Do black holes determine time's arrow?

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Does black holes determines time's arrow? Otherwise how to explain with time reversed objects been pushed out from black holes?
 

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  • #2
Ibix
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The maximally extended Schwarzschild black hole includes a black hole and a white hole. Under time reversal the black hole becomes the white hole and vice versa.
 
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Does "white hole" can be explained by the standard physics? Let's say we have a mass that collapse to a black hole - now under time reversal we also have the same mass concentration, so it should become a black hole under the same gravitational rules, so what make it white?
 
  • #4
Ibix
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Let's say we have a mass that collapse to a black hole - now under time reversal we also have the same mass concentration,
No - under time reversal, instead of a collapsing cloud of matter forming a black hole you have an expanding cloud of matter coming from a white hole with incoming radiation to warm it and expand it. You would not expect this to form a new hole, black or white.

The maximally extended Schwarzschild spacetime I spoke of is an eternal black hole. It didn't form from anything, which is why it needs a white hole to be plausible under time reversal.
 
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You would not expect this to form a new hole, black or white.
Can you please explain it a bit more?
If white hole is not creating mass out of nothing, it means that until the "explosion" we had all the mass in a singular point. So why does gravity is not working here as expected?
 
  • #6
Ibix
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Black hole singularities are not points. They are more like a moment in time, which is one way of explaining why you can't avoid them once you are inside the horizon.

If matter can hit a black hole singularity then matter can be emitted by a white hole singularity. Singularities don't really have a mass, but you could see it as the white hole singularity losing mass in the reverse of a black hole singularity gaining mass. However, the whole point of a singularity is that it's where our physical models have definitely gone wrong, so what happens there won't necessarily make sense. So a better way to put it is that whatever is actually where our models put a white hole singularity emits matter and radiation in a time-reverse of whatever is actually where our models put a black hole singularity.
 
  • #7
Dale
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Can you please explain it a bit more?
If white hole is not creating mass out of nothing, it means that until the "explosion" we had all the mass in a singular point. So why does gravity is not working here as expected?
Although your description is somewhat off as explained above, it is important to note that very often there are solutions to the equations of the laws of physics that are considered unphysical for one reason or another. This is one example. It is a mathematical solution that I don’t think anyone believes represents any part of the actual universe.
 
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It is a mathematical solution that I don’t think anyone believes represents any part of the actual universe.
The question is whatever a white hole can actually be formed in our universe if somehow we were able to set all the particles in the right place. If the answer is "no" - it means black hole is setting time's arrow.
 
  • #9
Dale
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The question is whatever a white hole can actually be formed in our universe if somehow we were able to set all the particles in the right place. If the answer is "no" - it means black hole is setting time's arrow.
Not really. A white hole is the initial condition, so you cannot set particles in the right place and have them evolve produce a white hole as a final condition. All you can do is to find one already existing naturally and see how it evolves.

Since its evolution would violate the 2nd law of thermodynamics, we don’t expect to see that naturally. But it is the 2nd law of thermo that is the issue, not the black/white hole. In other words, even here it is thermodynamics that provides the arrow of time.
 
  • #10
Ibix
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The question is whatever a white hole can actually be formed in our universe if somehow we were able to set all the particles in the right place.
Black holes evaporate through Hawking radiation. So arrange time-reversed Hawking radiation and you'll get a time-reversed black hole.
 
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PeroK
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The question is whatever a white hole can actually be formed in our universe if somehow we were able to set all the particles in the right place. If the answer is "no" - it means black hole is setting time's arrow.
To go back to the original question. I don't see the dependence of the arrow of time on black hole formation.
 
  • #12
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Not really. A white hole is the initial condition, so you cannot set particles in the right place and have them evolve produce a white hole as a final condition. All you can do is to find one already existing naturally and see how it evolves.
Let's say we have 2 holes, one is a black and the other is white. Both can have gravity fields (a satellite can orbit both bodies) , the difference between the two is that in the future the white will push out martials and disappear, but the black can become bigger and never push out martials.
The question is what is difference in the current internal state of these bodies that will make them so different in the future? Because from what I see, all we have is 2 identical bodies.
 
  • #13
PeroK
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Let's say we have 2 holes, one is a black and the other is white. Both can have gravity fields (a satellite can orbit both bodies) , the difference between the two is that in the future the white will push out martials and disappear, but the black can become bigger and never push out martials.
The question is what is difference in the current internal state of these bodies that will make them so different in the future? Because from what I see, all we have is 2 identical bodies.
The eternal black hole and white hole seem to be irrelevant to the question. They are part of the same maximally extended Schwartzschild solution. They are not things that evolve in our universe.

