"There is no escape from a black hole in classical theory," Hawking told Nature.
rjbeery said:PeterDonis, I should have clarified that the defense of black holes has not come from everyone, but certain people on this forum and others have defended them unequivocally.
rjbeery said:PeterDonis...I feel you're being selectively nuanced. Look at ZapperZ's comment above yours; this, despite the title of the article linked to, specifically quoting Hawking in a very clear manner. You don't recall all the mentions of "clear evidence of black holes in astronomy" in previous threads?
rjbeery said:PeterDonis, I should have clarified that the defense of black holes has not come from everyone, but certain people on this forum and others have defended them unequivocally.
DaleSpam said:I don't see anything in the arxiv preprint that is in dramatic disagreement with the conversations here.
However, I thought that one of the key assumptions is somewhat dubious. Specifically, the assumption that any quantum gravity theory would have global CPT symmetry. The argument requires global CPT symmetry, but GR is only locally Lorentz invariant and violates Lorentz symmetry globally. Maybe I am misunderstanding the assumption here.
Hi atyy! With respect, I think you and PeterDonis should be careful with comments regarding "nobody" and "everybody". I've had discussions of black holes with many physicists, both student and graduates, who would insist that event horizons exist. Perhaps when I get home from work tonight I'll search for a few examples...atyy said:That's a very important clarification you should have made.
No one bats an eye that in quantum gravity there may not be a true event horizon. The fuzzball picture is one such prominent proposal.
rjbeery said:Hi atyy! With respect, I think you and PeterDonis should be careful with comments regarding "nobody" and "everybody". I've had discussions of black holes with many physicists, both student and graduates, who would insist that event horizons exist. Perhaps when I get home from work tonight I'll search for a few examples...
Philosophically, I ask what exists in reality; this demands a rigorous definition of "existence". In a book on GR, where the concept of "now" is not considered, then of course event horizons exist. In an applied concept of GR, however, with a reasonable definition of what it means "to exist" relative to the "now"...YES I contest the idea that event horizons exist in general relativity...atyy said:Black hole event horizons unequivocally exist in classical general relativity. Are you contesting this?
It is definitely not a consensus at this time.rjbeery said:I would be happy with a consensus that event horizons do not exist due to QM effects.
Hi DaleSpam, do you even recall any of my specific objections?DaleSpam said:It is definitely not a consensus at this time.
However, even if such a consensus does develop, I don't think that it would place the conversations that I have seen with you on the topic in any need of revisions. IIRC, your objections to black holes were unrelated to those given by Hawking, and the explanations you received to your specific objections still stand.
I would be curious to see the math involved here, because this contradicts my understanding of black holes. I only bring it up because the same idea was mentioned on this site, and I twice neglected to ask to see the math.Either a black hole is entirely in your causal future (i.e. it "hasn't formed yet", as in the case of a large star that's still millions of years from collapsing), or it's not and part of it, which can include part of the singularity itself, are in your causal present. In the latter case, as the people on PhysicsForums are also explaining to you, regions within the event horizon in your causal present are a finite spacelike distance from you, in the sense that you can define a spacelike curve between you and such a region of the black hole, integrate ds along that curve, and you will get a finite value.
Does "interpolated" mean you're basically 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 deSitter metric" rather than the usual black hole interior spacetime? Does "periodically identified" refer to the idea of making a potentially infinite space (like the deSitter metric) into a finite one by picking a finite region and making a topological identification of the edges? (the sort of idea discussed here) Could the "interpolated" spacetime he's proposing be visualized in terms of something like a Penrose diagram?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.
rjbeery said:Philosophically, I ask what exists in reality; this demands a rigorous definition of "existence". In a book on GR, where the concept of "now" is not considered, then of course event horizons exist. In an applied concept of GR, however, with a reasonable definition of what it means "to exist" relative to the "now"...YES I contest the idea that event horizons exist in general relativity...
rjbeery said:PeterDonis...I feel you're being selectively nuanced.
rjbeery said:You don't recall all the mentions of "clear evidence of black holes in astronomy" in previous threads?
rjbeery said:I wouldn't call responses to this as "authoritative explanations" as much as "plausible explanations"; the latter possibly being used in defense of a particular viewpoint (i.e. that black holes currently exist).
rjbeery said:One issue that I regret not delving into further is the concept of the spacelike separation between an outside observer and both A) a point within the event horizon, and B) the singularity.
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I would be curious to see the math involved here, because this contradicts my understanding of black holes.
PeterDonis said:In other words, is there some way of showing that, at the classical level, there is no way of constructing a self-consistent model of an evaporating black hole? The answer to this question is "no" because such models can, and have, been constructed.
atyy said:Are there really classical models of black hole evaporation? My understanding is that black hole evaporation is semi-classical, and one gets a mixed state from a pure state, which is inconsistent with the assumption of unitarity.
PAllen said:Classical in this sense excludes quantum issues at all. What you need to do (to model a decaying BH) is violate at least the dominant energy condition - which is necessary for any classical model a radiating BH. Then you meld a distant Vaidya spacetime to a shrinking BH interior. In the junction region, the energy condition is violated, which is what allows energy to leave the horizon. Spacelike paths trivially cross the horizon, and violating energy conditions allows energy to follow spacelike paths.
atyy said:That's interesting. What's a good reference for this?
Also, is the radiation emitted "Hawking radiation" in the sense that it is thermal?
TrickyDicky said: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.
TrickyDicky said: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."
martinbn said:@PAllen: why can't an event horizon exist in a closed universe? Do you mean a compact space-time, not a spatially finite universe?
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.rjbeery said:Hi DaleSpam, do you even recall any of my specific objections?
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.PAllen said:I meant a closed, finite, universe; one where future null conformal infinity is undefined.
JesseM said: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.
Recent studies have shown that the traditional definition of a black hole, which states that it has an event horizon from which nothing can escape, may not be entirely accurate. This is based on the fact that the event horizon is a theoretical concept and has never been directly observed. Additionally, alternative theories such as modified gravity and gravastars have been proposed as possible explanations for the phenomena attributed to black holes.
The observed gravitational effects of black holes can be explained by alternative theories of gravity, such as modified Newtonian dynamics (MOND) or modified general relativity. These theories propose that gravity behaves differently at very large scales, which could account for the observed effects without the need for black holes.
The matter that was thought to have been consumed by black holes may actually be ejected back into the universe in the form of jets or other high-energy particles. This is supported by observations of active galactic nuclei, which show that matter is being expelled from the center of galaxies at high speeds, rather than being consumed by a black hole.
The non-existence of black holes would challenge our current understanding of gravity and the structure of the universe. It would also require a re-evaluation of the role that black holes play in the formation and evolution of galaxies. Additionally, it could lead to the development of new theories and models to explain the observed phenomena attributed to black holes.
Yes, there are ongoing studies and experiments, such as the Event Horizon Telescope, that aim to directly observe the event horizon of a black hole. If successful, this could provide strong evidence for the existence of black holes. Additionally, further research into alternative theories of gravity and the behavior of matter in extreme environments could also shed more light on the nature of black holes.