# Black Holes or Gravastars **URL:** \ **Category:** Factual Questions\ **Created:** [May 22, 2002, 6:39am UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263 "2002-05-22T06:39:15Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [May 24, 2002, 3:12pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/21 "2002-05-24T15:12:06Z")
I think the pressure is from something akin to a cosmological constant with positive pressure. From the article linked in the OP: > [@](#): > > The space trapped inside the Gravastar’s shell is a similarly uncanny conceptually. The interior of the Gravastar would be totally warped space-time (the traditional three dimensions plus time). According to the researchers, this interior space would exert an outward force on the shell, adding to its durability. You can also check out the actual paper: [http://xxx.lanl.gov/PS\_cache/gr-qc/pdf/0109/0109035.pdf](http://xxx.lanl.gov/PS_cache/gr-qc/pdf/0109/0109035.pdf) Here’s another take on it: [http://www.cosmiverse.com/space01170204.html](http://www.cosmiverse.com/space01170204.html) > [@](#): > > When an event horizon is about to form around a collapsing star, Mazur and Mottola believe that the huge gravitational field distorts the quantum fluctuations in space-time. These fluctuations would become so huge they would trigger a radical change in space-time, very similar to the formation of a BEC. > > This would create a condensate bubble. It would be surrounded by a thin spherical shell composed of gravitational energy, a kind of stationary shock wave in space-time sitting exactly where the event horizon of a black hole would traditionally be. The formation of this condensate would radically alter the space-time inside the shell. > > According to Mazur and Mottola’s calculations, it would exert an outward pressure. Because of this, infalling matter inside the shell would do a U-turn and head back out to the shell, while matter outside the shell would still rain down on it. ---
**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:** [May 24, 2002, 7:35pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/22 "2002-05-24T19:35:34Z")
> [@](#): > > I realize that. Chronos suggested that stars would have to shed their entropy, and then some, in order to form a Gravastar. In other words, they need to shed their entropy before forming anything, Gravastar, BH or any other potential object. No. You just have to shed entropy if you want to form something at a lower entropy. A gravastar has lower entropy than a “normal” star, so you need to shed entropy (and a lot of it, in this case). A black hole has higher entropy than a normal star, so you do not need to shed entropy. What the entropy of a gravastar implies about its temperature, I do not know. As for the tidal forces: What I said is valid for any gravitating object. Tidal forces are proportional to M/R[sup]3[/sup]. Since a black hole’s or gravastars radius is proportional to its mass, the tidal forces at the surface are proportional to 1/M[sup]2[/sup]. It happens that for a stellar-mass black hole, this leads to tides sufficient to squish humans, but that’s not because of the event horizon specifically. Make your hole (or gravastar) big enough, and you can make the tides at the surface as small as you like. This is not true near the center, however: As you approach the singularity, tidal forces get arbitrarily large, and you’re guaranteed to be ripped apart before reaching the center. ---
**Author:** ![Qeue](https://avatars.discourse-cdn.com/v4/letter/q/b5ac83/32.png) [@Qeue](https://boards.straightdope.com/u/Qeue)\ **Post date:** [May 25, 2002, 3:10pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/23 "2002-05-25T15:10:29Z")
rsa (from the link you provided: \*By extending the concept of Bose-Einstein condensation to gravitational systems, \*_a cold, compact object_ with an interior de Sitter condensate phase and an exterior Schwarzschild geometry of arbitrary total mass M is constructed. With no thermal emission the GBEC remnant then would be both **ultracold** and completely dark.\* So the temperature of the gravastar _is_ considered to be an ultra-cold object, which would agree with the massive shedding of entropy. \*The shell with its maximally stiff eq. of state p = pc^2 , where the speed of light is equal to the speed of sound could be expected to produce outgoing shock fronts when struck. The spectra of gravitational radiation from a struck gravastar should bear the imprint of its fundamental frequencies of vibration.\* I’m not absolutely sure I follow this. Can anyone explain ? ---
**Author:** ![Undead\_Dude](https://avatars.discourse-cdn.com/v4/letter/u/e19adc/32.png) [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)\ **Post date:** [May 25, 2002, 4:16pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/24 "2002-05-25T16:16:31Z")
Okay, so it is agreed that a gravastar would shed entropy. That sounds a like a minus for the theory to me. I am trying to be open-minded about it, but it really does seem much more likely that a phenomenon like this should increase in entropy in this situation (as a black hole would). So what is the supposed mechanism by which this entropy is shed? I can only imagine that there would need to be some sort of powerful explosion in the collapse process. That doesn’t sound very reasonable to me. Can anyone come up with a reasonable description of where this entropy would go after the collapse? ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [May 25, 2002, 6:55pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/25 "2002-05-25T18:55:02Z")
