Black Stars -- Not Holes

[Perpetual Student]

There is an interesting article on this topic in the October 2009 issue of Scientific American.
It is quite beyond my ability to evaluate the suggestions made by the authors, namely that it may not be possible for black holes to form and that black stars may be the ultimate density possible for such entities.
Here is a link (http://www.scientificamerican.com/article.cfm?id=black-stars-not-holes).
Any thoughts?

[ImaginalDisc]

The meat of the article is hidden behind a subscription. Boo.

[daenku32]

The meat of the article is hidden behind a subscription. Boo.

Just like a black hole.

[theprestige]

Any thoughts?
My only thought is that someone needs to post a coherent definition of "black star" in this thread, stat.

[Whiplash]

My only thought is that someone needs to post a coherent definition of "black star" in this thread, stat.


de4Gzf-TgqE
(this should irritate some of the guitar aficionados)

[Reality Check]

There is an interesting article on this topic in the October 2009 issue of Scientific American.
It is quite beyond my ability to evaluate the suggestions made by the authors, namely that it may not be possible for black holes to form and that black stars may be the ultimate density possible for such entities.
Here is a link (http://www.scientificamerican.com/article.cfm?id=black-stars-not-holes).
Any thoughts?
We cannot tell much from the preview.

The preview suggests to me that the authors think that vacuum polarization (http://en.wikipedia.org/wiki/Vacuum_polarization) can prevent the formation of "true black holes". This sounds like Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation) but maybe with the addition of an external electromagnetic field (thus a higher rate of evaporation?).

It is hard to tell what they mean by "true black holes". If an event horizon forms then it is a black hole. If an event horizon does not form then it is not a black hole.

[dasmiller]

We cannot tell much from the preview.

The preview suggests to me that the authors think that vacuum polarization (http://en.wikipedia.org/wiki/Vacuum_polarization) can prevent the formation of "true black holes".

I read the magazine article, but not carefully, and this isn't my field, so don't take my input too seriously.
But - sort of. The authors believe that vacuum polarization could prevent a slowly-collapsing star from forming a black hole. The resulting 'black star' would be slightly larger than the black hole would have been. In this context, I believe that a typical large star collapse would qualify as "slowly-collapsing."

However, they make it clear that the vacuum polarization wouldn't necessarily prevent all black hole formation. A big cloud of stars collapsing to a large, low-density black hole would still be permitted. Though I had the impression that they were a little grumpy about that.

This sounds like Hawking radiation (http://en.wikipedia.org/wiki/Hawking_radiation) but maybe with the addition of an external electromagnetic field (thus a higher rate of evaporation?).

It is hard to tell what they mean by "true black holes". If an event horizon forms then it is a black hole. If an event horizon does not form then it is not a black hole.

If anything, I think the evaporation rate was slightly lower. But I believe that they are consistent with your 'event horizon is necessary and sufficient' philosophy.

I was unconvinced by the article, but as I said, I really don't have the background to have a strong opinion.

[sol invictus]

The authors believe that vacuum polarization could prevent a slowly-collapsing star from forming a black hole. The resulting 'black star' would be slightly larger than the black hole would have been. In this context, I believe that a typical large star collapse would qualify as "slowly-collapsing."

Caveat: I didn't read it. But based on the description above, it's almost certainly wrong.

It's very very easy to get confused about effects like vacuum polarization near a BH horizon, because it often looks as though there are large effects there. There aren't. So no, I don't think vacuum polarization can prevent a star from collapsing in any even vaguely typical situation.

[edd]

Anything that's not a black hole has a surface. Things hitting surfaces look different from things passing through an event horizon.
That's generally speaking a good way to check you've got the real deal and not something close to being a black hole, but not a black hole.

[dasmiller]

Caveat: I didn't read it. But based on the description above, it's almost certainly wrong. It's very very easy to get confused about effects like vacuum polarization near a BH horizon, because it often looks as though there are large effects there. There aren't. So no, I don't think vacuum polarization can prevent a star from collapsing in any even vaguely typical situation.

Now I have it in front of me. From one of the sidebars in the article:
"If the matter's fall is slowed, vacuum polarization may grow, producing repulsion. The repulsion further slows the collapse, which allows the polarization to intensify. The collapse is delayed from ever forming an event horizon. The result is a black star."

