Speed of light enough to escape black holes ?

[PAllen]

Good try to discredit it (and already your second attempt, as it's what discovermagazine discussed) - but untrue according to Physical Review. The "analysis is in 3+1 dimensions and within conventional general relativity."

It provides perhaps the clearest and most up-to-date answer to the question of this thread, and from a quality source.
I'm still reading it.
[edit: it has 28 citations, so there may be a more relevant newer article on this]

Here is its abstract:

"We study the formation of black holes by spherical domain wall collapse as seen by an asymptotic observer, using the functional Schrodinger formalism. To explore what signals such observers will see, we study radiation of a scalar quantum field in the collapsing domain wall background. The total energy flux radiated diverges when backreaction of the radiation on the collapsing wall is ignored, and the domain wall is seen by the asymptotic observer to evaporate by non-thermal ``pre-Hawking radiation'' during the collapse process. Evaporation by pre-Hawking radiation implies that an asymptotic observer can never lose objects down a black hole. Together with the non-thermal nature of the radiation, this may resolve the black hole information loss problem. "

Hawking radiatiion and pre-Hawking radiation are outside of classical GR. I have read enough to see the obvious fact that every aspect of its conclusions is based on quantum corrections to GR. Even the topic: "Information Loss" is a problem that exists only for quantum mechanics + GR. Evaporation is not part of classical GR, and is fundamental to their argument. This paper is not a paper on classical GR at all.

 

[PAllen]

Good! - that is exactly my point. I will start a topic on that model vs Einstein's GR.

Do you consider spacetime diagrams or the block universe different theories than SR? To me, they are just different approaches to picturing SR. Hamilton's river model's are not a new theory, just a conceptual aid. They change not a single equation or rule for computing an observable.

 

[PAllen]

I saw that you gave the names of two methods that can be used to give different answers...

Thus answering the question at hand, which was "is there a way to say what event for a distant observer is simultaneous with reception of a signal by an interior observer?" The answer being: two well known possible conventions; plus the applicability of the general SR/GR rule that indicates there are infinite admissable answers and no way to say one is 'right'. If you want more specifics, pose a specific scenario with numbers, and I will try to compute a specific answer for you, using one or both specific conventions.

 

[harrylin]

OK, I now read sufficiently of http://arxiv.org/abs/gr-qc/0609024 to give a quick summary as concerns the topic here.

"Our analysis is in 3+1 dimensions and within conventional general relativity. [..] we find that Schwarzschild coordinates are sufficient to answer the very specific set of questions we ask from the asymptotic observer’s viewpoint."

"the standard result [is] that the formation of an event horizon takes an infinite (Schwarzschild) time if we consider classical collapse."

And they conclude:

"First, we studied the collapse of a gravitating spherical domain in [..] classical [..] theory, ignoring any evaporative processes. [...] our results show that [..] the horizon does not form in a finite time".

This is, as I mentioned earlier, also discussed on another forum:
http://blogs.discovermagazine.com/badastronomy/2007/06/19/news-do-black-holes-really-exist/

A follow-up paper states and explains the same, perhaps even more clearly:

"Black Hole - Never Forms, or Never Evaporates" (title)
- http://arxiv.org/abs/1102.2609

From that I conclude that the modern answer to the question of this thread is that according to GR it can never happen - similar to "what happens when I go faster than light".

I'm satisfied with that answer (and note that the original poster did not come back to this thread).

 

[harrylin]

[..] If you want more specifics, pose a specific scenario with numbers, and I will try to compute a specific answer for you, using one or both specific conventions.

Thanks for your kind offer; but it won't be needed!

PS. I started the new topic on "flowing space models" here:
https://www.physicsforums.com/showthread.php?t=647627

 

[PAllen]

First, one author does not represent the 'modern view'. Krauss is recognized as a cosmologist not a GR expert. Even so, I show how I believe you are over-interpreting the claims of this paper.

OK, I now read sufficiently of http://arxiv.org/abs/gr-qc/0609024 to give a quick summary as concerns the topic here.

"Our analysis is in 3+1 dimensions and within conventional general relativity. [..] we find that Schwarzschild coordinates are sufficient to answer the very specific set of questions we ask from the asymptotic observer’s viewpoint."

Yes, a specific set of questions. There is never any dispute about what light or radiation reaches a distant observer from the BH region.

The different question that came up in this thread was: can the distant observer send a message to an infaller? Where is the infaller when the message reaches them? Is there a way to talk about 'when' for the external observer the infaller gets the message? In classical GR, the first two of these are physical observables with absolutely indisputable answers; the last is purely a matter of convention - true for any question about distant simultaneity. The indisputable answer to the physical question:

- the infaller will receive signals from the distant observer at a well defined finite time on their watch.
- the infaller will continue receiving such signals past the EH all the way to the singularity.

