# Black Holes...or Gravistars? **URL:** \ **Category:** Factual Questions\ **Created:** [September 29, 2004, 10:16pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735 "2004-09-29T22:16:22Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [September 29, 2004, 10:16pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/1 "2004-09-29T22:16:22Z")
For years, the commonly accepted fate of a massive enough star was infinite collapse into a quantum singularity surrounded by point of no return, an event horizon. Now, apparently, a competing end result exists: a dark, cold, ultradense (but not infinitely dense) condensate material. Will **Bad Astronomer** please chime in and explain in better detail how Black Holes and Gravistars differ according to General Relativity, Quantum Mechanics, and M-Theory? ---
**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\ **Post date:** [September 29, 2004, 10:22pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/2 "2004-09-29T22:22:17Z")
The last time this topic was duked out, I pretty much recall it coming down to a simple fact: We don’t know what goes on inside a black hole’s event horizon. Everything could be filled with fluffy bunnies and tulips for all we know. A “gravistar” pretty much defines what a black hole would be, on the outside. ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [September 29, 2004, 11:10pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/3 "2004-09-29T23:10:09Z")
Basically the proposal hasn’t really ignited much interest. The [original preprint](http://uk.arxiv.org/abs/gr-qc/0109035) by Mazur and Mottola has yet to appear in print and has only [20 citations on Spires](http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=c+gr-qc+0109035), not all of which look terribly relevant. There’s nothing listed on arXiv for “gravistar” and only [one paper](http://uk.arxiv.org/abs/gr-qc/0310107) about “gravastar” physics, though that has appeared in _Classical and Quantum Gravity_. The topic’s thus hardly taboo, but hardly anybody thinks it’s worth pursuing. Personally, the idea always looked a little _ad hoc_ to me. ---
**Author:** ![Complex\_Conjugate](https://avatars.discourse-cdn.com/v4/letter/c/3bc359/32.png) [@Complex\_Conjugate](https://boards.straightdope.com/u/Complex_Conjugate)\ **Post date:** [September 29, 2004, 11:12pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/4 "2004-09-29T23:12:00Z")
GR tells us that there will defintely be a singulairty (Penrose’s singularity theorum), of course that’s provided you accept that the event horizon is a singularity that can be removed by choosing the righ coordinate system (acceptance of this pretty much unievrsal), otherwise GR says nothing abpout anything beyond the event horizon. Form what I understand string theorists would say you shouldnt use GR anyway to describe the regipn of space beypond the event horizon as according to string theory GR no longer provides a good approximation past this point (though I’d like someone to check this I’m not 100% sure abouyt that), i also think that in general string theory and quantum gravity models of black holes try to avoid singualrities. Quantum mechanics says bsolutely nothing about black holes as it says nothing about gravity, thouugh IIRC when Stephen Hawking first examined the entropy of a black hole he did it just using common-or-garden non-relativistic quantum mechanics. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [September 29, 2004, 11:40pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/5 "2004-09-29T23:40:38Z")
> [@Complex Conjugate](#): > > Form what I understand string theorists would say you shouldnt use GR anyway to describe the regipn of space beypond the event horizon as according to string theory GR no longer provides a good approximation past this point Not quite. GR and QM effects become comparable (i.e., QM adjustments to basic GR predictions become significant) at sufficiently high curvature. I believe it’s when the radius of curvature drops to the order of Planck length. A sufficiently massive black hole can still have very gentle curvature at its event horizon. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [September 29, 2004, 11:44pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/6 "2004-09-29T23:44:46Z")
> [@Complex Conjugate](#): > > Form what I understand string theorists would say you shouldnt use GR anyway to describe the regipn of space beypond the event horizon as according to string theory GR no longer provides a good approximation past this point (though I’d like someone to check this I’m not 100% sure abouyt that) … Not quite. I don’t think that most (or even many) string theorists think that GR can’t describe the region around an event horizon. What they think their theory describes better (and the regular GR folks will happily concede that their theory doesn’t describe very well at all) is the region around the central singularity, where the curvature becomes infinite. As for gravistars, I haven’t read the original paper, so I’m not too familiar with the mathematics. I have to run right now, but if **Chronos** or someone else doesn’t get to this thread before tomorrow I’ll take a look at the paper and see what my gut reaction is. ---
**Author:** ![Complex\_Conjugate](https://avatars.discourse-cdn.com/v4/letter/c/3bc359/32.png) [@Complex\_Conjugate](https://boards.straightdope.com/u/Complex_Conjugate)\ **Post date:** [September 30, 2004, 12:13am UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/7 "2004-09-30T00:13:24Z")
> [@Mathochist](#): > > Not quite. GR and QM effects become comparable (i.e., QM adjustments to basic GR predictions become significant) at sufficiently high curvature. I believe it’s when the radius of curvature drops to the order of Planck length. A sufficiently massive black hole can still have very gentle curvature at its event horizon. Yep, this is what I thought but i’ve been told otherwise by a quantmu physicist know. Of course it’s completly outside of his area of expertise so he may simply be confusing the fact that string theory offers a way of associating entropy with the microstates inside the event horizon (i.e. we’re looking at a stringy description of the inside of the evnt horizon) whereas GR really doesn’t. ---
