, not much doubt there. It's the white holes whose existence PeterDonis is questioning, on the rather reasonable grounds that they have never been observed and there are few plausible ways that one could come into existence.

PeterDonis
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what about the black holes existing now in space? the ones that we observe by means of space telescopes?
We don't see the actual black holes themselves; that's not possible, because light can't escape from a black hole. We see the gravitational effects of the holes on objects near them.

PeterDonis
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This is an article about a speculation that our universe could have formed from the interior of a black hole in some previous universe. That's not the same as saying that the Big Bang was a white hole; first, in the previous universe, a white hole would not be the same as a black hole; second, even if our Big Bang was caused by a "baby universe" being born inside a black hole from a previous universe, the Big Bang still would occupy *our* entire universe, and hence would not be a white hole.

stevendaryl
Staff Emeritus
Have you a source for that statement?
The possibility is mentioned in the Physics FAQ:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

There is one final twist in the answer to this question. It has been suggested by Stephen Hawking that once quantum effects are accounted for, the distinction between black holes and white holes might not be as clear as it first seems. This is due to "Hawking radiation", a mechanism by which black holes can lose matter. (See the relativity FAQ article on Hawking radiation.) A black hole in thermal equilibrium with surrounding radiation might have to be time symmetric, in which case it would be the same as a white hole. This idea is controversial, but if true it would mean that the universe could be both a white hole and a black hole at the same time. Perhaps the truth is even stranger. In other words, who knows?

PeterDonis
We don't see the actual black holes themselves; that's not possible, because light can't escape from a black hole. We see the gravitational effects of the holes on objects near them.
And so there is no doubt that they exist now and not “in the future”, right?
Nugatory
Those exist, not much doubt there. It's the white holes whose existence PeterDonis is questioning, on the rather reasonable grounds that they have never been observed and there are few plausible ways that one could come into existence.
Ok, but what do other scientists think about it? Is white hole’s existence under more doubt than black hole’s existence?

Nugatory
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Ok, but what do other scientists think about it? Is white hole’s existence under more doubt than black hole’s existence?
Yes, much more. Theory predicts ways for black holes to form and observation supports the existence of black holes formed in these ways. The theoretical case for white holes is much weaker and there are no observations that suggest they exist..

PAllen
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PeterDonis

And so there is no doubt that they exist now and not “in the future”, right?
Nugatory

Ok, but what do other scientists think about it? Is white hole’s existence under more doubt than black hole’s existence?
White holes existence is under much more doubt. My guess is that almost all astrophysicists assume that "the quantum analog of a classical BH" exists; and almost all astrophysicists would bet that no white holes exist (quantum analog or not). I make a distinction between an evaporating BH and a white hole. An evaporating BH still has a future horizon-like surface; a white hole would have past horizonlike surface and would have to be without cause.

PeterDonis
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And so there is no doubt that they exist now and not “in the future”, right?
In the sense of "now" that matters, yes, they exist now. But the sense of "now" that matters might not be the sense of "now" that you are accustomed to.

The black hole is an entire region of spacetime inside the event horizon. Strictly speaking, that region of spacetime does not exist "now" or "in the future"; it just is, just as spacetime in its entirety just is. To say that the black hole exists is just to say that the spacetime of our universe includes at least one region inside an event horizon. There is no time attached to that statement: it's just a statement about spacetime, period.

However, we here on Earth also occupy a region of spacetime, and if we ask the question, how is the region of spacetime we occupy related to the region of spacetime that is the black hole, the answer is that there is a portion of the black hole region that is in our future, here on Earth, but there is no portion that is in our past. The portion of the black hole that is not in our future, here on Earth, is in our "now", which just means it's spacelike separated from us. The fact that there *is* a portion of the black hole region that is in our "now" is why we can say that the black hole exists "now".

Conversely, if we were in a spacetime where a white hole was present, and we were on some planet far away from the hole, then a portion of the region of the spacetime that was the white hole would be in our *past*, and the remaining portion would be in our "now", but there would be *no* portion that was in our future. That's the key difference between a white hole and a black hole that I was alluding to when I said a black hole is in our future (strictly speaking, a portion of it is in our future but none of it is in our past), but a white hole would be in our past (strictly speaking, a portion of it would be in our past but none of it would be in our future).

Nugatory
PAllen
PeterDonis
Well, I did not know that the theory about black and white holes was so complicated, anyway thank you for your replies

Dont cross the streams...