Hawking Says, There are no black holes.

[zoobyshoe]

Hawking Says, "There are no black holes."

Most physicists foolhardy enough to write a paper claiming that “there are no black holes” — at least not in the sense we usually imagine — would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape.

In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form.

“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. 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...”

More:

http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583

 

[Enigman]

The war continues(?)...
The relevant Arvix paper: http://arxiv.org/pdf/1401.5761v1.pdf

Information Preservation and Weather Forecasting for Black Holes

S. W. Hawking
(Submitted on 22 Jan 2014)
It has been suggested [1] that the resolution of the information paradox for evaporating black holes is that the holes are surrounded by firewalls, bolts of outgoing radiation that would destroy any infalling observer. Such firewalls would break the CPT invariance of quantum gravity and seem to be ruled out on other grounds. A different resolution of the paradox is proposed, namely that gravitational collapse produces apparent horizons but no event horizons behind which information is lost. This proposal is supported by ADS-CFT and is the only resolution of the paradox compatible with CPT. The collapse to form a black hole will in general be chaotic and the dual CFT on the boundary of ADS will be turbulent. Thus, like weather forecasting on Earth, information will effectively be lost, although there would be no loss of unitarity.

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.

 

[dipole]

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?

 

[atyy]

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

 

[JesseM]

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 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?

 

[Baggins101]

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.

 

[JesseM]

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

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:

Thus the full geometry is flat at infinity, has a ‘throat’ type region at smaller r where it approximates the naive geometry (4.104), and then instead of a singularity at r = 0 it ends in a smooth ‘cap’. This particular geometry, corresponding to the profile (5.105), was derived earlier in [14, 15] by taking limits of general rotating black hole solutions found in [16]. We have now obtained it by starting with the particular NS1-P profile (5.105), and thus we note that it is only one member of the complete family parametrized by F⃗. It can be shown that all the metrics of this family have the same qualitative structure as the particular metric that we studied; in particular they have no horizons, and they end in smooth ‘caps’ near r = 0. We will review the argument for this smoothness below

Also this on p. 35:

There is a naive geometry that has a horizon, and we have ‘actual’ geometries that have no horizons but that differ from each other inside a region of the size of the horizon.

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"?

 

[|Glitch|]

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.

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?

 

[Chalnoth]

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.

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.

Also, if the event horizon becomes smaller than the apparent horizon, would not the black hole "radiate" light?

Black holes do radiate.

 

[Ken G]

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.

 

[marcus]

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

 

[JesseM]

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.

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.

 

[Ken G]

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.

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.

 

[JesseM]

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.

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.

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.

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.”'

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.

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:

The culmination was the celebrated Hawking-Penrose theorem (Hawking and Penrose, 1970), which since then is the singularity theorem par excellence. However,
all of the singularity theorems share a well-defined skeleton, the very same pattern. This is, succintly, as follows (Senovilla, 1998a)

Theorem 1 (Pattern Singularity Theorem) If a space-time of sufficient differentiability satisfies

1. a condition on the curvature
2. a causality condition
3. and an appropriate initial and/or boundary condition

then there are null or time-like inextensible incomplete geodesics.

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.

 

[Ken G]

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.

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.

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."

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!

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.”'

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 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:

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.

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.

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.

 

[|Glitch|]

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.

 

[JesseM]

Well, you need quantum mechanics to get any kind of radiation, classical physics has the ultraviolet catastrophe.

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.

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

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."

and if Hawking thinks that simple chaos provides a way out

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.

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.

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").

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.

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?

 

[Ken G]

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.

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.

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)?

But that's a pretty big "except"! That was the cause of the paradox.

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

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.

 

[Ken G]

Debate about what exactly?

Whether or not there "really are" black holes.

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 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 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.

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.

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."

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.

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.

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!

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?

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.

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?

More-- also AdS/CFT duality, the holographic principle, and even CPT symmetry.

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").

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, 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?

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."

 

[martinbn]

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.

 

[Ken G]

Yes, it's possible that singularity questions are a red herring in regard to the presence of formal event horizons, Hawking's main point seems to be twofold, that there's no reason to expect firewalls for a host of reasons, but also that you don't need firewalls to solve the information paradox, because you can solve that with chaos. But then he admits if you are solving it that way, then it is tantamount to saying that formal event horizons (which would produce information paradoxes) are precluded from forming by chaos, which in turn means you only get apparent event horizons, which further means that you don't get "actual" black holes, only apparent ones. My point is only that if we are inclined to believe the formation of black holes would be chaotic right out to the event horizon, then we would already have a very mundane solution to the information paradox that really doesn't seem to rely at all on the heavy machinery of quantum mechanics, because the same escape is available to us when applying Newtonian formalism to weather. Whether one regards that "generic" or "mundane" quality as a strength or weakness, it is certainly a bit of a rabbit-from-a-hat, with a lot of extra smoke and mirrors coming from the language of string theory and holographic thinking.

 

[atombuster]

I think that if black holes where not real than how did the Hubble space telescope see them or how did some astrounats see them? Really now I just think that Hawking is not right about it. You can also read about it on discover magazine online.

 

[Chalnoth]

I think that if black holes where not real than how did the Hubble space telescope see them or how did some astrounats see them? Really now I just think that Hawking is not right about it. You can also read about it on discover magazine online.

