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Hawking Says, There are no black holes.

  1. Jan 25, 2014 #1
    Hawking Says, "There are no black holes."

    More:

    http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583
     
  2. jcsd
  3. Jan 25, 2014 #2
    The war continues(?)...
    The relevant Arvix paper: http://arxiv.org/pdf/1401.5761v1.pdf
    Would be interesting to see how Susskind reacts to this...

    E: "Black holes do not exist"
    Methinks either Hawking is laymanizing or he really wants to get back his Penthouse collection from Thorne.
     
    Last edited: Jan 25, 2014
  4. Jan 26, 2014 #3
    I don't really know much about the information paradox stuff, but I don't see why what he suggests is so radical. Why would one expect that black holes exist exactly as classical GR predicts them?
     
  5. Jan 26, 2014 #4

    atyy

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    Here is a review of other considerations against taking event horizons as a very fundamental concept. Visser also notes that Hawking already proposed the non-existence of event horizons in 2004 at GR 17.

    http://arxiv.org/abs/0901.4365
    Black holes in general relativity
    Matt Visser (Victoria University of Wellington)
    (Submitted on 28 Jan 2009 (v1), last revised 5 Feb 2009 (this version, v3))

    "A common statement that one often encounters in the literature is this:

    “Horizons are not detectable with local physics”.

    The above statement is, of course, false. Note however, that it is almost true. Two closely related,
    but true, statements are:

    “Event horizons are sometimes not detectable with local physics”;

    “Apparent/ dynamical/ trapping horizons are not detectable with ultra-local physics”."


    Also, one prominent proposal against an event horizon is the fuzzball proposal, which has been around long before Hawking's latest paper.

    http://arxiv.org/abs/hep-th/0502050
    The fuzzball proposal for black holes: an elementary review
    Samir D. Mathur
     
  6. Jan 26, 2014 #5

    JesseM

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    This bit from Hawking's preprint seems like it's probably a crucial part of what he's proposing, I was curious if anyone here has a better understanding than me (or at least some well-informed speculation) about what it means:
    Does "interpolated" mean something like topologically stitching together a spacetime consisting of the exterior region of a standard evaporating black hole with a new inner region where the spacetime is a "periodically identified anti deSitter metric" rather than the usual black hole interior spacetime?
     
  7. Jan 27, 2014 #6
    Is this not exactly what Susskind has already proposed? Is Hawking merely agreeing (at last) with Susskind or is he proposing something notably different to Susskind?

    It seems that Hawking is being given credit for Susskind's work in the press simply by admitting Susskind was right.
     
  8. Jan 27, 2014 #7

    JesseM

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    I don't know enough to understand any of the details of this, but this part on p. 28, combined with the diagram on p. 32, sounds like it could be related to what Hawking was saying in the statement I quoted earlier:
    Also this on p. 35:
    Is this "smooth cap" the same as (or related to) the "periodically identified anti deSitter metric" that Hawking says "interpolates between collapse to a black hole and evaporation"?
     
  9. Jan 27, 2014 #8
    If there is matter being temporarily held behind the apparent horizon, would not this matter be packed even denser than neutrons? Perhaps the matter, once behind the apparent horizon, has been reduced to quarks due to temperature and pressure.

    Also, if the event horizon becomes smaller than the apparent horizon, would not the black hole "radiate" light?
     
  10. Jan 29, 2014 #9

    Chalnoth

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    I'm not sure "held" is the right term. I believe the picture that Hawking is presenting is essentially that black holes are regions of space-time with extreme turbulence. This turbulence both makes it take a very long time for matter to re-emerge, and also effectively randomized said matter.

    Black holes do radiate.
     
