B Do black holes determine time's arrow?

[Dale]

To me, the way to solve this confusion is to distinguish between "mass", a global geometric property, and "stress-energy", a local property.
…
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.

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.

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.” We are counting on the word to give a false impression in order to avoid discussing a challenging concept. I don’t think that is the way to go.

 

[PeterDonis]

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.”

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.

 

[martinbn]

I think the confusion comes from using mass when it is meant matter. The space-time is vacuum so there is no matter in it(the stress-energy tensor is zero) but the space-time has mass.

 

[shlosmem]

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.

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.

 

[PeroK]

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.

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.

It's more mathematics, than physics. It's purely a solution to the Einstein Field Equations. There is no quantum mechanics (or any other physics to complicate things). It's a hypothetical model of spacetime and nothing else.

My advice is to treat the whole thing as mathematics, not physics. And whatever you do, find some way to comprehend that it is not and cannot be part of the universe we live in.

 

[Dale]

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 ?

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.

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.

Hawking radiation is not a feature of the Maximally extended Schwarzschild solution.

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

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.

 

[martinbn]

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 seems to treat a specific solution in spherical symmetry. I don't see how they can make any general conclusions based on it.

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 what may cause confusion. I would say that matter collapses not mass. Mass is just a characteristic of matter.

 

[stevendaryl]

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

Are you using this paper as an example of one that is not rigorous, or one that is rigorous?

I'm very skeptical of the conclusions of that paper. The idea that Hawking radiation would prevent a black hole from ever forming in the first place has a certain intuitive appeal, but my feeling is that if it were true, researchers would not have spent such an enormous amount of effort on the information paradox.

 

[shlosmem]

I am not aware of any actual exploding white hole solutions,
...
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.

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]

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.

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.

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.

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]

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.

So you disagree with @Ibix on this matter but the original question is now back - do black hole sets a time arrow?

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.

About the second law of thermodynamics I don't mind, because we talking about reversed time which this law is not applied.

 

[PeroK]

So you disagree with @Ibix on this matter but the original question is now back - do black hole sets a time arrow?

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.

 

[martinbn]

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.

Explosion may be too strong. It seems to me like a white hole emitting and then becoming a star.

 

[stevendaryl]

This is incorrect. The black hole solution in which the time reverse is a white hole is the maximally extended Schwarzschild solution.

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. If you introduce a thermodynamic arrow of time, then the distinction between a black hole and a white hole becomes that for a white hole, the thermodynamic arrow of time points away from the singularity, while for a black hole, it points towards the singularity.

 

[stevendaryl]

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.

I don't think that the question was nonsensical, and I'm not sure I agree with your answer.

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]

I don't think that the question was nonsensical, and I'm not sure I agree with your answer.

Why don't you answer the OP's question, then?

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

 

[PeroK]

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.

I can see nothing there that directly addresses the OP's question.

 

[stevendaryl]

I can see nothing there that directly addresses the OP's question.

The direct answer is NO. Black Holes do not determine the arrow of time.

 

[Dale]

So you disagree with @Ibix on this matter

I am not sure. Where do you think we disagree?

the original question is now back - do black hole sets a time arrow?

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.

About the second law of thermodynamics I don't mind, because we talking about reversed time which this law is not applied.

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.

 

[Dale]

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.

100% agree

 

[PeterDonis]

in the reverse process this energy go in into the white hole

No. Nothing can go into a white hole.

 

[PeterDonis]

I am not aware of any actual exploding white hole solutions

I don't know that anyone has actually written a paper on it, but one obvious such solution would be the time reverse of the Oppenheimer-Snyder collapsing solution. Such a solution would be physically unrealistic since the white hole singularity would be in the past, not the future, and the white hole horizon would be a past horizon, and neither of those things have any physically reasonable way of coming into existence.

 

[PeterDonis]

there really is no distinction between a black hole and a white hole unless you introduce a thermodynamic arrow of time.