**Author:** ![Complex\_Conjugate](https://avatars.discourse-cdn.com/v4/letter/c/3bc359/32.png) [@Complex\_Conjugate](https://boards.straightdope.com/u/Complex_Conjugate)\ **Post date:** [September 30, 2004, 12:19am UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/8 "2004-09-30T00:19:21Z")
Actually this is what he was referring too, so it does appear that string theorists (I know Mathr’s idea was enthusiatcally received by string theoriests) do possibly think the entire region inside an event horizon is differnt from the classical description (And this is connected with the number of microstates of a black hole). [http://www.space.com/scienceastronomy/fuzzball\_blackholes\_040301.html](http://www.space.com/scienceastronomy/fuzzball_blackholes_040301.html) ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [September 30, 2004, 9:16am UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/9 "2004-09-30T09:16:42Z")
From the above link: > [@](#): > > In the classical model of how black holes form, a supermassive object, such as a giant star, collapses to form a very small point of infinite gravity, called a singularity. Infinite gravity?!! Run for it!! Ummm, you mean infinite ‘density,’ donchya? Well, actually, approaching infinite density until the density becomes undefined at an infinitesmal size. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [September 30, 2004, 9:28am UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/10 "2004-09-30T09:28:17Z")
Beware! I live! Seriously, that game gave me nightmares. And level 3 was, like, impossible. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [September 30, 2004, 4:14pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/11 "2004-09-30T16:14:18Z")
> [@moriah](#): > > Infinite gravity?!! Run for it!! > > Ummm, you mean infinite ‘density,’ donchya? Well, actually, approaching infinite density until the density becomes undefined at an infinitesmal size. Nope, he means infinite gravity. It’s entirely possible to have a black hole with **no** matter inside, but having things like the metric and the Riemann curvature tensor divergent. There is a single point at the center (i.e. the singularity) that could be said to have non-zero (in fact, infinite) density, but the density is zero as you approach it through the surrounding space. As for Mazur & Mottola’s paper, I took a quick look at it this morning. **bonzer** basically had it right - it’s possible that this is the case, since we don’t know (and, by definition, can’t know) about what goes on inside a black hole unless we actually enter it, and the exteriors of the black hole and gravistar solutions are almost identical.[sup]1[/sup] However, it also requires a great big _deus ex machina_, namely a “phase transition of gravity”: M&M postulate that the gravitational field undergoes a drastic change in structure near the event horizon, and acts like the “condensate material” mentioned in the OP. _A priori_, there’s no particular reason to believe that this phase transition should exist other than philosophical ones. The authors make a comment about quantum effects near the event horizon, which I’m unfortunately not qualified to comment on but seems a little suspect; and they note that this might help resolve the black hole information paradox, but it’s a less compelling candidate to resolve that than, say, string theory or loop quantum gravity, which also claim to be able to resolve the paradox, are much better-motivated, and are chock-full of other interesting phenomena. [sup]1[/sup] I say “almost” because there is, in fact, a small amount of this condensate that sticks out beyond where the event horizon would be in a black hole. However, M&M’s solution is only an approximation in which this shell pokes infinitesimally far out, and it’s not clear whether this shell would be observable in any practical sense. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 30, 2004, 6:10pm UTC](https://boards.straightdope.com/t/black-holes-or-gravistars/266735/12 "2004-09-30T18:10:12Z")
We have a very good handle on everything outside of a black hole of reasonable size (at least, what seems theoretically to be a very good handle). This does not actually tell us anything about what’s inside of the horizon. The simplest model is to just analytically continue the exterior geometry across the singularity. This should be true to at least some extent, since there are no indications from General Relativity that there’s anything at all locally significant about an event horizon. It’s possible to cross over the event horizon of a black hole without being able to determine, even theoretically, that there’s a black hole there at all (at least, not until some time later, when you’re already well and truly damned). Now, General Relativity predicts a singularity at the core of a black hole, but that, at least, is disputable. As you get closer to the core, eventually the curvature approaches the Planck scale, at which point we are quite certain that General Relativity must break down in some fashion. It is quite plausible to suppose, then, that quantum gravitational effects (whatever the heck those are) would act to prevent the formation of a singularity, and instead leave you with a very small but nonzero lump of stuff with a very high but finite density. This point could be at or even outside the event horizon, for a fantastically small black hole, but for any formed by any mechanism we know, it should be far, far within the event horizon by many orders of magnitude. The gravistar model, however, posits that there’s some form of quantum weirdness which manifests right at the event horizon (or rather, a miniscule amount outside the horizon). This is a complete handwave, since not only do we not know of any mechanism for the quantum effects to cause this, we don’t even know of any reason to suspect they might be relevant there. Furthermore, many of the problems which are claimed to be solved by gravistars are, in fact, made worse by them. For instance, a black hole has a much higher entropy than the supernova which creates it, which the proponents of the gravistar model claim is problematic. But instead, they propose a final state which has much _less_ entropy than a supernova. It’s quite consistent with the laws of thermodynamics for the entropy of a system to increase, even dramatically. But when it’s decreasing dramatically, you have to ask where all the rest of that entropy is going.