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trinitree88
2014-Jul-16, 05:33 PM
So the understanding has been that the gravitational field of one is so strong that escape velocity exceeds "c"...and that will render it black. Not so fast he says.....SEE:http://arxiv.org/abs/1407.3823


pete

Reality Check
2014-Jul-16, 09:38 PM
It is a preprint about a toy model that assumes matter "condenses on the apparent horizon" to resolve the information paradox and looks at dust solutions where that happens.
It is not about black holes in general or maybe not even real back holes.
So the title should really be "There's Nothing "Black" about the Black Holes We Consider".

Strange
2014-Jul-16, 09:41 PM
It is an interesting paper (although somewhat over my head). There is a lot of interesting work in this area at the moment.

But ...

the gravitational field ... is so strong that escape velocity exceeds "c"...and that will render it black.
... is a horribly inaccurate and misleading analogy.

Jeff Root
2014-Jul-17, 04:01 AM
I can see how it might be misleading. We can get to that later.

How is it inaccurate?

How is it horribly inaccurate?

How is it an analogy?

-- Jeff, in Minneapolis

Strange
2014-Jul-17, 08:23 AM
I can see how it might be misleading. We can get to that later.

How is it inaccurate?

How is it horribly inaccurate?

How is it an analogy?

-- Jeff, in Minneapolis

This should probably be a separate thread but ...

Black holes are not "black" because of escape velocity. If that were the reason then, for example, light could get some distance away from the surface before falling back (in the same way you can throw a ball in the air at less than escape velocity). For that reason, I don't think the (Newtonian) concept of escape velocity is even meaningful at (or within) the event horizon. That makes it an analogy. And also misleading because people who get their science articles from popular media may think that light can escape the surface, or you could escape if you moved faster than light or ....

(Even if it were a good analogy, it is irrelevant to the discussion in the paper.)

kzb
2014-Jul-17, 12:27 PM
This is the second paper in the last few weeks that calls into question the existence of black holes. The study linked to below was even featured on BBC Newsnight last month.

I don't understand it, but like the paper under discussion, the author is speculating that something happens to stop the collapse before a true BH is formed. The result is a collapsed object but the singularity is avoided.

Backreaction of Hawking Radiation on a Gravitationally Collapsing Star I: Black Holes?

Author Laura Mersini-Houghton

[/B]http://arxiv.org/abs/1406.1525

Jeff Root
2014-Jul-17, 03:53 PM
I started a new thread to discuss whether "escape velocity"
applies to light and black holes:

http://cosmoquest.org/forum/showthread.php?152359-Black-Holes-and-Escape-Velocity

-- Jeff, in Minneapolis

Jerry
2014-Jul-17, 05:47 PM
This is the second paper in the last few weeks that calls into question the existence of black holes.

Yes, and it is about time. The evidence of black holes is quite strong, but it is all based upon the idea that accelerations near a black hole are prodigiously brilliant. There are other possible causes of this brilliance, most notably acceleration in an extremely strong magnetic field. Polarity about a 'black hole' is necessary in the second case; but not the first.

To my memory, the early predictions of black hole behavior predicted that the inner most light bands emitted would be severely redshifted; and I don't think we have seen evidence of this reddening.

Reality Check
2014-Jul-17, 10:45 PM
This is the second paper in the last few weeks that calls into question the existence of black holes.

This preprint does not really say that black holes do not exist - it points out that one way to resolve a paradox about them is to assume that matter does not cross the event horizon ("condenses on the apparent horizon").
The preprints are slightly dubious since they are written by single authors (and are preprints!). The authors seem to have broken away from their colleagues in previous published papers.

But the big problem is physical: There is evidence that black holes do not have surfaces.
Neutron stars emit Type I X-ray bursts. Type I X-ray bursts fit the models of what happens when matter falls onto the surface of a neutron star, is compressed and heated as it accumulates which leads to thermonuclear reactions (and X-rays). The proposed objects are a form of "condensed" neutron star.
No Type I X-ray bursts have been observed from candidate stellar black holes or super-massive black holes. Note that this is not conclusive evidence that there are event horizons - it could be that we are only looking at black holes that have no or rarely infalling matter.

