Black holes may not exist after all!

Answers and Replies

  • #2
PeterDonis
Mentor
Insights Author
2019 Award
30,073
9,256
The "unequivocal defense" in your previous threads has been of a statement which Hawking agrees with; from the article:

"There is no escape from a black hole in classical theory," Hawking told Nature.
Nobody has "unequivocally defended" the claim that a correct theory of quantum gravity must also include black holes with event horizons, which is the claim that Hawking's paper is attempting to refute (and the article notes that the paper is still being reviewed). Everyone agrees that we don't (yet) know for sure whether black holes with event horizons are still possible when quantum gravity is taken into account; there are plausible arguments both ways, but we have no general theory and no way to test the question experimentally.
 
Last edited:
  • #3
346
8
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.
 
  • #4
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,791
4,569
But it seems that you have read MORE into this than what has been claimed! He didn't say "Black holes may not exist", as you stated in the title of this thread. He's saying that these blackholes may have different characteristics, with regards to the 'even horizon' than we thought.

This is FAR from saying these things don't exist!

And let's not forget that Hawkings was WRONG when he bet against information being able to escape a black hole. If you put all your eggs in this still-unpublished basket, you might end up in the same fate.

Zz.
 
  • #5
PeterDonis
Mentor
Insights Author
2019 Award
30,073
9,256
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.
Well, I've been one of the people defending unequivocally the classical proposition, which I quoted from the Nature article, and which Hawking agrees with. I don't think anyone has unequivocally defended the quantum proposition (i.e., the claim that quantum gravity must also allow black holes with event horizons); people have made arguments in its favor, but that's not the same as "unequivocally defending" it. Can you give any specific examples of people "unequivocally defending" the claim that quantum gravity must allow black holes with event horizons?
 
  • #6
346
8
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?
 
  • #7
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,791
4,569
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?
I think that it is YOU who have been "selective". Re-read those quote of Hawkings again. But better yet, read his preprint and understand the physics!

Please note that I did not make any argument FOR anything. I pointed out what YOU did and how you made the wrong conclusion based on what you are citing. You turned it around and somehow concluded that I was giving "clear evidence" for black holes.

You have a very strange ability in understanding what you read.

Zz.
 
  • #8
29,736
6,070
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.
 
  • #9
atyy
Science Advisor
13,906
2,175
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.
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.
 
  • #10
atyy
Science Advisor
13,906
2,175
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.
In AdS/CFT, the non-perturbative definition of the theory is given by a Lorentz invariant quantum field theory on Minkowski spacetime, which we can think of as the boundary of the AdS gravitational bulk which emerges at low energy. The boundary non-gravitational theory is D dimensional, but the gravitational theory is D+1 dimensional. I think Hawking means CPT of the boundary theory. Edit: Apparently not.
 
Last edited:
  • #11
346
8
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.
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...
 
  • #12
atyy
Science Advisor
13,906
2,175
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...
Black hole event horizons unequivocally exist in classical general relativity. Are you contesting this?
 
  • #13
346
8
Black hole event horizons unequivocally exist in classical general relativity. Are you contesting this?
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...

That being said, I would be happy with a consensus that event horizons do not exist due to QM effects. :thumbs:
 
  • #14
29,736
6,070
I would be happy with a consensus that event horizons do not exist due to QM effects.
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.
 
  • #15
346
8
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.
Hi DaleSpam, do you even recall any of my specific objections?
For example, 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).

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. In a similar thread on Sciforums it was said that
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.
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.
 
  • #16
JesseM
Science Advisor
8,496
12
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:
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.
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?
 
Last edited:
  • #17
member 11137
I would say: let us wait for the next coming observations of the center of our galaxy. See (e.g.) informations on the ESO website or where you prefer. I have recently participated to a conference concerning that item and came out full of doubts in my head. After having seen the simulation I got the strange impression that we shall observe "nothing" or more precisely the topology of our universe, as if it would have an invisible shape or frontier...
 
  • #18
atyy
Science Advisor
13,906
2,175
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...
This is not a forum to discuss your personal philosophy.
 
Last edited:
  • #19
PeterDonis
Mentor
Insights Author
2019 Award
30,073
9,256
PeterDonis...I feel you're being selectively nuanced.
I'm going on my best recollection of the various threads in which questions like this have come up. My best recollection is that the only claim that has been "unequivocally defended" is the one I quoted from Hawking: *classically*, event horizons are unavoidable. But we all know that reality is not classical.

