Black Holes and Information Loss

Discussion in 'Physics & Math' started by RJBeery, Mar 21, 2017.

  1. Schmelzer Valued Senior Member

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    First, why should I do this?

    BHs in such popular statements are sloppy speaking anyway. BH means some object which in classical GR would be described by a Schwarzschild or Kerr solution. If one believes in HR, very small BHs would evaporate. But this would have to be a quantum gravity process, thus, naming this "BH evaporation" is nothing but sloppy speech.
     
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  3. RJBeery Natural Philosopher Valued Senior Member

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    Because it isn't possible to reconcile them yet you seem reluctant to acknowledge this.
     
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  5. przyk squishy Valued Senior Member

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    I don't see that there's anything that needs to be reconciled. As far as I know you had it at least partially right when you said this:
    Classical GR does not predict any such thing as a collapsing black hole. So, yes, the whole idea of a collapsing black hole contradicts GR.

    In classical GR, a black hole is always in the causal present or future of an external observer. The picture people have in mind for a collapsing black hole is different. Googling for "Kruskal evaporating black hole" turns up diagrams like this one, which seems to be the sort of model people speculating about this stuff have in mind:

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    So, according to this picture at least, the black hole does not remain in the outside observer's causal future or present indefinitely; instead, at a certain finite time for the outside observer, the black hole's event horizon drops into their causal past while the interior of the black hole just gets cut off for them.
     
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  7. Schmelzer Valued Senior Member

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    Acknowledge???? I have maid clear that I don't believe they are correct, nor about the possibility that BHs may be created there, nor about HR, thus, as the BHs themselves, as their evaporation are their fantasy, not supported by established physics.

    The creation of such microscopic BHs is afaik a string theory speculation, thus, supported by nothing but speculation. But, of course, if there exists such a speculation, and had made its way into a scientific journal, and the public had heard about it, they have to respond. One reasonable response would have been that this is simply speculation based on nothing but a speculative theory which has no observational support at all. But there are too many scientists supporting this speculation, so that this is not possible. Similar for HR. So, they have to give an explanation, which allows for string theory being possible, so if something in string theory allows for such micro BHs, they have to say yes, this cannot be excluded. And once HR is believed by many, they have to say that it would immediately evaporate.

    As I have explained, the language for this, with talking about BHs being created and evaporating, is sloppy language. But established language. Naming conventions in physics are in general sloppy, in comparison with mathematics, which used precise definitions. It does not really matter how something is named, the relevant point is that everybody knows what one means.

    Nonsense. A "collapsing BH" makes not much sense, the correct phrase is a gravitational collapse which leads to a BH. After horizon creation, the inner part would continue to collapse to create a singularity. According to classical GR.
    The second claim is simply wrong, see #26 for the details.
     
  8. RJBeery Natural Philosopher Valued Senior Member

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    I object to the notion that can evaporating black hole of any sort can contain an event horizon. It's a logical contradiction. As I posted in the other thread on Hawking radiation:

    1) HR requires an event horizon
    2) HR causes BH evaporation
    3) An evaporating BH has a finite life
    4) A BH with a finite life cannot contain an event horizon because it would be capable of existing in our past light cones

    Either Hawking radiation suffers a logical contradiction or event horizons form in a way other than predicted by general relativity.

    And, Schmelzer, I think my wording was provocative. It wasn't intentional.
     
  9. przyk squishy Valued Senior Member

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    Um, okay. "Evaporating black hole" then.

    I meant for an observer who remains outside the black hole, i.e., pick an event A outside the black hole and an event B somewhere inside it. Then B is either in the causal present or future of A.


    No, that is an assumption.

    Well the picture of an evaporating black hole I gave above has an event horizon and it does drop into an outside observer's causal past at some finite time from their perspective.

    Is that picture accurate or reliable? I don't know. Like Schmelzer says, ideas about evaporating black holes come from mixing classical GR and quantum physics in a way that is known can't be entirely consistent.

    But it is the (or at least, one) picture people have in mind for black hole evaporation, and it illustrates something you have not logically ruled out.
     
  10. RJBeery Natural Philosopher Valued Senior Member

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    Yes I have logically ruled it out. Wald's definition of an event horizon (from here) is:
    The "last breath" of an evaporating black hole is a specific point in spacetime which, without question, is contained in the causal past of future null infinity. There is a clear and obvious contradiction here. Perhaps you should supply the source of your graphic?
     
  11. przyk squishy Valued Senior Member

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    No, this is just a question of applying a definition. There isn't a contradiction unless you make one up.

    If you pick an event that you are considering calling "last breath of the evaporating black hole" then either it is not in the causal past of future infinity, and so part of the black hole by that definition, or it is in the causal past of future infinity, and so maybe associated with the black hole but not technically part of it by that definition.


    I gave a link in the post that used it, though it doesn't seem to show up well with the fonts/colours used here: https://inspirehep.net/record/1400849/plots.

    I don't see why this is important though. I didn't include that picture because I thought it was necessarily correct or the source authoritative. I included it because it shows a logical possibility that you don't seem to have considered, let alone ruled out.

    So my real point here is, you need to be careful about assuming things that you haven't actually proved in general. For example, dichotomies like "if it's not in my causal present or future then it must be in my causal past" might hold in classical GR but not necessarily in some theory of gravity that predicts evaporating black holes. (And I'm not sure that particular dichotomy always holds even in classical GR.)
     
  12. RJBeery Natural Philosopher Valued Senior Member

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    So in order for currently accepted concepts surrounding Hawking radiation to be shown to be logically contradictory, *I* need to prove that there isn't "some theory of gravity that predicts evaporating black holes"...which isn't general relativity?