Gravitational waves? How do neutron stars shed entropy? BTW, the temperature of a gravastar should be about 10 billionths of a degree above absolute zero. ---
**Author:** ![Undead\_Dude](https://avatars.discourse-cdn.com/v4/letter/u/e19adc/32.png) [@Undead\_Dude](https://boards.straightdope.com/u/Undead_Dude)\ **Post date:** [May 26, 2002, 12:47am UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/26 "2002-05-26T00:47:46Z")
But gravitational waves require changes in gravitation. If we are stationary relative to a collapsing star, gravitational changes would only be a local phenomenon. Thus, I don’t think gravitational waves could carry entropy out of the system. Neutron stars are created in supernovas. Now _that_ is certainly an explosion capable of carrying away a great deal of entropy. 🙂 ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [May 26, 2002, 6:43am UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/27 "2002-05-26T06:43:45Z")
This whole entropy thing is beyond me. I read a few comments between a couple of really smart guys (John Baez was one) that got different results trying to calculate entrophy gain or loss from a cloud of dust condensing to a star. If they’re a bit confused, I’m just clueless. ---
**Author:** ![Qeue](https://avatars.discourse-cdn.com/v4/letter/q/b5ac83/32.png) [@Qeue](https://boards.straightdope.com/u/Qeue)\ **Post date:** [May 26, 2002, 1:24pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/28 "2002-05-26T13:24:37Z")
\*The shell with its maximally stiff eq. of state p = pc^2 , where the speed of light is equal to the speed of sound could be expected to produce outgoing shock fronts when struck. \* How can the speed of light be equal to the speed of sound ? ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [May 26, 2002, 4:51pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/29 "2002-05-26T16:51:15Z")
The speed of sound in a material is affected by, among other things, the stiffness of the material. The speed of sound through diamond is 35 times the speed of sound in air for example. So in a gravistar, apparently the stiffness of the shell would be so great that the speed of sound would be equal to (or nearly so) the speed of light. In hunting around a bit, I found that this property is one of the things that is used to place an upper limit on the mass of a neutron star. Above a certain mass the speed of sound in a neutron star would exceed the speed of light. Since this is not possible, neutron stars above a certain mass can be excluded. ---
**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:** [May 26, 2002, 6:23pm UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/30 "2002-05-26T18:23:05Z")
> [@](#): > > _The spectra of gravitational radiation from a struck gravastar should bear the imprint of its fundamental frequencies of vibration._ > > I’m not absolutely sure I follow this. Can anyone explain ? What this means is a potential way to distinguish between gravastars and black holes. When matter hits a gravastar, it would ring it like a bell, except that instead of sound, we’d be “listening” to it using gravitational waves. Further, the exact way in which a gravastar rings would be unique to gravastars, so if we “heard” one, we’d be able to tell that it was a gravastar. The first gravitational wave detectors are just now starting operation, with several more planned over the next few decades, so this is a reasonable way to detect them (I’m not sure what sort of detector would be necessary, though). Spherically-symmetric collapse cannot produce gravitational waves, but outside of textbooks, the collapse is never perfectly spherically symmetric. In fact, we expect collapsing stars to be a major source of gravitational waves. I think it’s a bit naive, though, to make it a law of nature that collapse _cannot_ be spherical, so as to provide an entropy-release mechanism. As soon as you can come up with _any_ mechanism which would produce a black hole rather than a gravastar, Occam tells us we should just drop gravastars pending further evidence (such as hearing one ring). ---
**Author:** ![Qeue](https://avatars.discourse-cdn.com/v4/letter/q/b5ac83/32.png) [@Qeue](https://boards.straightdope.com/u/Qeue)\ **Post date:** [May 27, 2002, 1:46am UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/31 "2002-05-27T01:46:39Z")
I thought a gravitational wave was an effect on space, sort of a shortening of space in the direction of the wave. ---
**Author:** ![Qeue](https://avatars.discourse-cdn.com/v4/letter/q/b5ac83/32.png) [@Qeue](https://boards.straightdope.com/u/Qeue)\ **Post date:** [May 27, 2002, 2:11am UTC](https://boards.straightdope.com/t/black-holes-or-gravastars/110263/32 "2002-05-27T02:11:36Z")
Here we are: _Because of their intense gravitational fields, two nearby black holes would attract each other like magnets. They would collide with great fury, and some small fraction of their masses would be converted to gravity waves. The detectability of these cosmic collisions from afar will depend on just how much energy is released. But this detection is bizarre in that it involves sensing distortions wherein space gets momentarily shorter in the direction of the wave._ [http://www.nature.com/nsu/991111/991111-3.html](http://www.nature.com/nsu/991111/991111-3.html) [Previous page](https://boards.straightdope.com/t/black-holes-or-gravastars/110263.md?page=1)