Reality Check
2014-Jul-17, 10:54 PM
The evidence of black holes is quite strong, but it is all based upon the idea that accelerations near a black hole are prodigiously brilliant.

What is a "prodigiously brilliant" acceleration, Jerry?
It sounds like you think the existence of active accretion discs is evidence for black holes which is a new one to me. Or that the models of accretion discs being wrong means that black holes do not exist.
The evidence that I know about is
* There are objects that emit no detectable light.
* These objects are too heavy to be neutron stars and what we know about physics says that there is nothing to stop the collapse.
* There is no evidence of a surface for candidate stellar black holes or super-massive black holes.

ETA: X-Ray burst papers:
•Advection-dominated Accretion and Black Hole Event Horizons; Narayan, Garcia & McClintock, Astrophysical Journal Letters 478(2): L79-L82, April 1997.
•Quasi-regular X-Ray Bursts from GRS 1915+105 Observed with the IXAE: Possible Evidence for Matter Disappearing into the Event Horizon of the Black Hole; Paul, et al., Astrophysical Journal Letters 492(1): L63-L66, January 1998.
•New Evidence for Black Hole Event Horizons from Chandra; Garcia, et al., Astrophysical Journal 553(1): L47-L50, May 2001.
•On the Lack of Type I X-Ray Bursts in Black Hole X-Ray Binaries: Evidence for the Event Horizon?; Narayan & Heyl, Astrophysical Journal 574(2): L139-L142, August 2002.
•Observing the effects of the event horizon in black holes; Done & Gierlinski, Monthly Notice of the Royal Astronomical Society 342(4): 1041-1055, July 2003.
•The Rates of Type I X-Ray Bursts from Transients Observed with RXTE: Evidence for Black Hole Event Horizons; Remillard, et al., Astrophysical Journal 646(1): 407-419, July 2006.

Jeff Root
2014-Jul-17, 11:04 PM
* These objects are too heavy to be neutron stars
Can you explain that a bit?

-- Jeff, in Minneapolis

Reality Check
2014-Jul-17, 11:10 PM
There is a thing called gravity (:D). What holds neutrons apart in a neutron star is the degeneracy pressure of neutrons. Add enough mass and gravity increases until the degeneracy pressure is overcome.
Tolman–Oppenheimer–Volkoff limit (http://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_limit)

There is speculation that quarks also have a degeneracy pressure. But once again add enough mass and gravity increases until that degeneracy pressure is overcome.

Jeff Root
2014-Jul-17, 11:49 PM
I asked about the first part of the bullet point, not the
second. What I was primarily wondering was how it is
known that the mass is greater than the neutron star limit.

Since the *possibility* of black holes is being questioned,
the possibility that the neutron star limit is wrong would
seem to be an option. Or maybe just that the quark star
limit is wrong?

And since the possibility of black holes is being questioned,
isn't it possible that several neutron stars close together
could equal the gravity required?

-- Jeff, in Minneapolis

Reality Check
2014-Jul-18, 12:20 AM
What I was primarily wondering was how it is
known that the mass is greater than the neutron star limit.

We measure the mass by looking at the orbit of the other visible star in a binary pair, the accretion disc or other stars: http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q14.html

And yes - if everything we know about nuclear physics is wrong then the Tolman–Oppenheimer–Volkoff limit could be wrong.
And yes - if we speculate that quarks have degeneracy pressure and speculate again that it is so strong that even the mass of billions of Suns (the heaviest super-massive BH is NGC 4889, weighing in at 21 billion solar masses) cannot overcome it then even supermassive black holes cannot exist.

The "close together" in "Several neutron stars close together" is ~30 km radius for a stellar black hole (http://en.wikipedia.org/wiki/Black_hole). A neutron star is ~10 km in radius. So those several neutron stars would quickly collide into on neutron star and collapse to a black hole.

kzb
2014-Jul-18, 03:40 PM
Yes, and it is about time. The evidence of black holes is quite strong, but it is all based upon the idea that accelerations near a black hole are prodigiously brilliant. There are other possible causes of this brilliance, most notably acceleration in an extremely strong magnetic field. Polarity about a 'black hole' is necessary in the second case; but not the first.