(Not posting this as a attempt to dispute Sol, just wanted to make sure my description of the article was accurate. I had some serious concerns with the article, but I really do lack the technical depth here)

[sol invictus]

Now I have it in front of me. From one of the sidebars in the article:
"If the matter's fall is slowed, vacuum polarization may grow, producing repulsion. The repulsion further slows the collapse, which allows the polarization to intensify. The collapse is delayed from ever forming an event horizon. The result is a black star."

Right, that sounds like what I expected. Vacuum polarization (and other similar quantum effects) can produce regions that look like they have negative energy, and negative energy repels positive energy.

The problem is that such effects are (in any macroscopic scenario like a star collapsing) incredibly small, temporary, and local, even though certain sloppy treatments of them near trapped surfaces or event horizons might make them appear to be large. So while I haven't read the paper or done any calculations, I'm willing to bet it's wrong.

[Perpetual Student]

The authors state that the conventional description of a black hole violates "a fundamental feature of quantum mechanics called unitarity." i.e.: an apparent loss of information. The concept of black stars avoids this problem as well as avoiding the problems associated with the infinite density of singularities.
It seems that since black holes are mathematical constructs with inherent problems and with no supporting experimental evidence, such alternative proposals should not be so readily dismissed.

[Third Eye Open]

de4Gzf-TgqE
(this should irritate some of the guitar aficionados)

I was expecting this one ;)

http://www.youtube.com/watch?v=Wh_0fofEpbw

[sol invictus]

The authors state that the conventional description of a black hole violates "a fundamental feature of quantum mechanics called unitarity." i.e.: an apparent loss of information.

The authors are wrong.


It seems that since black holes are mathematical constructs with inherent problems and with no supporting experimental evidence, such alternative proposals should not be so readily dismissed.

There's tons of experimental evidence for black holes. For example, astronomers have imaged complete orbits of stars around the one at the center of the milky way, and within a few years may even be able to achieve a resolution nearly down to the horizon itself, so that they can see light orbiting the hole and other strong gravity effects.

[geni]

People have proposed other options that produce something less collapsed than a classic black hole but I don't think any of them have really caught on.

[Perpetual Student]

The authors are wrong.

OK, but could you elaborate a bit?

There's tons of experimental evidence for black holes. For example, astronomers have imaged complete orbits of stars around the one at the center of the milky way, and within a few years may even be able to achieve a resolution nearly down to the horizon itself, so that they can see light orbiting the hole and other strong gravity effects.

The authors feel that black stars would also produce those same effects. I was referring to evidence that would distinguish between black holes and the alternative. Again, since this is such a highly theoretical area, I am a little surprised by your certainty. My own problem is that I am not able to do anything but rely on others in this area.

[sol invictus]

OK, but could you elaborate a bit?


They have in mind Hawking radiation, which, when the calculation is done the way Hawking did it, is perfectly thermal (and hence independent of the stuff the hole was formed from, which if true would indeed violate unitarity). However it is now understood (from examples in which one can do the calculation precisely) that this is an artifact of the approximations he used. Those approximations are vastly less accurate than necessary to see the non-thermality in the radiation, so they tell you nothing about whether information is lost (and in the examples I referred to where you can do the calculation, it's not lost).


The authors feel that black stars would also produce those same effects. I was referring to evidence that would distinguish between black holes and the alternative.

Either there's no experimental support for black holes or there is, regardless of whether it distinguishes between black holes and "black stars". You can't have it both ways.

The fact is that there is lots of evidence for black holes. That's the starting point. The burden of proof is on these guys to either demonstrate that the theory predicts these black stars, find experimental evidence that distinguishes them from holes, or both. It's not the other way around.

[Perpetual Student]

They have in mind Hawking radiation, which, when the calculation is done the way Hawking did it, is perfectly thermal (and hence independent of the stuff the hole was formed from, which if true would indeed violate unitarity). However it is now understood (from examples in which one can do the calculation precisely) that this is an artifact of the approximations he used. Those approximations are vastly less accurate than necessary to see the non-thermality in the radiation, so they tell you nothing about whether information is lost (and in the examples I referred to where you can do the calculation, it's not lost).

That is precisely what the authors discuss. It's strange that they did not address the point you make.


