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B Do Black Holes exist?

  1. Oct 27, 2017 #1

    wolram

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    Do Black Holes exist? after all the maths breaks down at the singularity Like the maths of the singularity breaks down at the beginning of the universe, what if there is no need for the maths to break down and these singularities do not exist?
     
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  3. Oct 27, 2017 #2

    FactChecker

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    A black hole begins long before the math of a singularity breaks down. Many have been "seen" by observing the behavior around them. Although the math at the singularity in the middle of a black hole might be a problem, that is only at the very center.
     
  4. Oct 27, 2017 #3

    wolram

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    It is also a problem at the universe singularity, why should we belie they exist, may be Black holes exist but they do not have the properties we attributes give to them
     
  5. Oct 27, 2017 #4

    PeterDonis

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    This is a vague question. Let me try to answer some more precise versions of it.

    (1) Do the singularities that show up in idealized models in GR (of black holes and of the universe) describe something that actually exists in our universe? Answer: almost certainly not. Physicists regard these features of the models in GR as an indication that GR breaks down in this regime and needs to be replaced with a more comprehensive theory

    (2) Do objects exist in our actual universe that, in practical terms, are equivalent to the idealized black holes that show up in the models, even if there aren't actual singularities inside them? Almost certainly yes. In other words, there are almost certainly regions of spacetime into which objects can fall and never come out, at least not in any practical sense. (Coming out in ##10^{67}## years as Hawking radiation doesn't count.)
     
  6. Oct 27, 2017 #5

    PeterDonis

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    See my post #4.

    The same answer I gave in post #4 applies to this: there is almost certainly not an "initial singularity" in our universe like the one that appears in the idealized models, but that does not mean the early universe was not very hot, very dense, and with very strong spacetime curvature. So the idealized model is still a very good approximation back to a very early time, which is all we need it for in practical terms anyway.
     
  7. Oct 27, 2017 #6

    wolram

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    What can replace the idealised model, how can we do away with the singularity.
     
  8. Oct 27, 2017 #7

    PeterDonis

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    By finding a more comprehensive theory that covers the regime where GR appears to break down. In the meantime, as I said, we can still use the GR models in the regime where they don't break down, so in practical terms not having a solution (yet) to this issue has no real impact on the work scientists are doing. It's just one of many open questions for research.
     
  9. Oct 27, 2017 #8
    There are lots of ways to deal with singularities. If scientists knew how to do it for this case, they would, they just haven't figured it out yet though.

    Aren't there singularities in Standard Model too? I was under the impression that QM had point interactions, which happen at a single point in space and time, which causes mathematical problems. String theory doesn't have the same problem because the extra dimension removes any points and allows you to do time slicing in a relative way.
    three.jpg

    I'm pretty sure neither String theory or LQG have singularities at the hearts of black holes, so there are ways around it. The theories will approximate to the standard model though for any energy level that we are able to test with, so I don't think we'll have any idea for a while.
     
  10. Oct 27, 2017 #9

    PeterDonis

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    Nobody knows how to construct a string theory or LQG model of a black hole (yet), so we don't know if this is true (although I know most workers in the field expect it to be true).
     
  11. Oct 28, 2017 #10

    timmdeeg

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    Am I right that nevertheless geodesics are incomplete in this case, even if there are no singularities in the sense you mentioned ?
     
  12. Oct 28, 2017 #11

    PeterDonis

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    No, because geodesic incompleteness and the presence of singularities in the sense I mentioned are equivalent, since the defintion of "singularity" in the sense I mentioned is geodesic incompleteness.
     
  13. Oct 29, 2017 #12

    timmdeeg

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    Ah, yes. So the timelike geodesic of a particle which falls into such a (real) black hole is complete, because it remains a part of spactime. Kindly correct if wrong.
     
  14. Oct 29, 2017 #13

    PeterDonis

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    "Remains a part of spacetime" is not the definition of geodesic completeness. An incomplete geodesic is still "part of spacetime".

    The definition of geodesic completeness is that the geodesic can be extended to any value of its affine parameter. In the case of a timelike geodesic, that means that every possible value of proper time, from ##- \infty## to ##+ \infty##, corresponds to some point on the geodesic. An incomplete geodesic is one for which that is not true--for example, a timelike geodesic that falls into an idealized black hole with a singularity at the center, for which values of proper time greater than or equal to some finite number do not have any corresponding point on the geodesic.
     
  15. Oct 29, 2017 #14

    timmdeeg

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    Saying "Remains a part of spacetime" I meant the particle, not the geodesic it describes. Would this make sense?

    EDIT after overthinking I doubt it. The definition for geodesic completeness you gave is quite clear and particles are not included.
     
    Last edited: Oct 29, 2017
  16. Oct 29, 2017 #15

    PeterDonis

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    You doubt correctly. :wink:
     
  17. Oct 29, 2017 #16

    Chronos

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    Modern arguments over the existence of black holes generally center on whether or not a singularity is necessary. Most theoreticians question the existence of singularities, while perfectly willing to concede the existence of event horizons. The math behind an event horizon without a singularity is not pretty, nor is that of a singularity. A change in the way we think about the fundamentals of geometry is probably necessary to resolve both issues.
     
  18. Oct 29, 2017 #17

    PeterDonis

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    I would not put it quite that way. I think most theoreticians are perfectly willing to concede the existence of apparent horizons, i.e., surfaces where radially outgoing light, locally, does not move outward. But I don't think most theoreticians are willing to concede that the presence of apparent horizons necessarily entails the presence of event horizons. Many of the hypotheses about how quantum gravity effects change things end up amounting to: quantum gravity effects prevent the formation of actual event horizons, while still allowing the formation of apparent horizons.

    Since, experimentally, the only way to tell the difference between an apparent horizon and an event horizon is to wait a very, very long time (on the order of the Hawking evaporation time, i.e., about ##10^{67}## years for a solar mass black hole, and the time goes up as the cube of the mass), practically speaking there is no way to know if the various black hole candidates we observe really have event horizons, or only apparent horizons.
     
  19. Oct 29, 2017 #18

    PeterDonis

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    That's quite true, and it's one of the reasons why many theoreticians appear to be hoping that, when quantum effects are taken into account, all of those horizons will turn out to only be apparent horizons, not event horizons. The difficulties you refer to all go away if the horizons are only apparent horizons.
     
  20. Nov 1, 2017 #19

    zonde

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    There is chain of theoretical arguments that lead to black hole model of GR with some assumptions here and there. There can be flaws at different stages of the argument.
    The place where I see the flaw in that chain of arguments is treating Pauli exclusion principle (degeneracy "pressure") as a source of additional force.
     
  21. Nov 1, 2017 #20

    PeterDonis

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    Why do you think this is a flaw? Do you have any references to back up this claim?
     
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