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    Maybe you missed this
    What you're saying would only have merit if Stephen Hawking wrote in his paper that he was taking as a given that event horizons exist, possibly outside the framework of general relativity. He did no such thing, and no one has ever mentioned that as a possibility. You're actually agreeing with me here, you just don't know it. Either Hawking radiation is a logical contradiction or general relativity is wrong.
     
  13. przyk squishy Valued Senior Member

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    It is your responsibility to prove what you claim.

    When you write a paper challenging "currently accepted concepts surrounding Hawking radiation" then your target audience is people who do research on this and related topics, and it is also your responsibility to make sure that what you are challenging is the pictures and ideas these people have about Hawking radiation and evaporating black holes and not some strawman of it. It doesn't look to me like you've done that.


    I haven't read Hawking's paper so I don't know what he says or does not say. But the prediction of Hawking radiation comes from quantum physics and I'd say that as soon as you mix quantum physics with GR you are accepting up front that the classical GR picture can't be correct in every respect.

    Having said that, I don't see a substantial difference between the event horizons that bound classical GR black holes and the event horizon appearing in the Kruskal diagram for the evaporating black hole that I randomly dug up.
     
    Last edited: Mar 28, 2017
  14. RJBeery Natural Philosopher Valued Senior Member

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    Speaking of strawmen, you're trying to say that I'm claiming Hawking radiation is wrong which is not true -- I'm saying that Hawking radiation and general relativity, together, produce a logical contradiction. I don't need to "prove" anything beyond that -- I've already stated (three times now) that event horizons could be produced in some other manner...which is the same thing as saying general relativity is wrong. If that's your stance then so be it.
     
  15. RJBeery Natural Philosopher Valued Senior Member

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  16. przyk squishy Valued Senior Member

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    Sure, then, but then what's your point? Classical GR predicts only black holes that don't evaporate and don't emit Hawking radiation.

    Sure but, again, what's your point? As far as I know that's already been the consensus for decades.
     
  17. RJBeery Natural Philosopher Valued Senior Member

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    What's my point? You're better than this przyk. This means that the information loss paradox may not be a paradox at all, event horizons don't exist, and that Hawking radiation's role in resolving the paradox may be redundant. Nuclear Physics B found it interesting enough...

    http://www.sciencedirect.com/science/article/pii/S0550321316301274
     
  18. Schmelzer Valued Senior Member

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    Ok, let's try:
    Formally even true, but afaiu not even RJBeery claims this.
    There is no such notion as "causal present" in GR. The GR notion closest in its meaning to "causal present" would be "space-like separated".

    Fine. If the BH would evaporate, there would be no horizon.
    No. A Hawking-like radiation exists during the collapse even before horizon formation. If the radius is close to the horizon radius, the temperature of the radiation will be close to Hawking temperature.
     
  19. RJBeery Natural Philosopher Valued Senior Member

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    You cannot just say "no" here Schmelzer. Stephen Hawking mentions the event horizon 66 times in his original paper. This is from the summary:
    If black holes radiate / evaporate then it is not in a manner explicitly envisioned and described by Hawking.
     
  20. Schmelzer Valued Senior Member

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    Lol. Do you really think I have to care about what Hawking claims? What I have said is correct, this is all what matters.
     
  21. RJBeery Natural Philosopher Valued Senior Member

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    I agree with you that the following is possible
    But you can't just announce it as fact, that you're correct, that Hawking is irrelevant, and give no other supporting evidence, logic or reasoning. While I agree that a Hawking-like radiation (occurring in the absence of an event horizon) makes the most sense, I think it's important to emphasize that we can't really truly know. The only thing I have said with certainty is that a logical contradiction cannot describe reality.

    Correct me if I'm wrong but I thought your position was that micro black holes didn't exist anyway? If that's true then what evidence of or need for radiation, which obviously differs from Hawking's, do we have?
     
  22. Schmelzer Valued Senior Member

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    What we can know is what semiclassical quantum field theory predicts for this situation. Because it is simply math. And the fundamental math here is even quite simple: The classical background changes, therefore the corresponding vacuum state changes in time too, thus, the old vacuum state, which simply evolves, is no longer a vacuum state, thus, radiation appears. The question is only how much radiation with which frequencies. Pure math.
    Nobody thinks that semiclassical theory is a correct description of reality. It is inconsistent as a theory. This does not change the fact that as an approximation it seems quite reasonable. But only if one cares about the question if semiclassical theory is applicable at all. During the collapse itself, it is. For some time. If the surface time dilation reaches factors where Planck time on the surface becomes, say, a year outside, this is quite obviously no longer the case.
    We have no observational evidence for Hawking radiation. And no hope to get some. All one can hope for is that for some effects in fluid dynamics, which are, in some weak sense, analogical to HR, we may have a chance to observe it. This is a logic of type "assume ether theory is true. Then all this gravity is analogical to some usual condensed matter. In condensed matter, we observe XYZ. So we have observational evidence for this XYZ effect for gravity too."
     
  23. przyk squishy Valued Senior Member

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    Sure, that's a possibility, but what does it have to do with what you were saying? Your change of topic to the information loss paradox makes no sense to me unless you were under the impression that it is a paradox in classical GR and Hawking radiation was supposed to "fix" it. But information loss isn't a problem for classical physics. Unusual, maybe, but not inherently paradoxical. Information loss (non-unitarity) is specifically a problem for quantum physics, and the black hole information paradox refers to a problem that arises when you mix GR and quantum physics, at least in a certain naive way, where you apparently get evaporating black holes and Hawking radiation. So it is the conception people have of evaporating black holes that is relevant to that discussion. The question there is what the correct and consistent way to model that physics is. It is not a problem of classical GR or how you define "information loss" in that context.
     
    Last edited: Mar 30, 2017
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