It's a little more complicated than that.

In any system of time-symmetric laws, such as GR, there will be two types of solutions. One type is itself time-symmetric; the maximally extended Schwarzschild spacetime is an example of this type. In such a solution, one has to make an arbitrary choice of which time direction is the "future" direction. (Since the solution is globally hyperbolic, one only has to make one such choice, which holds for the entire spacetime.)

The other type of solution comes in pairs, each of which is the time reverse of the other. The Oppenheimer-Snyder collapsing solution, which describes a spherically symmetric star collapsing to a black hole, is one of such a pair; it will have a time reverse that describes a spherically symmetric white hole exploding and becoming a star. For this type of solution, "choosing a time direction" corresponds to choosing which solution of the pair you want to look at, rather than choosing which direction of time is the "future" in a time symmetric solution (as with the first type above).

 

[Dale]

I don't know that anyone has actually written a paper on it, but one obvious such solution would be the time reverse of the Oppenheimer-Snyder collapsing solution.

D’oh! Yes that is obvious (once someone points it out). There is no corresponding black hole in that one. Just the white hole that spews dust out of one part of the singularity.

 

[Ibix]

I don't know that anyone has actually written a paper on it, but one obvious such solution would be the time reverse of the Oppenheimer-Snyder collapsing solution. Such a solution would be physically unrealistic since the white hole singularity would be in the past, not the future, and the white hole horizon would be a past horizon, and neither of those things have any physically reasonable way of coming into existence.

I was thinking along these lines earlier (which is why @shlosmem thinks I disagree with @Dale - I don't, we're talking about different things), although considering a time reverse of an evaporating black hole. The white hole would come about from a very violent inrushing of blackbody radiation (time reversed Hawking radiation) and grows from a steady infall of blackbody radiation before starting to emit matter and radiation then finally un-collapsing into a star. You have a point about the singularity, though, which remains difficult to explain. Although we don't really expect the singularity to be a real thing, so perhaps this shouldn't worry us too much.

 

[PeterDonis]

The white hole would come about from a very violent inrushing of blackbody radiation (time reversed Hawking radiation)

You can't create a white hole this way. The white hole's singularity (as you appear to realize) would have to magically already be there, as would its horizon.

To see this, look at the Penrose diagram for a black hole that is formed by gravitational collapse of a null shock wave and then emits Hawking radiation and evaporates. For example, see Fig. 10 of this paper:

https://arxiv.org/pdf/hep-th/9501071.pdf

If we time reverse this (flip the diagram over so $i^+$ is at the bottom and $i^-$ is at the top), we can see that the singularity and the region inside the event horizon are not "reachable" from the flat region at the bottom of the flipped diagram (which is the time reverse of the region at the top of Fig. 10 as given). So while you could add a "shock wave" that was the time reverse of the last pulse of Hawking radiation in the evaporating black hole (i.e., the time reverse of extending the "event horizon" line in Fig. 10 all the way up and to the right, to $I^+$), that shock wave could not create the white hole singularity or the region inside the white hole horizon (since that horizon is now a past horizon, i.e., nothing can get inside it). So those things would have to magically already be there.

 

[PeterDonis]

we don't really expect the singularity to be a real thing, so perhaps this shouldn't worry us too much

Sure it should, because models in which the singularity is replaced by something else, but still otherwise look just like the models we've considered so far at the classical (or semi-classical) level, simply don't exist. That is pretty much the upshot of the last few decades of research on black hole evaporation and the information paradox. In other words, the original view that, since the singularity is hidden deep inside the horizon, it doesn't really affect anything outside, is not really tenable any more; any model that replaces the singularity with something else will have at least some effect on the region outside the horizon (or it will not even have an event horizon at all, only an apparent horizon).

 

[stevendaryl]

It's a little more complicated than that.