Either there's no experimental support for black holes or there is, regardless of whether it distinguishes between black holes and "black stars". You can't have it both ways.

The fact is that there is lots of evidence for black holes. That's the starting point. The burden of proof is on these guys to either demonstrate that the theory predicts these black stars, find experimental evidence that distinguishes them from holes, or both. It's not the other way around.

I understand your point; however, it remains true that current observations cannot distinguish between black holes and black stars. Unfortunately, the authors hint at some differences but are not definitive about what future observations might provide evidence for black stars.

[Lennart Hyland]

This is maybe OT but as I read the article in my Scientific American I wonder:

Light cannot escape a black hole, but photons are massless so in what way are the incredible gravity from the black hole forcing not to leave? I guess you guys have heard this question alot of times before :P

[dasmiller]

The authors state that the conventional description of a black hole violates "a fundamental feature of quantum mechanics called unitarity." i.e.: an apparent loss of information. The concept of black stars avoids this problem as well as avoiding the problems associated with the infinite density of singularities.
It seems that since black holes are mathematical constructs with inherent problems and with no supporting experimental evidence, such alternative proposals should not be so readily dismissed.

This brought up one of my major issues with the article. 'Black stars' solved the unitarity problem for slow-collapse black holes, but not for all black holes. If this is correct, then either:
1) there's some other mechanism that preserves unitarity for black holes, so there isn't actually a unitarity problem for slow-collapse black holes, so the 'black stars' would be solving a non-problem (I believe this is Sol's take), or

2) there's no other mechanism. Fast-collapse black holes really do violate unitarity, so the unitarity problem doesn't actually need a solution and black stars would be solving a non-problem.

In either case, the unitarity argument seemed pointless to me.

Of course, there's a fundamental level at which I disunderstand unitarity, so maybe there was more there than I understood.

(my other concern with the article was that I don't think they addressed rotating stars. IIRC, this turned out to be a really big deal in modeling how stars collapse to black holes).

[Reality Check]

I understand your point; however, it remains true that current observations cannot distinguish between black holes and black stars. Unfortunately, the authors hint at some differences but are not definitive about what future observations might provide evidence for black stars.
I take it that black stars do not have event horizons (otherwise they would be black holes).


In that case there are observations that can tell the difference. Matter vanishes into the event horizon. This means that astronomers can compare observations of black hole candidates to objects that definitely have surfaces, i.e. neutron stars. Type I X-ray bursts are a characteristic of matter hitting a surface, e.g. they are seen when matter from accrual disks impact the surface of neutron stars. These X-ray bursts are not observed from any in-falling matter from the accrual disk of Sagittarius A* and the observed black hole candidates. A black star will have a surface and so should have X-ray bursts. So either there is no in-falling matter (unlikely) or we have an event horizon.

Advection-dominated Accretion and Black Hole Event Horizons (http://adsabs.harvard.edu/abs/1997ApJ...478L..79N); 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 (http://adsabs.harvard.edu/abs/1998ApJ...492L..63P); Paul, et al., Astrophysical Journal Letters 492(1): L63-L66, January 1998.
New Evidence for Black Hole Event Horizons from Chandra (http://adsabs.harvard.edu/abs/2001ApJ...553L..47G); 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? (http://adsabs.harvard.edu/abs/2002ApJ...574L.139N); Narayan & Heyl, Astrophysical Journal 574(2): L139-L142, August 2002.
Observing the effects of the event horizon in black holes (http://adsabs.harvard.edu/abs/2003MNRAS.342.1041D); 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 (http://adsabs.harvard.edu/abs/2006ApJ...646..407R); Remillard, et al., Astrophysical Journal 646(1): 407-419, July 2006.

[theprestige]

This is maybe OT but as I read the article in my Scientific American I wonder:

Light cannot escape a black hole, but photons are massless so in what way are the incredible gravity from the black hole forcing not to leave? I guess you guys have heard this question alot of times before :P
Light, being massless, always travels the straightest possible line through spacetime. Gravity warps spacetime. The gravity of a black hole warps spacetime so much that even the straightest possible lines lead right back to the black hole itself. Light, being massless, follows the straightest line possible, but it still can't get away from the vicinity of a black hole.

