shlosmem
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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?
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.shlosmem said:Let's say we have a mass that collapse to a black hole - now under time reversal we also have the same mass concentration,
Can you please explain it a bit more?Ibix said:You would not expect this to form a new hole, black or white.
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.shlosmem said: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?
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.Dale said:It is a mathematical solution that I don’t think anyone believes represents any part of the actual universe.
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.shlosmem said: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.
Black holes evaporate through Hawking radiation. So arrange time-reversed Hawking radiation and you'll get a time-reversed black hole.shlosmem said: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.
To go back to the original question. I don't see the dependence of the arrow of time on black hole formation.shlosmem said: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.
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.Dale said: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.
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.shlosmem said: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.shlosmem said: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.
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.shlosmem said: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?
Excuse me - I addressed this when I said:shlosmem said: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.
Ibix said:Black holes evaporate through Hawking radiation. So arrange time-reversed Hawking radiation and you'll get a time-reversed black hole.
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).shlosmem said:But if this is the case how does the same mass states causes 2 different behavior?
Not just topology, it is spacetime topology and curvature.shlosmem said:It seems like you all treating black and white holes as spacetime topologies
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.shlosmem said:without paying attention to the cause of that topology which is the mass.
https://en.wikipedia.org/wiki/White_holeshlosmem said: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) .
Who said it has no mass?shlosmem said:But if white hole has no mass
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.shlosmem said: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) .
I didIbix said:Who said it has no mass?
Dale said:There is no mass anywhere in the manifold.
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.Dale said:I did
Yes, since it is a vacuum solution there is no mass anywhere in the manifold. There is a characteristic length scale for the manifold, the Schwarzschild radius. In geometrized units mass has dimensions of length, so you can turn this length scale into a mass scale, but it is important to recognize that this mass isn’t in the Schwarzschild spacetime.Ibix said:it also means that a black hole has no mass
While I understand what you mean by this, it is non-standard terminology and will cause confusion (as, indeed, it already has in this thread).Dale said:since it is a vacuum solution there is no mass anywhere in the manifold. There is a characteristic length scale for the manifold, the Schwarzschild radius
But the white hole still has the same attractive gravity as any other object with the same mass. The difference is that you can't actually fall into the white hole; you end up falling into the black hole region of the same global spacetime geometry instead.Dale said:as described earlier a white hole cannot be formed from collapse. So we are left only with the length scale.
I disagree with your assessment. The confusion is caused by the standard terminology. Because the standard terminology calls the length scale a mass there is a reasonable but incorrect assumption that there is actually mass in the spacetime and that the mass is the cause of the gravitation. I am trying to clarify for the OP a point that is unclear precisely due to the standard terminology.PeterDonis said:it is non-standard terminology and will cause confusion (as, indeed, it already has in this thread).
Yes, as the OP saidPeterDonis said:But the white hole still has the same attractive gravity as any other object with the same mass.
shlosmem said:Black hole can have a satellite , it means that on time reversed e.g. white hole will have also that satellite
To me, the way to solve this confusion is to distinguish between "mass", a global geometric property, and "stress-energy", a local property. In any physically realistic spacetime, if there is mass (global property), there is stress-energy somewhere (local property). But there are physically unrealistic solutions of the Einstein Field Equations that have mass without having stress-energy, one of which is the maximally extended Schwarzschild spacetime that contains both a white hole and a black hole. There are no physically realistic spacetimes that contain white holes (for the reasons you gave earlier in the thread), which is why it is never possible to explain the mass of a white hole in terms of stress-energy somewhere.Dale said:The confusion is caused by the standard terminology. Because the standard terminology calls the length scale a mass there is a reasonable but incorrect assumption that there is actually mass in the spacetime and that the mass is the cause of the gravitation.
Well, I don’t feel this is much of an improvement. You said “‘mass’, a global geometric property”. That begs the question: what kind of geometric property is “mass”? It is a length scale. So your explanation is the same as mine, but with sneaky terminology.PeterDonis said:To me, the way to solve this confusion is to distinguish between "mass", a global geometric property, and "stress-energy", a local property.