You don't recall all the mentions of "clear evidence of black holes in astronomy" in previous threads?
The clear evidence is of compact objects that, classically, must be black holes, i.e., must have event horizons. Obviously whether or not they *actually* have event horizons depends on how significant quantum gravity effects are, and we don't have a theory of quantum gravity (yet) that can tell us that. We only have various educated speculations.

But there's also another point that probably should be clarified; Hawking, characteristically, only hints at it in his paper (the one on arxiv that is linked to in the Nature article) and leaves the reader to fill in. The point is this: in order to know for sure whether there is an event horizon present, and if so, where it is, you have to know the entire future of spacetime. We obviously don't. As far as our actual evidence is concerned, we can't distinguish the case where a compact object has an actual event horizon from the case where it only has an apparent horizon--a surface where outgoing light stays at the same radius--but not an actual event horizon, because the apparent horizon eventually disappears due to Hawking radiation.

Hawking is basically claiming that the compact objects we call "black holes" actually only have apparent horizons; if we knew the entire future of spacetime, we would see that these apparent horizons eventually disappear and all the quantum information that was hidden behind them gets back out again, so there is no actual event horizon. But there's no way to test this by observations at our current state of knowledge. We obviously can't directly observe the entire future of spacetime, and we don't have a quantum gravity theory that tells us what indirect observations might shed light on the question.

I can't say how carefully all the above points were observed in the various past threads on this topic, but I would point out that, given the state of our knowledge as I've described it above, asking whether black holes "really" exist, in the sense you appear to be using that term, is pointless: we just don't know. Various people have made various educated speculations, but we have no way of resolving the question at present. So if that's the question you really want an answer to, you're not going to get an answer, no matter how many times you ask it.
 
  • #20
PeterDonis
Mentor
Insights Author
2019 Award
30,073
9,256
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).
The topic question of that thread is another example of a pointless question given our current state of knowledge: at this point we simply don't know whether Hawking radiation prevents the formation of an event horizon.

However, you appear to be confusing that question (which has no answer at present) with another question, which does have the answer "no": is there some *classical* way of using Hawking radiation to rule out the formation of an event horizon? 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. They may or may not represent what actually happens in reality; we don't know at this point. But as models, they are perfectly consistent. That's what people kept trying to explain to you and you kept on not understanding.

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.

...

I would be curious to see the math involved here, because this contradicts my understanding of black holes.
And here is a key reason why, IMO, you kept on not understanding: you don't really understand the purely classical model of black holes, on its own terms, as a model, independent of the question whether or not any real region of spacetime realizes the model. In terms of the model, what DaleSpam is saying here is perfectly obvious. The simplest way to see it mathematically is to use Painleve coordinates; the simplest way I know of to see it graphically is to look at a Kruskal diagram, which makes it obvious that the future light cone of any event E outside the horizon divides the region inside the horizon into two regions, a region inside the future light cone, which is to the future of event E, and a region outside it which is spacelike separated from event E (and therefore allows a spacelike curve to be drawn from event E to any point in this spacelike separated region along which ds can be integrated to get a finite value, as DaleSpam says).
 
  • #21
atyy
Science Advisor
13,906
2,175
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.
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.
 
  • #22
PAllen
Science Advisor
2019 Award
8,095
1,381
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.
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.
 
  • #23
atyy
Science Advisor
13,906
2,175
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.
That's interesting. What's a good reference for this?

Also, is the radiation emitted "Hawking radiation" in the sense that it is thermal?
 
  • #24
PAllen
Science Advisor
2019 Award
8,095
1,381
That's interesting. What's a good reference for this?

Also, is the radiation emitted "Hawking radiation" in the sense that it is thermal?
No, it is not thermal, it is null dust - purely classical. I will try to dig up references this weekend.
 
  • #25
3,507
26
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 in finity."

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.
 

Related Threads on Black holes may not exist after all!

Replies
44
Views
12K
Replies
61
Views
10K
  • Last Post
Replies
9
Views
3K
Replies
1
Views
2K
Replies
10
Views
3K
Replies
5
Views
2K
  • Last Post
Replies
10
Views
3K
Top