  11. Feb 3, 2014 #10

    Ken G

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    Looking at all the recent flap about the black hole information paradox, and the more recent "firewall paradox", it is hard not to conclude that we have a tempest in a teacup the size of an event horizon! Look at Hawking's own admission: "The correct treatment remains a mystery." So what we have is, theorists arguing about the right way to do calculations in the overlap of two theories that are known to be inconsistent with each other. What we don't have is people saying "solution X leads to observable effect Y, so let's look for Y," or "we already have observation Y, so we know we should be looking for X in the correct solution." So this is a pretty sorry state of affairs. What makes it really clear how bad the problem is, is that Hawking's recent paper is essentially concluding that the resolution of the black hole information paradox is something completely mundane: deterministic chaos. This means it is the same resolution as the resolution of the "information paradox" in understanding the movements of the air around us right now, despite all the tours de force invoking AdS/CFT duality and holographic speculation. That should be giving us some pause, I would say.
     
  12. Feb 3, 2014 #11

    marcus

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    Dearly Missed

    Incidental intelligence:
    Nobel physicist Frank Wilczek's comment on the Hawking paper was
    "I think the kind thing to do is to pass this over in silence."

    Hawking's paper was mentioned briefly at the start of a 24-minute panel discussion of broader topics by three physicists on the 31 January edition of PRI's "Science Friday" program
    http://www.sciencefriday.com/segment/01/31/2014/could-there-be-a-crisis-in-physics.html

    the three were

    Lawrence Krauss
    Foundation Professor
    Director, The ASU Origins Project
    Arizona State University

    Frank Wilczek
    Nobel Laureate in Physics, 2004
    Herman Feshbach Professor of Physics
    Massachusetts Institute of Technology

    Brian Schmidt
    Nobel Laureate in Physics, 2011
    Professor
    Australian National University (Canberra)

    Wilczek's comment comes around minute 4:00
     
    Last edited: Feb 3, 2014
  13. Feb 3, 2014 #12

    JesseM

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    I don't think Hawking was suggesting deterministic chaos alone could explain it without also invoking some theory of quantum gravity--I thought the Penrose-Hawking singularity theorems showed definitively that in classical general relativity, if a mass collapses beyond a certain point there's no way to avoid the collapsing mass getting crushed into a singularity.
     
  14. Feb 3, 2014 #13

    Ken G

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    You need quantum mechanics to have Hawking radiation in the first place, but if you accept that you have that, then Hawking's resolution of the information paradox is just like a resolution of the information paradox in a Newtonian description of weather. Newtonian mechanics respects CPT too, and seems to have all the essential elements Hawking quotes in his paradox resolution, as long as we stipulate the existence of Hawking radiation. Indeed, if chaos precludes the creation of real event horizons, then the Penrose-Hawking theorems are invalid anyway, even without Hawking radiation or quantum mechanics.
     
  15. Feb 3, 2014 #14

    JesseM

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    It's not clear that's true--Hawking radiation would ultimately be derived from a theory of quantum gravity but the current derivation uses semiclassical gravity, which from what I've read still involves a classical spacetime geometry obeying the rules of general relativity, rather than a superposition of different geometries or something. So I don't think any of the conclusions about when event horizons and singularities become inevitable would be different in semiclassical gravity than they are in general relativity with classical matter fields. However, as I mention below the singularity theorems do depend on certain energy conditions, and quantum fields can violate them in certain cases, so this might be a way out.
    What are the essential elements you're referring to? It seems to me that Hawking is invoking ideas beyond just classical chaos + Hawking radiation + CPT invariance--for example look at the section from his paper I was asking about earlier, where he said "I take this as indicating that the topologically trivial periodically identified anti deSitter metric is the metric that interpolates between collapse to a black hole and evaporation." Also, the Nature article here suggests Hawking is just trying to sketch how he thinks things would work in a future theory of quantum gravity: 'A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”'
    I don't think it's true that the Penrose-Hawking singularity theorems depend on the assumption that there's a "real event horizon". The wikipedia article does at one point describe the singularity theorem in terms of event horizons--"The singularity theorems prove that this cannot happen, and that a singularity will always form once an event horizon forms"--but there is no citation for this claim and I suspect it's incorrect, because Hawking's theorem dealt with the Big Bang singularity which wouldn't have an event horizon, and anyway I thought event horizons were defined in terms of the boundary between points where all lightlike worldlines hit a singularity and points where some can escape to infinity. The review on singularity theorems at http://arxiv.org/pdf/physics/0605007.pdf gives an outline on pages 7-8 of what conditions are used to derive the conclusion that a singularity forms, and event horizons aren't among them:
    As explained on p. 8, #1 is satisfied as long as the matter field doesn't violate certain energy conditions like the strong energy condition, and p. 5 of this presentation by Matt Visser mentions that the Penrose singularity theorem which is relevant to black holes (as opposed to the Big Bang, which Hawking's dealt with) requires the weak energy condition. Also note that p. 6 of Visser's presentation mentions that the averaged null energy condition (ANEC) is used in the "generalized Penrose singularity theorem" by Roman (which seems to be this paper, which says "we show that Penrose’s singularity theorem will still hold if the weak energy condition is replaced by a weaker (nonlocal) energy condition and if the null generic condition holds"), and that "ANEC is the weakest averaged energy condition in common use." So although quantum fields like those involved in Hawking radiation can violate various energy conditions, it sounds like the conclusion of an inevitable singularity would still apply provided Hawking radiation didn't violate ANEC--I'm not sure if current theory says anything definite about this one way or the other.