[Checkmite]

I'm not sure I understand what the authors are proposing - a new type of star? We already have a name for stars that collapse, but not past the Schwarzschild radius - they're just neutron stars, aren't they? And without event horizons, such stars would surely still be giving off light. So the "black star" label confuses me.

[edd]

Well a neutron star is of a rather specific kind of composition and therefore density ranges. There's something of a history of people trying to come up with something other than a black hole (quark stars, gravastars). They don't exactly have great support for them, and I can't see how this new idea really avoids the same difficulties they suffer.

[ben m]

I take it that black stars do not have event horizons (otherwise they would be black holes).


In that case there are observations that can tell the difference. Matter vanishes into the event horizon. This means that astronomers can compare observations of black hole candidates to objects that definitely have surfaces, i.e. neutron stars. Type I X-ray bursts are a characteristic of matter hitting a surface

No, type I x-ray bursts occur (probably) when matter accumulates on a surface under circumstances where it periodically "ignites" and burns in a runaway fashion. The circumstances that make the burning run away like this---rather than reaching some equilibrium, like stars do---are rather complicated; I would not make the generalization that "anything with a surface shows type-I bursts". Heck, I wouldn't even generalize it to all neutron stars, as it may be mass-dependent.

If the new "surface" is really, really close to the event horizon (as was the case for gravastars, IIRC) then it'd be possible that runaway burning events do occur, just like on a neutron star, but they're so deep down in the gravity well that the initial x-ray emission is redshifted into the UV or infrared or something, where we can't monitor as sensitively.

[sol invictus]

This brought up one of my major issues with the article. 'Black stars' solved the unitarity problem for slow-collapse black holes, but not for all black holes. If this is correct, then either:
1) there's some other mechanism that preserves unitarity for black holes, so there isn't actually a unitarity problem for slow-collapse black holes, so the 'black stars' would be solving a non-problem (I believe this is Sol's take), or

2) there's no other mechanism. Fast-collapse black holes really do violate unitarity, so the unitarity problem doesn't actually need a solution and black stars would be solving a non-problem.

In either case, the unitarity argument seemed pointless to me.

That's precisely right.

Of course, there's a fundamental level at which I disunderstand unitarity, so maybe there was more there than I understood.

What don't you understand about it?

(my other concern with the article was that I don't think they addressed rotating stars. IIRC, this turned out to be a really big deal in modeling how stars collapse to black holes).

Most black holes are thought to be rapidly rotating, as are most stars. But I think the mistake is more fundamental than that - I don't think vacuum polarization is capable of stopping an ant from falling in, let alone a star's worth of collapsing matter.

[dasmiller]

What don't you understand about it?

I suspect that explaining the subtleties of unitarity to someone with my understanding of quantum mechanics is a lot like explaining the implications of en passant capture in king-and-pawn endgames to someone who's only vaguely aware that chess is a game played on a square board.

Let me research it a bit so that I can at least ask a coherent question.

[Brian-M]

What exactly is the difference between a "black star" and a neutron star? Or are they just using a different term for the same thing?


Light, being massless, always travels the straightest possible line through spacetime. Gravity warps spacetime. The gravity of a black hole warps spacetime so much that even the straightest possible lines lead right back to the black hole itself. Light, being massless, follows the straightest line possible, but it still can't get away from the vicinity of a black hole.


And even if the light could travel directly away from the black hole without curving, it would still red-shift down to nothing as it went.

[tusenfem]

As long as SciAm writes stupid things like:


These regions—black holes—consist of a location where matter densities approach infinity (a “singularity”) surrounded by an empty zone of extreme gravitation from which nothing, not even light, can escape.


why should we take that seriously? Apparently, they don't even know what "singularity" means. In the early days this would never have been written, they should rename the magazine to PopSciAm or PopAm (even better).

[sol invictus]

What exactly is the difference between a "black star" and a neutron star?

Neutron stars aren't held up by vacuum polarization.

[Lennart Hyland]

Light, being massless, always travels the straightest possible line through spacetime. Gravity warps spacetime. The gravity of a black hole warps spacetime so much that even the straightest possible lines lead right back to the black hole itself. Light, being massless, follows the straightest line possible, but it still can't get away from the vicinity of a black hole.

Ah ofcourse! Thanks!