In any system of time-symmetric laws, such as GR, there will be two types of solutions. One type is itself time-symmetric; the maximally extended Schwarzschild spacetime is an example of this type. In such a solution, one has to make an arbitrary choice of which time direction is the "future" direction. (Since the solution is globally hyperbolic, one only has to make one such choice, which holds for the entire spacetime.)

The other type of solution comes in pairs, each of which is the time reverse of the other. The Oppenheimer-Snyder collapsing solution, which describes a spherically symmetric star collapsing to a black hole, is one of such a pair; it will have a time reverse that describes a spherically symmetric white hole exploding and becoming a star. For this type of solution, "choosing a time direction" corresponds to choosing which solution of the pair you want to look at, rather than choosing which direction of time is the "future" in a time symmetric solution (as with the first type above).

I’m not sure I understand the distinction you are making.

The issue is whether an arrow of time is an intrinsic attribute of the spacetime geometry or is additional structure imposed by us. Given a spacetime geometry corresponding to a star collapsing into a black hole, we can choose how we describe that geometry using coordinates. Flipping the sign of the time coordinate is just changing the description, not the geometry. So the white hole solution isn’t a different solution, it’s the same solution using different coordinates.

If you include an observer complex enough to have a thermodynamic arrow of time, then relative to such an observer, you could define a black hole to be a solution in which there is a future-directed timelike path reaching the central singularity. A white hole would be a solution where there is a past-directed timelike path reaching the singularity. But that distinction is relative to the is observer’s own arrow of time, rather than being something intrinsic in the geometry.

 

[PeterDonis]

The issue is whether an arrow of time is an intrinsic attribute of the spacetime geometry or is additional structure imposed by us.

Yes. And for the first type of solution (time symmetric), the answer is that it's imposed by us, because we have to decide which direction of the solution to call the "future" direction. While for the second type of solution (comes in pairs, each the time reverse of the other), it's an intrinsic attribute of the spacetime geometry, because each individual spacetime geometry is not time symmetric; one "end" of time is different from the other.

Given a spacetime geometry corresponding to a star collapsing into a black hole, we can choose how we describe that geometry using coordinates. Flipping the sign of the time coordinate is just changing the description, not the geometry.

No. The two time-asymmetric solutions (star collapsing into black hole, white hole exploding into star) are not the same solution with the sign of the time coordinate flipped. They are a pair of distinct solutions, each of which is the time reverse of the other. The fact that you can take a description of one in a particular coordinate chart, flip the sign of the time coordinate, and have a description of the other, does not change the fact that they are two distinct solutions. Physically, the two distinct solutions correspond to two distinct physical scenarios.

If you include an observer complex enough to have a thermodynamic arrow of time, then relative to such an observer, you could define a black hole to be a solution in which there is a future-directed timelike path reaching the central singularity. A white hole would be a solution where there is a past-directed timelike path reaching the singularity. But that distinction is relative to the is observer’s own arrow of time, rather than being something intrinsic in the geometry.

No. The spacetime geometry in any time asymmetric solution has an intrinsic direction; as I have already said, such solutions always come in pairs (since the underlying laws in GR are time symmetric), each of which is the time reverse of the other. One solution has the singularity in the future; the other has it in the past. That is an intrinsic property of each spacetime geometry. The fact that we, as observers with a thermodynamic arrow of time, choose the solution that matches our actual physical experience in order to build models of our actual universe, i.e., in which the future direction that is intrinsic to the spacetime geometry is the same as the future direction of our thermodynamic arrow of time, does not mean the spacetime geometry does not have an intrinsic direction of time.

 

[PeterDonis]

Given a spacetime geometry corresponding to a star collapsing into a black hole, we can choose how we describe that geometry using coordinates.

Note, btw, that while you can indeed choose any coordinates you like, that doesn't change invariants, and there are invariants, like the expansion scalar of the matter, that distinguish the two solutions (star collapsing into black hole vs. white hole exploding into star--the former has a negative expansion scalar, the latter has a positive expansion scalar).