    #2 is discussed on p. 8 of the paper, they call it the "most reasonable and well-founded condition" and it sounds as though it just means the spacetime doesn't contain closed timelike curves, which wouldn't be expected in any real-world model of conditions where matter was collapsing into a black hole.

    On #3, the "boundary condition", the paper says on p. 10 that the most commonly used one is the existence of a "trapped surface", which is different from an event horizon. One such trapped surface would be the apparent horizon, which is defined as the outermost trapped surface around a black hole, and can differ from the event horizon--and Hawking says in the abstract that his proposal involves the claim that "gravitational collapse produces apparent horizons but no event horizons behind which information is lost". So there is no assumption of an event horizon here, only an apparent horizon, which Hawking still assumes would exist.

    If I'm understand the above summary of the Penrose-Hawking singularity theorems correctly, it shouldn't be possible in general relativity or semiclassical gravity to have such a trapped surface and to avoid a singularity, at least not unless the spacetime contains closed timelike curves or violates ANEC. It might be that current knowledge doesn't rule out the idea that Hawking radiation violates ANEC and that this means semiclassical gravity alone can give a model where there are no singularities and no true event horizons, but I doubt Hawking was trying to argue for such a purely semiclassical explanation, since he doesn't even mention energy conditions in his paper.
     
  16. Feb 3, 2014 #15

    Ken G

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    Well, you need quantum mechanics to get any kind of radiation, classical physics has the ultraviolet catastrophe. But I don't know if these details are that important, what matters is that even the issue of whether or not one expects Hawking-Penrose singularities in the first place is a matter of debate, and if Hawking thinks that simple chaos provides a way out, like it allows information to be lost in the practice of weather prediction, then what it means to me is that we have yet another example of physicists taking their physical theories too seriously, essentially because they can.