1. Nobody has "seen" a black hole. There are many indirect measurements that demonstrate that objects near to the density of black holes must exist.
2. Hawking's idea isn't that there is no extremely dense object, but rather that the surface of the object isn't quite a horizon as has been supposed for many decades.

 

[Ken G]

It should also be mentioned that in astronomy, the only kinds of black holes we currently have evidence of are so large that AFAIK they produce no observable effects that have anything to do with Hawking radiation, and no observable effects that have anything to do with the difference between the kinds of event horizons Hawking is talking about, i.e., real vs. apparent event horizons. So in regard to actual astrophysical black holes, none of these issues have any demonstrable relevance whatsoever, and certainly there is no practical difference between a "real" black hole and an "apparent" black hole. Hence, there is no reason to think about them any differently. We really are going to have to find evidence for black holes with much much smaller masses than any we've seen evidence of, before any of this debate is going to have the slightest physical importance, unless the Big Bang itself can be framed as some type of Hawking radiation (and I'm not sure what is the status of that kind of thinking).

Of course the reason people are interested in this is because it seems like a place where quantum mechanics and general relativity merge, so it seems relevant for unification, but I would argue it isn't the right place for that at all, for the simple reason that we do not seem to have any useful observations related to these issues in the context of astrophysical black holes. We should look somewhere else.

 

[Chalnoth]

It should also be mentioned that in astronomy, the only kinds of black holes we currently have evidence of are so large that AFAIK they produce no observable effects that have anything to do with Hawking radiation, and no observable effects that have anything to do with the difference between the kinds of event horizons Hawking is talking about, i.e., real vs. apparent event horizons. So in regard to actual astrophysical black holes, none of these issues have any demonstrable relevance whatsoever, and certainly there is no practical difference between a "real" black hole and an "apparent" black hole. Hence, there is no reason to think about them any differently. We really are going to have to find evidence for black holes with much much smaller masses than any we've seen evidence of, before any of this debate is going to have the slightest physical importance, unless the Big Bang itself can be framed as some type of Hawking radiation (and I'm not sure what is the status of that kind of thinking).

Of course the reason people are interested in this is because it seems like a place where quantum mechanics and general relativity merge, so it seems relevant for unification, but I would argue it isn't the right place for that at all, for the simple reason that we do not seem to have any useful observations related to these issues in the context of astrophysical black holes. We should look somewhere else.

Though depending upon the scale of the deviations from a true event horizon, part of me wonders if this sort of experiment might be able to put Hawking's idea to the test:
http://www.eventhorizontelescope.org/

The experimental idea is to use very long baseline interferometry to allow a series of radio telescopes to have a resolution sharp enough to actually view the silhouette of a black hole (specifically, the supermassive black hole at the center of our galaxy, Sagittarius A*). I sincerely doubt any deviation would be visible from a first attempt at this kind of experiment, but maybe sometime in the future we might be able to do even better, for example with an array of orbiting satellites at extremely large distances from one another.

Edit: Bear in mind that the scale of corrections could very easily be on the order of a few Planck lengths, making it essentially impossible to test experimentally. That would be unfortunate, but is entirely possible.

 

[Ken G]

Certainly any kind of direct measurement would be fantastic!

 

[Manraj singh]

Hawking Says, "There are no black holes."

The thing as that when an object is absorbed by a black hole, it gives out radiations. Mainly x rays. These are detected, proving the existence of black holes. And it is impossible to see a black hole, visible light can not escape from it. You would just end up looking through it.

 

[madboy]

black holes

Well i think that black holes do exist right giant objects in the universe who have gravity that bends space time actually i thought that stars turn into black holes once their energy finished so black holes must contain compact stars at the middle of the black holes so how come the concept of singularity comes into play i want to know it for sure. And yeah i read about it hawking is proposing a theory that black holes don't exist instead objects called grey holes exist well theese objects are quite interesting given that their event horizon contains all the particles unfortunate enough to enter it.

 

[Edward Wij]

I just came across the Nature article separately and saw this thread. After one year, what is your thoughts of it?

Black hole is one solid proof of the geometric underpinning of General Relativity and they say you can't model black holes using gravity as field or force. But with this admission by Hawking there are no black holes. Does this deduct one proof of General Relativity and makes possible a non-geometric theory of gravity ?

 

[Ken G]

When Hawking said there were no black holes, he was speaking somewhat figuratively. He did not mean we no longer have the observational support for GR that says something strange happens to spacetime when you create a Schwarzschild radius, he just meant that what happens is a bit more complicated, and hard to predict, than the usual simple black-hole image of a singular point of mass, surrounded by a nice smooth spacetime with an event horizon that light cannot escape from. The bottom line is, because we cannot observe beneath the event horizon, it becomes difficult to apply the standard process of the scientific method to decide what is going on in there, and the experts disagree rather completely on what they think is happening there. Even Hawking radiation, for normal black holes, is completely impossible to observe, so the observational status of the whole business is kind of a mess-- with the one important exception that all the different approaches used by the experts predict that something odd happens when a Schwarzschild radius is generated, and that's something that the observations can address-- and it checks out nicely.

 

[Quds Akbar]

In his paper Information Preservation and Weather Forecasting for Black Holes Hawking wrote:
"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. There are however apparent horizons which persist for a period of time. This suggests that black holes should be redefined as metastable bound states of the gravitational field"
Black holes still exist. Watch this:

 

[Dale]

Let's not necropost threads that were sketchy to begin with.