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What happens if there is no singularity inside a black hole?

  1. Jul 16, 2014 #1
    Hello. I'm a layperson curious about physics.

    I've read that Quantum Gravity could eliminate the predicted singularity inside a black hole based on General Relativity.

    If that's true, what would happen from the frame of reference at the center of a black hole? Would that cause a tremendous amount of energy to be released as the star collapses? (Which, of course, would be contained within the event horizon.)

    I've read that a supernova can shine as bright as 10 billions stars. If there was an explosion at the center of a black hole, instead of a singularity, roughly how bright would it be?

    I'm also curious about degenerate matter. Could quark degeneracy (a quark star) or hypothesized preon degeneracy stop an imploding star within the event horizon?
     
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  3. Jul 16, 2014 #2

    bcrowell

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    The existence of exotic states of matter could prevent an event horizon from forming. But if these exotic states of matter don't violate an energy condition (I believe it's the strong energy condition), then once an event horizon forms, you're guaranteed in classical gravity that you'll get a singularity; this is basically the Penrose singularity theorem. Also, if the system forms a stationary equilibrium with an event horizon, then the black hole no-hair theorems guarantee a singularity.

    That's all in classical gravity, but classical gravity should remain valid until the object gets as dense as the Planck density, which means it can't be any of the hypothetical stable forms of matter you have in mind.

    Since we don't have any theory of quantum gravity, I don't think we can predict anything specific about what happens when you get to the Planck density.

    Observationally, we see objects that have all the predicted characteristics of black holes, which suggests that sufficiently massive objects do form event horizons when they collapse gravitationally.
     
  4. Jul 17, 2014 #3
    Are you talking about whether a singularity means all the matter occupies the same space or just that the matter is compacted as tightly as is physically possible?
     
  5. Jul 17, 2014 #4

    pervect

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    I've found Nikodem Popławski "Big bounce" ideas, based on ECKS theory (sometimes called Einstein-Cartan theory) interesting. As far as I know it's not "a quantum theory of gravity", though it does incorporate modifications to make it more friendly to particles with spin, which is an important quantum mechanical property.

    See http://en.wikipedia.org/w/index.php?title=Nikodem_Popławski&oldid=589198285 for an overview of the author and the ECKS theory of gravity.

    I'm not overly familiar with the theory, but it is touted as a "minimal addition to GR which can handle spin".

    Wiki , for example, claims that torsion is required to properly handle spin. GR assumes there is no torsion.

    In the ECKS theory the torsion apparently doesn't propagate as a field but is just carried around by the spinning matter. The gravitational effects of this torsion also apparently prevent the formation of singularities by "the dirac field", which I take to be by fermions. See http://arxiv.org/pdf/0910.1181.pdf

    You might get better answers in the "beyond the standard model" forum, because your interests are outside of classical GR. ECKS is just one of a broad range of non-GR theories. ECKS theory is very difficult to distinguish from GR experimentally, but it makes some significantly different predictions about what happens inside singularities.
     
  6. Jul 19, 2014 #5
    Thanks for your responses. Very informative. In searching "plank density" I came across an interesting article that proposes, when taking quantum mechanics into account, a black hole never forms an actual event horizon. Only an apparent one. At the center a plank star forms which is eventually revealed after the (apparent?) black hole radiates away due to Hawking Radiation.

    But my interest in a hypothetical explosion inside a black hole might still be relevant and its energy would be predictable according to General Relativity.

    From my understanding, when a star uses up its fuel it collapses. Due to the Pauli Exclusion Principle (particles of half-integer spin can't occupy the same quantum state simultaneously - essentially the same space) the collapse of low mass stars is halted due to electron degeneracy producing white dwarfs. With medium mass stars, neutron degeneracy halts the collapse forming a neutron star.

    From here, I surmise, black hole theory is based on the concept that there is no further degeneracy pressure that can halt the collapse. So a black hole will form. And according to the Penrose Singularity Theorem, all objects inside the event horizon will fall to the center which is a singularity.