    Now, there is nothing wrong with deriving the ramifications of a theory, the problem is in thinking that reality is beholden to those ramifications. The theories are supported by experiments, so ultimately, a derivation using a theory is nothing more than a means of finding other experimental outcomes that would be consistent with existing ones via some particularly simple or aesthetically unified connection. That doesn't mean the unobserved consequences are required by the observed ones, it only means they are consistent with what has been observed. But here we have the real problem: what unobserved consequences are we talking about? In the context of astrophysical black holes, the clear answer is, unfortunately, none-- there are no unobserved consequences that any of this debate seems to be trying to motivate us to look for. As long as that continues to be true, the entire field is sadly sterile. So rather than debating information paradoxes and firewall paradoxes, I feel the theorists should be madly looking for observable predictions they can use the theory to make. Note that Hawking radiation itself does not even appear to be one of those observable predictions, not as long as we are stuck with astrophysical black holes.
    My point is, the details of the local theory being used are really not terribly important if chaos is being invoked as its solution. We have all the same issues with a Newtonian description of weather. This is the subtext of Hawking's own analogy, that the resolution is similar to unpredictability of weather. It is nice to have an analogy that everyday people can find meaning in, but in this case, the analogy is very much a double-edged sword: to whatever extent it is a valid analogy, that is the extent to which the heavy-duty theory being invoked to support the conclusion is not terribly relevant. In other words, if you believe you need that heavy-duty theory to get a believable outcome, but the believable outcome ends up looking just like weather prediction, then the actual answer to the paradox transcends the heavy-duty theory, it is a resolution that is available in a broad range of local theories, including ones we haven't even come up with yet. It is a perfectly mundane resolution, the one we should probably have expected all along: that formal physics theories do not always accurately reflect the behavior of the usable information that physicists need to manipulate in order to make testable predictions, of the kind you need to even decide if you will regard that formal theory as accurate. This is not exactly a new discovery!
    Right, that's exactly my point. In my view, Hawking is basically saying that since we don't know the right theory to use, the best we can do is hit it with the biggest sledgehammer we have around, but even when we do that, we end up staring a very simple and old problem right in the face: the formal theory doesn't work in practice, because of simple chaos. That is a conclusion that is quite separate from the formal theory chosen, so if we expect that black hole formation is chaotic, we don't need a formal theory at all, to reach the conclusion that Hawking reaches.
    I'm not saying an event horizon is needed to be assumed, I'm saying that whatever you need to assume to get a singularity is going to produce an event horizon. A "real" event horizon, as opposed to an "apparent" event horizon, seems to be the difference between an eternal black hole, and a temporary one. I believe the idea is that in pure GR, you expect black holes to be eternal if they create real event horizons, which creates a paradox (which is essentially the black hole information paradox) if you tack on Hawking radiation. In other words, the paradox is, how do you marry the quantum mechanical expectation of Hawking radiation with the Hawking-Penrose singularity theorems, which seems related to the problem of marrying unitarity with no-hair theorems.

    I'd say the entire landscape of what assumptions lead to what expectations is somewhat overshadowed by the deeper problem of not really having any idea what assumptions are even valid in the first place-- in the absence of better observational constraints.
     
  17. Feb 4, 2014 #16
    I am confused, which I admit is not an uncommon occurrence. I have always been under the impression that anything smaller than the Schwarzschild radius was considered a black hole (at least for the uncharged, non-rotating variety), and that the surface at this radius was effectively the event horizon. I do not understand how a black hole could not have an event horizon. Is not an event horizon the point were nothing can escape the gravitational pull of the black hole (except for Hawking radiation)?

    Also, could someone explain the difference between the event horizon and the apparent horizon?

    Thank you.
     
  18. Feb 4, 2014 #17

    JesseM

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    I was talking about the fact that Hawking radiation has been derived using semiclassical gravity, which involves modeling quantum fields on the curved spacetime of classical general relativity.
    Debate about what exactly? The Penrose-Hawking singularity theorems only claim anything about what happens in general relativity, they don't claim that general relativity's predictions will perfectly model our actual physical reality. I don't think there's any debate that the theorems are correct about what must happen in general relativity. Since semiclassical gravity still uses the rules of general relativity to model spacetime--it just treats the "matter field" in a quantum way--the singularity theorems should still apply to it. And as far as I can tell the only relevant loophole in the singularity theorems is that if the averaged null energy condition isn't satisfied in the neighborhood of a black hole the singularity theorems might not apply, but Hawking doesn't even mention energy conditions so it doesn't seem that this is that argument he is making.