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This seems to me to be a better way to address the confusion because then we don't have to explain how a length scale can gravitate, which is the confusion that emerged earlier in this thread.
I would say we used the word "mass" because that's what we use for objects that can gravitate. But I agree that we still have to explain how that can correspond to a length scale, so we're stuck with that either way.Dale said:The only reason that we wouldn’t “have to explain how a length scale can gravitate” is because we snuck the length scale in by using the word “mass.”
So when the white hole explode it means that we get material out of vacuum which brings new quotations.Dale said: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.
The maximally extended Schwarzschild solution is a hypothetical universe in which there is only a vacuum - by definition. It wasn't created by a collapsing star in our universe; it wasn't created by an explosion.shlosmem said:So when the white hole explode it means that we get material out of vacuum which brings new quotations.
1. What about the energy preservation law ?
2. As mentioned, black holes eventually disappearing dou to Hawking radiation, in other words the original mass that fails into the black hole find itself out as radiation energy. So in the reverse process this energy go in into the white hole and since energy is equivalents to mass, we must assume that the space geometric is not just a vacuum.
I am not aware of any actual exploding white hole solutions, the usual white hole is the vacuum one that we have been discussing so there is no exploding involved. However, local energy conservation is built into the Einstein field equations, so any solution must locally conserve energy at every event in the manifold. The singularity itself is not part of the manifold.shlosmem said:So when the white hole explode it means that we get material out of vacuum which brings new quotations.
1. What about the energy preservation law ?
Hawking radiation is not a feature of the Maximally extended Schwarzschild solution.shlosmem said:2. As mentioned, black holes eventually disappearing dou to Hawking radiation, in other words the original mass that fails into the black hole find itself out as radiation energy. So in the reverse process this energy go in into the white hole and since energy is equivalents to mass, we must assume that the space geometric is not just a vacuum.
This seems to treat a specific solution in spherical symmetry. I don't see how they can make any general conclusions based on it.Dale said:Unfortunately, many treatments of Hawking radiation are not rigorous and use an approach that mistakenly leads to problems like what you mention. Here is a paper which shows that if the black hole evaporates then it actually never formed in the first place:
https://arxiv.org/abs/1102.2609
This is what may cause confusion. I would say that matter collapses not mass. Mass is just a characteristic of matter.Dale said:So if you have mass which collapses, forms an apparent horizon, and evaporates, then there is no true event horizon at all, only the apparent horizon. Hence there is no evaporating black hole and the time reverse is not a white hole.
Dale said:Unfortunately, many treatments of Hawking radiation are not rigorous and use an approach that mistakenly leads to problems like what you mention. Here is a paper which shows that if the black hole evaporates then it actually never formed in the first place:
https://arxiv.org/abs/1102.2609
I'm bit confused now, since the all reason we brought up the white hole to this thread is to say that black hole collapse is time reversal as a white hole. In simple words, if we see a reversed video clip of a star falling into a black hole we can say that the events in the clip obeying the laws of physics because what we see is exploding white hole.Dale said:I am not aware of any actual exploding white hole solutions,
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So if you have mass which collapses, forms an apparent horizon, and evaporates, then there is no true event horizon at all, only the apparent horizon. Hence there is no evaporating black hole and the time reverse is not a white hole.
This is incorrect. The black hole solution in which the time reverse is a white hole is the maximally extended Schwarzschild solution. This is a vacuum solution with no mass anywhere in the spacetime and therefore neither collapse nor evaporation.shlosmem said:I'm bit confused now, since the all reason we brought up the white hole to this thread is to say that black hole collapse is time reversal as a white hole.
You could probably make such a valid solution, but I have never seen one actually done. However, I suspect that it would violate the second law of thermodynamics.shlosmem said:In simple words, if we see a reversed video clip of a star falling into a black hole we can say that the events in the clip obeying the laws of physics because what we see is exploding white hole.