To my memory, the early predictions of black hole behavior predicted that the inner most light bands emitted would be severely redshifted; and I don't think we have seen evidence of this reddening.

I can't pretend that I understand the topic at all, but I do think it is good that questions are being asked. I just have this feeling that "something" will happen to prevent BH formation, perhaps something that we have no way of knowing about yet.

We don't get to hear much about the quantum world implications. For example, the event horizon position will have an associated fuzziness. Likewise, subatomic particles have uncertain positions.

Hornblower
2014-Jul-18, 04:33 PM
I can't pretend that I understand the topic at all, but I do think it is good that questions are being asked. I just have this feeling that "something" will happen to prevent BH formation, perhaps something that we have no way of knowing about yet.

We don't get to hear much about the quantum world implications. For example, the event horizon position will have an associated fuzziness. Likewise, subatomic particles have uncertain positions.My bold. I have no "feeling" one way or another. The history of physics has shown that such feelings frequently are unreliable.

caveman1917
2014-Jul-21, 07:33 PM
We don't get to hear much about the quantum world implications. For example, the event horizon position will have an associated fuzziness. Likewise, subatomic particles have uncertain positions.

Hawking radiation seems relatively well-known.

Strange
2014-Jul-21, 07:45 PM
There is a huge amount of work and debate around these areas at the moment - we had a thread here on the firewall paradox and possible solutions, and then there is Hawking's recent thing suggesting the event horizon is not just fuzzy but porous, and Rovelli's paper saying that a black hole could could "bounce" and turn onto a white hole, and idea relating wormholes and entanglement, and ....

Hlafordlaes
2014-Jul-22, 10:30 AM
Strange, on the BH bounce comment, is that related to loop quantum gravity, or something like that?

Strange
2014-Jul-22, 10:36 AM
Strange, on the BH bounce comment, is that related to loop quantum gravity, or something like that?

That's the one. I don't know if there is a thread on it already.
http://www.nature.com/news/quantum-bounce-could-make-black-holes-explode-1.15573

kzb
2014-Jul-22, 05:57 PM
Hawking radiation seems relatively well-known.

It's not quite the same I don't think. Hawking radiation (if it actually exists) originates from the formation of a virtual pair of particles close to an event horizon. One gets sucked in and the other escapes. By my understanding, nothing material has crossed the EH from the inside.

What I am saying is, does the uncertainty principle apply to the position of the EH.

caveman1917
2014-Jul-22, 08:56 PM
What I am saying is, does the uncertainty principle apply to the position of the EH.

Yes and no, in the sense of "it should, but it can't". It's one of the issues in finding a consistent theory of quantum gravity, QM basically requires the uncertainty principle to apply to the position of the EH but GR basically falls apart at the core when you allow any uncertainty there (you'd be able to locally create mass-energy out of nothing). Any actual answer there would need a theory of quantum gravity.

Reality Check
2014-Jul-22, 09:14 PM
The basic answer is no because GR which describes a EH is not a quantum mechanical theory and does not contain the uncertainty principle.
We expect there to be a unified theory in which EH and the uncertainty principle exist together. But the uncertainty principle is that there are pairs of measurements that are "complementary" - the less uncertainty in one, the more uncertainty in the other. The complementary measurement for position is momentum. EH do not have any momentum - they are a mathematical surface.

What should make EH fuzzy is that the union of GR and EH is expected to make spacetime discrete and fluctuating at a very small scale. So that mathematical surface cannot be smooth or constant. Think about trying to draw a circle on paper with a grid of dots using just the dots. You will not get an exact circle. Now let the dots jiggle about.

kzb
2014-Jul-23, 11:49 AM
The other thing I have read is that it is academic whether the EH position is fuzzy or not, because inside the EH there is no such direction as "up". All directions have a "down" component vector, i.e away from the EH.

However, from what is being said here and the fact that these publications are appearing, I think there is a lot more to the BH story yet.