Black holes may not exist after all!

  • #26
atyy
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S. Hawking:"The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can't escape to infinity."

If Hawking weren't one of the most authoritative voices on the subject of black holes since its modern conceptualization in the late sixties, this same sentence coming from almost anyone else would have been deemed as pure crackpottery and if affirmed around here the author most likely banned no matter how he had argumented it, as justified by a strict adherence to the guidelines. This simple observation is probably what spurred the apparent irritation in the OP.

I must say I very much concur with the nuanced explanations by Peter Donis, but it is also fair to say that most people in those threads discussing black holes made an "unequivocal defence of the existence of black holes" probably based on pop-sci accounts(or at the very least not so carefully nuanced as Peter's explanations here) of black holes that have been taking for granted their existence for many years.
But the same sentence can indeed be nonsense, and it is unclear what the previous arguments defending the possible nonexistence of black holes the OP referred to are. The OP does not direct us to discuss Hawking's new paper per se, but implicitly requires us to discuss it with respect to his previous discussions. Hawking's new paper is not required to conclude that black holes may not exist. Referring to Hawking's new paper is poor justification of previous claims for or against the existence of black holes.
 
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  • #27
PAllen
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S. Hawking:"The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can't escape to infinity."
Actually, I don't think it is crackpottery at all, but simply definitional. A true horizon has light never escaping to infinity. An apparent horizon is a local trapping surface. Already, with a closed universe, there are no 'true' black holes, because a true horizon can't exist, only an apparent horizon can. Similarly, as soon as any form of BH decay is posited, the horizon is formally an apparent horizon.

To me, the sense in which it is at all meaningful to talk about BH in the non-classical universe we live in is:

- Do collapsed objects have a surface which externally, and macroscopically behaves like an apparent horizon? Most quantum treatments (including fuzzball and firewalls and Hawking's new paper) say yes. One example that doesn't is the Krauss et. al. paper. Experiment may decide this soon.

- Does fall through the (apparent) horizon behave macroscopically like the classical model? Many quantum models (including fuzzball and Hawking's new paper) say yes, but firewalls says no. It is not clear to me how this would ever be directly verified. Other predictions of theories would have to decide.

- Is there a singularity inside the apparent horizon? I think no one would bother publishing a model that says yes. For this issue, also, I don't see any plausible way this would be subject to verification.

- Do BH's actually decay or emit Hawking radiation? Unless we eventually make or find a very small BH, this will likely never be settled experimentally. Astronomical BH emissions are way, way, to small to detect; and decay times dwarf the life of stars.
 
  • #28
martinbn
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@PAllen: why can't an event horizon exist in a closed universe? Do you mean a compact space-time, not a spatially finite universe?
 
  • #29
PAllen
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@PAllen: why can't an event horizon exist in a closed universe? Do you mean a compact space-time, not a spatially finite universe?
I meant a closed, finite, universe; one where future null conformal infinity is undefined.
 
  • #30
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Hi DaleSpam, do you even recall any of my specific objections?
Yes, I clearly recall the thread where you made a definition of exist which tautologically resulted in black holes not existing. I didn't follow all of your other threads closely, but your objection to black holes in that thread had nothing to do with Hawking's arxiv paper and the explanations you received would still stand even if his idea becomes the consensus.
 
  • #31
JesseM
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I meant a closed, finite, universe; one where future null conformal infinity is undefined.
Even in a closed universe, is it meaningful to distinguish between worldlines that terminate in a black hole singularity and worldlines that terminate in the Big Crunch singularity? If so, then even if the standard definition of "event horizon" in terms of escaping to infinity wouldn't apply, perhaps one could still define a different sort of horizon marking the boundary between points in spacetime where all lightlike worldlines through that point end in the black hole singularity, and points where at least some lightlike worldlines terminate in the Big Crunch singularity. But maybe a distinct black hole singularity doesn't exist in this case, I don't know.
 
  • #32
PAllen
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Even in a closed universe, is it meaningful to distinguish between worldlines that terminate in a black hole singularity and worldlines that terminate in the Big Crunch singularity? If so, then even if the standard definition of "event horizon" in terms of escaping to infinity wouldn't apply, perhaps one could still define a different sort of horizon marking the boundary between points in spacetime where all lightlike worldlines through that point end in the black hole singularity, and points where at least some lightlike worldlines terminate in the Big Crunch singularity. But maybe a distinct black hole singularity doesn't exist in this case, I don't know.
But then some events inside the apparent horizon (near the big crunch) would be considered outside the true horizon. An 'outgoing' null ray from them might be caught be the big crunch without reaching what was the BH singularity.

I think, for a universe without the appropriate infinity it makes more sense to define an 'effective BH' in terms of the local trapping surface plus classically predicted singularity.
 
  • #33
Vanadium 50
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This is drifting into the realm of personal theories.
 

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