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B Black holes and the singularity

  1. Dec 21, 2016 #1

    wolram

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    AFAIK, scientists do not believe in the singularity, one way out of the singularity is quantum gravity, but as we have no testable theory of quantum gravity what do scientists think a black hole is?
     
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  3. Dec 21, 2016 #2

    martinbn

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    Is there a reason to say black holes (plural) and use singular (ha ha, no pun here) the singularity? Why not the singularities?
     
  4. Dec 21, 2016 #3

    Jonathan Scott

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    I don't think there's a "mainstream" answer to this; current physics simply breaks down at a singularity. Mainstream science can say what a black hole does, but not what it is, and given that mainstream science also expects a singularity to be behind an event horizon, there is no way to distinguish between alternative ideas.

    I do not consider it proven yet that black holes actually occur. There is certainly solid observational evidence that extremely massive but compact objects exist which must be black holes if General Relativity remains accurate even in such extreme cases. The first LIGO detection also confirmed that the physical sizes of such objects must be close to the predictions for black holes of a few stellar masses. However, some of those objects behave in ways which are difficult to reconcile with the GR predictions of black holes, for example apparently having intense magnetic fields or having X-ray luminosity which appears to far exceed the theoretical maximum from current black hole models.

    If these objects aren't in fact black holes, then one way to tell would be that a collision between them would give off a significant amount of electromagnetic radiation as well as gravitational radiation, so when the Fermi Gamma-Ray Space Telescope team claimed an apparent observation of a gamma-ray burst (close to the limit of detectability) associated with the first LIGO detection I found that very interesting, although that result was controversial and marginal. I hope that LIGO will get some more strong results soon to allow further checking for possible correlated GRBs.
     
  5. Dec 21, 2016 #4

    wolram

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    It would certainly be a step forwards if the [black hole singularity] is found to be non existent, but we are left with some thing with masses of a few, to millions solar masses that distorts space time, It has been said that the black hole leads to a worm hole to another universe, would that be a universe for each BH or the same for all BHs
    I think this is string theory, but as string theory seems to have been put on the back burner i can not believe it has much credence.
     
  6. Dec 21, 2016 #5

    Jonathan Scott

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    If you're interested in wormholes, see the Wikipedia article: https://en.wikipedia.org/wiki/Wormhole

    Although the concept of wormholes has been investigated mathematically in various ways, I think that all potentially interesting ideas (such as being able to fall into a black hole and survive a journey to somewhere else) rely on non-standard theories or a large pinch of unobtainium.
     
  7. Dec 21, 2016 #6

    Ssnow

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    Hi, I can say that a '' black hole is a rift in the continuum space-time'' as we heard in science fiction movies ... :smile:
    I prefer the entity created when a lot of mass implodes on itself ...
     
  8. Dec 21, 2016 #7

    PeterDonis

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    Currently, the best answer is a region of spacetime from which light, locally, cannot escape. The more technical term would be a "trapped region". Such a region is bounded by a trapped surface, which is also called an apparent horizon. The open question is whether these apparent horizons are associated with actual event horizons (regions from which light can never escape, not just regions from which it can't escape at present). The technical definition of a black hole requires there to be an actual event horizon; but that just means that, if we end up discovering that there are no actual event horizons, only apparent ones, we'll probably redefine the term "black hole" to cover that case.

    The question of whether there is an actual singularity inside the horizon is not the same as the question of whether the horizon is an actual event horizon or only an apparent horizon. The singularity theorems in GR are actually based on the condition that there is an apparent horizon, not an event horizon. So in order for there not to be singularities, some other condition of those theorems has to be violated. The most likely candidate is the energy conditions: quantum fields are known to violate them under certain conditions (for example, this is what makes Hawking radiation possible, since the theorem that the area of a black hole must increase also assumes that the energy conditions are valid).
     
  9. Dec 21, 2016 #8

    PeterDonis

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    In the highly speculative models where a black hole gives birth to a "baby universe" instead of having a singularity at the center, there would be a new baby universe for each BH, and all of them would be disconnected from each other.

    There are no models in which a black hole contains a traversable wormhole. The wormhole in the maximally extended Schwarzschild solution pinches off too fast for anything to get through it (in more technical language, the only paths through the wormhole are spacelike, there are no timelike or null paths through it). And anyway, that portion of the maximally extended Schwarzschild solution doesn't appear in realistic models of how a black hole would form by gravitational collapse.