So you disagree with @Ibix on this matter but the original question is now back - do black hole sets a time arrow?Dale said:This is incorrect. The black hole solution in which the time reverse is a white hole is the maximally extended Schwarzschild solution. This is a vacuum solution with no mass anywhere in the spacetime and therefore neither collapse nor evaporation.
About the second law of thermodynamics I don't mind, because we talking about reversed time which this law is not applied.Dale said:You could probably make such a valid solution, but I have never seen one actually done. However, I suspect that it would violate the second law of thermodynamics.
The arrow of time is a fundamental aspect of our universe and is not dependent on a specfic phenomenon such as black hole formation. If we discovered tomorrow that, in fact, black holes cannot form, then we still have an arrow of time.shlosmem said:So you disagree with @Ibix on this matter but the original question is now back - do black hole sets a time arrow?
Explosion may be too strong. It seems to me like a white hole emitting and then becoming a star.shlosmem said:I'm bit confused now, since the all reason we brought up the white hole to this thread is to say that black hole collapse is time reversal as a white hole. In simple words, if we see a reversed video clip of a star falling into a black hole we can say that the events in the clip obeying the laws of physics because what we see is exploding white hole.
Dale said:This is incorrect. The black hole solution in which the time reverse is a white hole is the maximally extended Schwarzschild solution.
PeroK said:The arrow of time is a fundamental aspect of our universe and is not dependent on a specfic phenomenon such as black hole formation. If we discovered tomorrow that, in fact, black holes cannot form, then we still have an arrow of time.
There is no physical or logical sense to your question.
You might as well ask whether the growing of potatoes sets the arrow of time.
Why don't you answer the OP's question, then?stevendaryl said:I don't think that the question was nonsensical, and I'm not sure I agree with your answer.
shlosmem said:Does black holes determines time's arrow? Otherwise how to explain with time reversed objects been pushed out from black holes?
I can see nothing there that directly addresses the OP's question.stevendaryl said:Since the laws of physics have no preferred direction in time (okay, there are processes that violate T invariance, but there is nothing about such violations that determine which direction in time should be "future" and which should be "past"), it's not clear to me that the arrow of time is a fundamental aspect of our universe. The arrow of time seems to be a matter of boundary conditions. There was a Big Bang in our past, so that determines a cosmological arrow of time. However, that doesn't completely solve the problem. In addition to the Big Bang cosmology, where the universe has a finite past and an infinite future, there is an equally valid solution to the equations of General Relativity in which the universe has an infinite past and a finite future, with all matter accelerating toward a Big Crunch in the future. To distinguish between these, we have to rely on the thermodynamic arrow of time. The Big Bang cosmology is the one in which the thermodynamic arrow of time points away from the time of infinite density, while the Big Crunch cosmology is the one in which the thermodynamic arrow of time points towards the time of infinite density.
So the real mystery is the thermodynamic arrow of time. Where did it come from? General Relativity certainly doesn't answer that. You could just say it's an arbitrary initial/final condition; of all possible Big Bang/Big Crunch cosmologies, some have very low entropy immediately after/before the time of infinite density and the entropy rises farther away. In such cosmologies, there is a thermodynamic arrow of time pointing away from the time of infinite density.
PeroK said:I can see nothing there that directly addresses the OP's question.
I am not sure. Where do you think we disagree?shlosmem said:So you disagree with @Ibix on this matter
My answer to that has not changed. No, the arrow of time is due to the second law of thermodynamics. Black holes do not define it.shlosmem said:the original question is now back - do black hole sets a time arrow?
Then about the only thing you can do is use the maximally extended Schwarzschild solution. All of the collapse or evaporation spacetimes will have a change in entropy. You could have something like the maximally extended Schwarzschild solution with a small test object (small enough to only perturb the spacetime negligible) leaving the white hole and going to the black hole.shlosmem said:About the second law of thermodynamics I don't mind, because we talking about reversed time which this law is not applied.
100% agreestevendaryl said:I pointed this out before, but there really is no distinction between a black hole and a white hole unless you introduce a thermodynamic arrow of time. The spacetime geometry itself has no preferred direction in time.