This paper simply chose not address these question, so in no way does it refute them.

"the standard result [is] that the formation of an event horizon takes an infinite (Schwarzschild) time if we consider classical collapse."

Schwarzschild time is not an observable of GR, it is a coordinate quantity. The only relevant observable for classical GR is the never disputed observation that no signal or influence will propagate from the EH or inside to a distant observer.

And they conclude:

"First, we studied the collapse of a gravitating spherical domain in [..] classical [..] theory, ignoring any evaporative processes. [...] our results show that [..] the horizon does not form in a finite time".

Now this I would say is wrong without qualification. The question is whose time? If you follow an infalling dust particle in a collapse, the horizon forms in finite time, the particle crosses it. Further, the area of the externally observed, ultra-redshift boundary grows as new matter falls into the region. Each new piece of matter you follow in its local frame crosses a larger surface area horizon.

So here, this phrasing (at least the snippet you've given, taken in isolation) is at least misleading. It certainly does not represent consensus view of classical GR.

This is, as I mentioned earlier, also discussed on another forum:
http://blogs.discovermagazine.com/badastronomy/2007/06/19/news-do-black-holes-really-exist/

A follow-up paper states and explains the same, perhaps even more clearly:

"Black Hole - Never Forms, or Never Evaporates" (title)
- http://arxiv.org/abs/1102.2609

As I read that paper, the titular conclusion follow from proposed behavior of Hawking radiation:

"it shows that so long as the mechanism of black hole evaporation satisfies a quite loose condition that the evaporation lifespan is finite for external observers, regardless of the detailed mechanism and process of evaporation, the conundrums above can be naturally avoided.'

Evaporation is outside the scope of classical GR.

From that I conclude that the modern answer to the question of this thread is that according to GR it can never happen - similar to "what happens when I go faster than light".

I'm satisfied with that answer (and note that the original poster did not come back to this thread).

 

[harrylin]

First, one author does not represent the 'modern view'. Krauss is recognized as a cosmologist not a GR expert.

I used the pragmatic Wikipedia method: check out the citations over 4 years.

Even so, I show how I believe you are over-interpreting the claims of this paper.
[..] The different question that came up in this thread was: can the distant observer send a message to an infaller? Where is the infaller when the message reaches them? Is there a way to talk about 'when' for the external observer the infaller gets the message?

Instead I only referred to the topic as in the title question of this thread.

Now this I would say is wrong without qualification. The question is whose time? [..]

That is specified: "the asymptotic observer" - which corresponds to the time I referred to in the 10 foregoing posts. They elaborate: "More concretely, if a black hole is formed in the Large Hadron Collider, it has to be observed by physicists sitting on the CERN campus."

This is how the more recent paper by Yi phrases the same (and with that I'll leave this thread):

"According to Oppenheimer & Snyder [7], there are two influencing conclusions of black hole(BH) formation: “[..] an external observer sees the star asymptotically shrinking to its gravitational radius”. It must be mentioned that the word “see” means measuring by coordinates, not “watching” the light emitted from the star".

 

[PAllen]

I used the pragmatic Wikipedia method: check out the citations over 4 years.

Counting citations without looking at the goals and content of the papers doesn't mean much. Here is later summary by a universally recognized GR expert on these issues at the boundary of classical and quantum gravity (it cites this paper and many others):

http://arxiv.org/pdf/0901.4365v3.pdf

I note the following, which I view as the continuing, unchanged, majority consensus for classical GR:

Classical general relativist: Eternal black holes certainly exist mathematically, as stationary
vacuum solutions to the Einstein equations. (See, for example, [10, 11, 12, 13], or any of the
many standard textbooks in general relativity [14].) Furthermore classical astrophysical black holes
(future event horizons) certainly exist mathematically as the end result of classical collapse based
on certain physically plausible equations of state. (See, for example, [15].)

That is specified: "the asymptotic observer" - which corresponds to the time I referred to in the 10 foregoing posts. They elaborate: "More concretely, if a black hole is formed in the Large Hadron Collider, it has to be observed by physicists sitting on the CERN campus."

Since a defining feature of such micro-black holes is quantum behavior, this is ipso facto completely outside of classical GR.

This is how the more recent paper by Yi phrases the same (and with that I'll leave this thread):

"According to Oppenheimer & Snyder [7], there are two influencing conclusions of black hole(BH) formation: “[..] an external observer sees the star asymptotically shrinking to its gravitational radius”. It must be mentioned that the word “see” means measuring by coordinates, not “watching” the light emitted from the star".