    I hypothesize an exotic matter degeneracy pressure that can halt a star's collapse inside its event horizon, producing a core that is smaller than its black hole. It should be theoretically possible given the entire universe was once smaller than the size of an atom!

    So say a 10-solar-mass star collapses to the size of a beach ball (30 cm radius.) The size of its black hole would be about 30 km in radius (Swartzchild Radius.) From the reference frame of the hypothetical core, the speed of in-falling particles should be near the speed of light. So what would their relativistic gamma (Lorentz factor) be? (That would, of course, indicate the amount of kinetic energy released in the collision.)

    What would gamma be if the core was the size of an atom? (About 300 pm radius.)
     
    Last edited: Jul 19, 2014
  7. Jul 19, 2014 #6
    I'm interested in knowing the energy of an explosion within a black hole if a star can collapse to a core smaller than its Swartzchild Radius.

    My understanding of a non-rotating black-hole singularity is that would contain all the mass of a black hole in a single point which has infinite density.
     
  8. Jul 19, 2014 #7

    Nugatory

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    That's the prediction that the theory of general relativity makes. However, the appearance of this infinity density in the calculation is usually interpreted as a very strong hint that the theory breaks down when enough mass is concentrated into a small enough volume. Thus "We don't know what's really at that central point" would be more correct.
     
  9. Jul 19, 2014 #8
    I'm curious: in this theory, where does the matter "bounce" into? What happens to it as the black hole undergoes quantum-mechanical Hawking decay?
     
  10. Jul 19, 2014 #9

    pervect

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    From the popularizations I've read, the theory is the bounce creates a new universe. Essentially instead of collapsing to a singularity, the black hole forms a wormhole, an Einstein-Rosen bridge. See for instance http://news.nationalgeographic.com/...rnate-universe-multiverse-einstein-wormholes/

    Caution: As I've indicated, while I find the theory interesting, my knowledge of it is less than complete.
     
  11. Jul 20, 2014 #10
    Thanks! This is exactly the idea I was looking for. My hypothesis is that if something halts the collapse of a star within the event horizon a tremendous amount of kinetic energy will be released, enough to form another universe.

    Since the unification of quantum gravity and the grand unified force is at a very high energy, that implies (I'm guessing) dark matter particles are very high energy. So my hypothesis is that there is a chain of degeneracy based on the Pauli Exclusion Principle where the collapsing core transforms into higher and higher energy matter, eventually forming a dark matter core, which can be compressed no further.

    I think the Pauli Exclusion Principle is another conflict between General Relativity and Quantum Mechanics which a black hole singularity would violate.

    Of course, the mechanism of black hole "big bounces" could be more complex like Nikodem Popławski's idea. Of course, there may be no white hole on the other side of a black hole. But it's very interesting to think about!
     
  12. Jul 20, 2014 #11

    pervect

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    It's not really a matter of energy release, it's more a matter of the gravitational effects of concentrating enough energy in a small enough location.

    Standard GR predicts a singularity on collapse, the ECKS modification predicts a bounce due to the effects of spin-induced torsion. Idealized Newtonian mechanics would predict a bounce back to our universe. The ECKS theory predicts something different, it's rather more like the maximally extended Schwarzschild geometry of a black hole, an Einstein Rosen bridge.

    You can think of this geometry as a black hole joined to a white hole via a wormhole.

    See for instance
    http://en.wikipedia.org/wiki/Kruskal–Szekeres_coordinates
    http://en.wikipedia.org/wiki/Wormhole#Schwarzschild_wormholes

    I suspect that the "bounce" predicted by the ECKS theories isn't the only possibility, but I really couldn't say. I thought the idea was interesting, and I'm presenting it as one interesting possibilty on a topic that is not only still being researched, but is speculative in the sense that the experimental data to test the theory is absent due to the extreme physical conditions required before the theory departs from GR.
     
  13. Jul 20, 2014 #12
    @pervect: So how do these ideas square with the quantum-mechanical Hawking decay? What is happening to the "other universe" and the matter it contains (which is the matter from the original star, right?) as the BH undergoes Hawking decay?
     
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