    To get a traversable wormhole, you have to thread the wormhole with exotic matter, i.e., the "unobtainium" that Jonathan Scott mentioned; such models have been studied as a matter of theoretical interest, but AFAIK nobody is claiming they can actually exist.
     
  10. Dec 21, 2016 #9

    mathman

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    There is plenty of evidence that black holes exist. However what happens inside is essentially unknown. Current physics (quantum theory +general relativity) breaks down.
     
  11. Dec 21, 2016 #10

    PeterDonis

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    The definition of "black hole" is precisely the point at issue, so you can't just help yourself to this statement as it stands. You have to actually describe what there is plenty of evidence for the existence of, without using the term "black hole". I tried to do that in post #8; a quick summary would be that there is plenty of evidence for apparent horizons into which matter can or has collapsed.
     
  12. Dec 21, 2016 #11
    According to the best theories we have it's possible to describe the state of a neutron star.
    In that case there no longer are atoms, only a sea of a mainly neutrons with some free electrons and light nuclii mixed in.
    We have no idea what happens when even neutron degeneracy cannot hold up against further infalling matter.
     
  13. Dec 22, 2016 #12

    mathman

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    At the center of all (or almost) all galaxies there is an invisible object, the size of which is in the millions or billions of solar masses. Black holes are the only description that fits according to current theory.
     
  14. Dec 22, 2016 #13

    PeterDonis

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    Once again, the point at issue is the definition of "black hole". If you are defining "black hole" to mean "event horizon and the region inside it", then your claim is not justified; an apparent horizon is a description that also fits according to current theory. The only way to distinguish an apparent horizon from an actual event horizon, according to current theory, is to know the entire future of the spacetime, and we don't know that.

    If you just define "black hole" to mean "a region inside either an apparent horizon or an event horizon", then your statement is true. But if that were your definition, you wouldn't be (apparently) disputing my previous post.
     
  15. Dec 23, 2016 #14

    Chronos

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    The simple and obvious definition of a black hole is a bounded region of space wherein photons cannot escape. Any further details are irrelevant.
     
  16. Dec 23, 2016 #15

    PeterDonis

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    Cannot escape just right now, or cannot escape forever? That's the difference between an apparent horizon and an event horizon.

    That would come as a great surprise to all the participants in what Susskind calls the Black Hole War, in which the question of whether there are any actual event horizons, or only apparent ones, is critical.
     
  17. Dec 23, 2016 #16

    Chronos

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    While typically robust, all event horizon are strictly virtual. On these grounds, it can be argued a black hole is only a black hole so long as the walls of its EH remain standing in the same sense that ice is only ice until it unfreezes. In theory, every black hole will face its own spring and ultimately unfreeze allowing photons to once again stream freely through.
     
    Last edited: Dec 23, 2016
  18. Dec 23, 2016 #17

    PeterDonis

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    What does "virtual" mean here? If you mean we don't know if it's an event horizon or an apparent horizon, that's true, but that doesn't mean the event horizon is "virtual". The definition of an event horizon is perfectly precise: it's the boundary of the causal past of future null infinity. If there is no such boundary in a spacetime, i.e., if the entire spacetime is in the causal past of future null infinity--which will be the case if all horizons are only apparent horizons--then it's not correct that there is a "virtual event horizon". There is no event horizon, period.

    This is one way of describing the model in which there is an event horizon, but the region inside it only exists until the black hole evaporates away completely (instead of existing infinitely into the future as in classical GR). I'm not up to date on the latest in the Black Hole War, but AFAIK Susskind a few years ago was claiming that his side had won, which if true would mean the model just described is not viable (Susskind's side was the side that said the presence of any event horizon at all, no matter how small the region inside it, would violate unitarity and thus be inconsistent with QM).
     
  19. Dec 23, 2016 #18

    Chronos

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    Perhaps the word 'virtual' is less than ideal. The sense I had in mind was that of a temporary construct. Since black holes are generally believed to evaporate, it is destined to return [albeit with little urgency] its disconnected volume of space back to the causal universe at large.
     
  20. Dec 27, 2016 #19
    Nice pun
     
  21. Dec 27, 2016 #20
    Love the humor!
     
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