"Measuring coordinates" is nonsense. Coordinates are not observable, in SR or GR. Further, IMO, the introduction to the Yi paper announces him as crank (unlike Krauss et.al., who make no equivalent statements). Schwarzschild time, per se, is not an observable for anyone. All you can talk about is light and signals received by some observer, at various proper times along their world line. Coordinates are computed quantities, defined for various purposes.

A geometric translation of "SC time becomes infinite as an infall trajectory approaches the horizon" is:

If I compute simultaneity using one particular set of spacelike-hypersurfaces (distant simultaneity itself being inherently unsobservable), then I never find an event on an external observer's world line to be simultaneous with an infaller crossing the horizon.

However, if I make any of an inifnite number of different choices for simultaneity surfaces, I can declare which external event I compute to be simultaneous with an EH crossing event for an infaller.

 

[harrylin]

[..] Here is later summary by a universally recognized GR expert on these issues at the boundary of classical and quantum gravity (it cites this paper and many others):

http://arxiv.org/pdf/0901.4365v3.pdf

[..]

Popped in for one last look at this thread, and see there: Nice - I had missed that one!
I see that that expert imagines general relativists to be mathematicians: "exist mathematically" isn't physical existence.

Thanks again,
Harald

 

[PAllen]

Popped in for one last look at this thread, and see there: Nice - I had missed that one!
I see that that expert imagines general relativists to be mathematicians: "exist mathematically" isn't physical existence.

Thanks again,
Harald

Well part of this is that nearly everyone, myself included, think the universe is quantum in nature; thus GR pushed to extreme regimes is unlikely to be correct. I simply clearly separate this not-yet understood realm, from classical GR. In EM, we have little difficulty saying: according to Maxwell's equations, xyz happens (e.g a stable atom cannot exist); accounting form quantum theory, it can. Similarly, I believe (almost certainly) that (due to quantum corrections):

- singularities don't actually exist in our universe
- compact objects (collapsed stars; active galactic centers) share many broad properties, observed from a distance, as classical event horizons; however, the details are likely different. How different, I don't know. For example, the question of whether BH horizons produce 'firewalls' is currently being debated between Polchinksi and Susskind (both prominent string theorists).

 

[pervect]

Observation of Incipient Black Holes and the Information Loss Problem
Tanmay Vachaspati, Dejan Stojkovic, Lawrence M. Krauss
(Submitted on 7 Sep 2006 (v1), last revised 7 Jun 2007 (this version, v3))

We study the formation of black holes by spherical domain wall collapse as seen by an asymptotic observer, using the functional Schrodinger formalism. T

"functional Schrodinger formalism", whatever else it may be is NOT classical.

While I'm not particulary fond of at least one of the authors (Krauss, as in my opinion he messed the"Physics of StarTrek" with some quirky popularizations) the paper probably deserves some serious study. But it is no sense about CLASSICAL black holes.

(And I'm sorry, but I can't really address the non-classical aspects of the paper due to a lack of familiarity. Perhaps someone else can. But I feel quite comfortable with talking about the CLASSICAL theory of black holes.)

One thing I think should be pretty clear. The paper will NOT answer the experiences of someone who falls into a black hole. Which is really the question under discussion - and not at all related to the topic of the paper. At least I thought that's what the question was. It seems like the topic is morphing around, never a good sign.

From what I gather, you seem to think that someone who falls into a black hole just vanishes or something? And you ignore all the various published papers about what happens inside the event horizon - for reasons that I don't follow. I"m really not sure what you think happens when there is "geodesic incompleteness". Imagine that you drew a space-time diagram, and you came to the edge of the paper, and you just stopped drawing the worldline. That is an informal description of "geodesic incompletness".

Geodesic incompletenss is NOT a required feature of the event horizon, though you can put it in if you really want to (you could have geodesics vanish somewhere other than the event horizon too - you simply decide where they vanish, and declare that it happens). It's rather like sketching a map of the universe on some piece of paper, and saying that anything that goes "off the map" just vanishes from existence.

You can do it, but it's silly.

I can point out (as I have many times in the past) the analogous situation of someone falling behind the Rindler horizon. The situation provides much insight. While I doubt there are any papers on the topic, there may be a textbook question or two. It's quite easy for the whole Earth to fall behind the Rindler horizon of an acclerating spaceship, and from the spaceship you'll see all the usual reshifting and apparent "freezing" of time, just as you will see with a black hole horizon, or any other event horizon. But, it should be reasonably obvious that nothing that happens on the remote spaceship is going to affect the Earth - if the Earth falls behind the Rindler horizon of the spaceship in 2012, the Earth is not going to vanish from existence, or cease to exist, in any meaningful sense. The spaceship, however, won't be able to see any events subsequent to 2012 as long as it continues to accelerate.