    Then there is also the point, which you didn't respond to, that Hawking seems to be talking about something more than just chaos when he says "I take this as indicating that the topologically trivial periodically identified anti deSitter metric is the metric that interpolates between collapse to a black hole and evaporation."
    That is precisely the thing I am disputing! I don't think that Hawking in fact says anything like "simple chaos provides a way out" in the paper, I think you are misreading it. And my point, again, is that the phrase "simple chaos provides a way out" seems to imply that if we take chaos into consideration, even our existing theoretical model which we use to analyze black hole formation and Hawking radiation (semiclassical gravity) might allow for the possibility that no event horizon or singularity would form. Is that what you're arguing? If so, again my point is that the singularity theorems still apply to semiclassical gravity, they have been proved mathematically so there's no chance that "simple chaos" would negate the conclusion that if certain conditions are satisfied (conditions 1-3 that I quoted), a singularity is inevitable.
    When you say "biggest sledgehammer we have around" and "the formal theory", what theory are you referring to? General relativity with quantum fields on the spacetime, i.e. semiclassical gravity? That's the only approach I know of to dealing with quantum effects in curved spacetime that doesn't get into speculations about quantum gravity, and we don't have a completed theory of quantum gravity yet (which I assume is what you meant when you said "we don't know the right theory to use").
    Again, are you claiming that even if the "formal theory chosen" is general relativity (including semiclassical gravity), chaos can negate the conclusion of the Penrose-Hawking singularity theorems that if certain conditions are satisfied, a singularity is inevitable?
     
  19. Feb 4, 2014 #18

    Ken G

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    That's all still true, especially since you said "effectively." Hawking isn't saying there aren't things that act like we expect astrophysical black holes to act, he is drawing a formal distinction between things that act for all practical purposes like black holes, and "actual" black holes that could present an information paradox.
    But that's a pretty big "except"! That was the cause of the paradox.
    It seems to be a very technical point, perhaps involving whether the black hole has a true singularity inside there, or some kind of turbulent super-high density but that somehow escapes the singularity theorems. Others who know the details better might chime in.
     
  20. Feb 4, 2014 #19

    Ken G

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    Whether or not there "really are" black holes.
    I agree, but you wouldn't think the theorists believe that, the way they talk about the ramifications of their calculations. Let's start with Hawking radiation itself!
    I don't know if he equates the presence of singularities with the presence of real event horizons, but that seems like a natural connection to make. If so, then he is indeed saying the singularity theorems must not apply, and he agrees that GR says they should, so he is looking for a way that reality can avoid obeying GR. His conclusion seems to be that reality avoids obeying GR much like the way air avoids obeying Newtonian theorems of time-reversible motion, which has to do with the difference between a result of a formal theory, and a result of how information actually works in practice. But I don't see how that directly relates to the singularity theorems, he seems to only connect the issue directly with the information paradox.
    I think that all comes under the heading of trying to find the appropriate solution. But my point is, who cares what is the appropriate solution if one is going to invoke chaos anyway-- chaos is an extremely generic, and rather mundane at that, form of dynamics! Certainly not something that requires the sledgehammers that Hawking invokes elsewhere in his paper, such as the one you mention.
    Then cite the place where Hawking justifies his appeal to chaos, and show me why it wouldn't apply to simple Newtonian physics just as well-- bearing in mind that the "weather" analogy is Hawking's, not mine!
    It's not what I'm arguing, it's what Hawking is arguing. He is arguing that somewhere along the line, the proper physics will go chaotic. He does not cite any special attributes of the proper physics that will do that, he mostly uses the proper physics to defeat the firewall idea (which Hossenfelder probably already defeated anyway). Indeed, he makes an analogy with a completely different set of physics to say what chaos is doing here, I think that puts it pretty clearly into perspective that the proper physics is rather incidental to that aspect of his argument.
    More-- also AdS/CFT duality, the holographic principle, and even CPT symmetry.
    Exactly, Hawking admits that freely-- we don't really know the proper theory to use. That's yet another reason why Hawking's argument has a very "generic" flavor, underneath all the formal references to advanced theories and speculations.
    Again, that is Hawking's argument. He feels that certain aspects of the "right theory" will supercede the GR theorems. My point is, if that opens the door for chaos, as he claims, then the information paradox resolution is just as mundane as the resolution of the "weather information paradox."
     
  21. Feb 5, 2014 #20

    martinbn

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    I don't think he equates presence of singularities with that of event horizons. This would depend on some form of cosmic censorship, and probably there is no need for that to hold outside of classical general relativity(may be not even there) for example with the presence of Hawking radiation.
     
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