If you want to understand this Schwartzschild solution, you'll need to do quite a bit of research, as the whole solution and the white hole in particular are non trivial concepts.

The white hole is nothing like you are imagining!
 
  • #14
Dale
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Let's say we have 2 holes, one is a black and the other is white. Both can have gravity fields (a satellite can orbit both bodies) , the difference between the two is that in the future the white will push out martials and disappear, but the black can become bigger and never push out martials.
The question is what is difference in the current internal state of these bodies that will make them so different in the future? Because from what I see, all we have is 2 identical bodies.
It doesn’t work this way at all. There are no 2 identical bodies.

A Schwarzschild black and white hole are not objects. They are two different moments in time for the same spacetime, the maximally extended Schwarzschild solution.

The difference between the black and white holes is that the white hole is the beginning of the spacetime and the black hole is the end. When you reverse it then what was the end becomes the beginning and vice versa.
 
  • #15
stevendaryl
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I would like to point out that as far as spacetime geometry, there is no difference between a black hole and a white hole, if by "geometry" you mean topology plus spacetime curvature. The difference comes into play when you try to assign an arrow of time to each pair of timelike separated points. Two points in spacetime are timelike separated if it is possible for a slower-than-light observer (or other massive object) to travel between the points. If you choose an arrow of time so that for objects that are inside the event horizon of a black hole, the singularity is in the future, then consistency would require you to make the singularity of the white hole take place in the past. In the Schwarzschild geometry with both a black hole and a white hole, there are timelike paths that start at the white hole singularity and end at the black hole singularity. Which is the "start" and which is the "end" is purely conventional, until you introduce a thermodynamic arrow of time, as well.
 
  • #16
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It seems like you all treating black and white holes as spacetime topologies without paying attention to the cause of that topology which is the mass. We must have mass in order to have black hole and vise versa - a black hole means there is a mass inside. But if this is the case how does the same mass states causes 2 different behavior?
 
  • #17
PeroK
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It seems like you all treating black and white holes as spacetime topologies without paying attention to the cause of that topology which is the mass. We must have mass in order to have black hole and vise versa - a black hole means there is a mass inside. But if this is the case how does the same mass states causes 2 different behavior?
There is no cause (and can be no cause) for an eternal black hole. It is a spacetime geometry. It is not a black hole as formed by the collapse of a large star.
 
  • #18
Ibix
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It seems like you all treating black and white holes as spacetime topologies without paying attention to the cause of that topology which is the mass.
Excuse me - I addressed this when I said:
Black holes evaporate through Hawking radiation. So arrange time-reversed Hawking radiation and you'll get a time-reversed black hole.
But if this is the case how does the same mass states causes 2 different behavior?
It doesn't. A collapsing gas cloud (precursor to a black hole) is not the same state as an expanding gas cloud (time reversed black hole final state).
 
  • #19
Dale
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It seems like you all treating black and white holes as spacetime topologies
Not just topology, it is spacetime topology and curvature.

without paying attention to the cause of that topology which is the mass.
This is incorrect. A white hole is a feature of the maximally extended Schwarzschild spacetime, which is a vacuum solution. There is no mass anywhere in the manifold.

There may be white holes in other spacetimes that I am not aware of, where there is mass in the manifold. However, even if such spacetimes do exist the white hole cannot be caused by the mass or anything else since the white hole is literally the beginning of time. There is no part of the manifold before a white hole, nothing is before it so nothing can cause a white hole.
 
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  • #20
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But if white hole has no mass , how come it has gravity? (Black hole can have a satellite , it means that on time reversed e.g. white hole will have also that satellite) .
 
  • #23
Dale
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But if white hole has no mass , how come it has gravity? (Black hole can have a satellite , it means that on time reversed e.g. white hole will have also that satellite) .
Gravity is more complicated than it is in Newtonian theory. In GR, there are non-trivial vacuum solutions, including the maximally extended Schwarzschild solution. A vacuum solution is one that describes a way that gravity can exist without any source.

In other words, spacetime can simply curve this way even without mass. That is what it means for something to be a vacuum solution.
 
  • #25
Ibix
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I did
I see. But if we interpret what you said as meaning that a white hole has no mass it also means that a black hole has no mass.

Your point, I think, is that the mass of a Schwarzschild black/white hole is a feature of the spacetime geometry, and not associated with any matter or radiation. I think that seems to have passed @shlosmem by.
 

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