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B Is the association of QG with BH a fake

  1. Dec 27, 2017 #1

    ftr

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    I mean, first GR is a semi-classical theoryt which makes BH neither classical nor quantum. And since it was formed from many particles so it seems in the least it is closer to classical(so no QM should apply) and I don't know if it makes sense to treat it as manyparticle nonrelativistic qft. Also we don't know if any natural quantum particle that is in the Planck's region i.e. no quantum BH exists.

    So BH does not generally makes sense physically.
     
  2. jcsd
  3. Dec 27, 2017 #2

    Nugatory

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    The word "classical" is casually used in two ways: To describe the pre-relativistic and pre-quantum physics of the 19th century; and to distinguish quantum physics from non-quantum physics. This leads to some confusion about exactly what you mean when you say that "GR is a semi-classical theory" - it's classical under the latter meaning and non-classical under the former, but never "semi-classical".

    When these squishy adjectives create problems, it's often best to be specific about the range in which we're applying the theory. Difficulties at the Planck scale don't prevent us from applying GR to the vacuum region above, at, and well inside the event horizon of a stellar-mass black hole; and many observations suggest that this makes sense physically. Similarly the lack of a Planck-scale theory doesn't prevent us from considering quantum mechanical effects at the horizon; the results make physical sense even though it is impractical to observe the predicted phenomenon.
     
  4. Dec 27, 2017 #3
    To say one is supreme while the other is worthless is folly. The realm of applicability is set by necessity.
     
  5. Dec 27, 2017 #4

    ftr

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    Ok thanks. I have seen the word semi (describing GR)used since the the system is neither Newtonian nor quantum, but never mind the word.

    But may be I should clarify. I am not saying that BH exist or not, what I am saying is that it seems that the physical situation is unclear to apply to it some model. Ok, you can guess some hypothetical, but it just looks too much. Even if you take neutron or (quark ?!!!) stars the modeling looks fetchy, but BH there is nothing even conjectured to be inside it at least AFAIK.
     
  6. Dec 28, 2017 #5

    zonde

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    ftr, I agree that it is not very meaningful to apply QM to BH. QM can describe particles in potential well, but GR does not describe the gravity using potentials. However you can speak indirectly about potential differences in GR by looking at relative time dilation (in stable physical configuration). The limit of applicability for that approach is event horizon where potential would approach infinity. So if we would consider that potentials have some correspondence to physical reality (QM type approach) we should conclude that BH is unphysical speculation.
     
  7. Dec 28, 2017 #6

    Demystifier

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    No, GR is a fully classical theory.

    There are many systems of many particles that cannot be explained without QM: superconductivity, superfluidity, white dwarfs, neutron stars ...

    Why nonrelativistic?

    BH's do not need Planck's region.

    If something doesn't make sense, that's your arguments above.
     
  8. Dec 28, 2017 #7

    Dale

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    I am not sure from your comments if you are concerned about the singularity or the event horizon. The region near the singularity is where quantum gravity is expected to be important. The event horizon should be purely classical, so the issues you mention don’t even arise there. Usually the term “black hole” refers to the event horizon.
     
  9. Dec 28, 2017 #8

    Dale

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    I have not seen this specific objection to black holes in the professional scientific literature. Do you have a reference?
     
  10. Dec 28, 2017 #9

    ftr

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    Let me clarify my heavily summarized post. I meant when the star exploded it contained many particles clumped together as plasma, so to explain the details of how eventually the core became a black hole with singularity with unknown material in the inside of the event horizon there seem to be no easy model classical or quantum.Even explaining what goes inside the sun seem to be an evolving science dependent on data from probes. If a non physicist had conjectured a quark star he would simply be ignored.

    I meant it in this sense
    https://en.wikipedia.org/wiki/Micro_black_hole

    I do feel that way , all the time.:frown:



     
  11. Dec 28, 2017 #10

    Dale

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    Ok, so I am still confused. Are you focused on the singularity (quantum gravity needed) or on the event horizon (quantum gravity not needed)?
     
  12. Dec 28, 2017 #11

    ftr

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    I am talking about a different issue. You are assuming that the black hole has already formed with the specific prediction of GR. My problem is how it got there through a step by step physical process.
     
  13. Dec 28, 2017 #12

    PeterDonis

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    We have both an exact solution and numerical simulations that describe this in detail. The exact solution is the Oppenheimer-Snyder spherically symmetric dust collapse model, which dates back to 1939; it is discussed in many GR textbooks and has also been discussed here at PF. The numerical simulations cover all kinds of more realistic collapse processes, including the effects of pressure, radiation, ejection of gas jets, etc. All of these models show black holes being formed from the collapse of massive objects. So your "problem" has long since been solved.
     
  14. Dec 29, 2017 #13

    Dale

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    As @PeterDonis mentioned, there is the Oppenheimer-Snyder collapse, as well as numerous unnamed numerical solutions.

    However, you still haven’t clarified! This is getting rather frustrating. When you say “how it got there” is “it” the event horizon or is “it” the singularity?

    Please answer clearly!
     
  15. Dec 29, 2017 #14

    zonde

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    No, I don't have a reference. Actually I can't find discussions about QM part in Oppenheimer-Volkoff argument on maximum mass of neutron stars. Maybe it's too speculative for serious researchers and not providing much prospects on generating some observable/testable predictions. And of course validity of this maximum mass calculation can not exactly confirm or falsify GR or QM as long as there are assumptions unrelated to either theory. On the other hand it seems related to possible developments of QG.

    Anyways, are you concerned that my conclusion is too speculative as per PF rules?
     
  16. Dec 29, 2017 #15

    Dale

    Staff: Mentor

    Yes, very much so. GR has a Lagrangian and a Hamiltonian formulation. So the objection that you brought up seems fundamentally wrong to me. But of course, I don’t know all the literature on the topic, so I wanted to check first.

    Since you do not have a reference supporting that point, you should not post it further. It seems wrong to me, but at a minimum it is speculative.
     
  17. Dec 29, 2017 #16

    PeterDonis

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    In all of the analyses that I'm aware of on neutron star maximum mass, gravity is treated classically; in other words, whatever quantum effects are included in the analysis are done in a classical background spacetime, with no quantum degrees of freedom corresponding to gravity.

    But in such a case the potential well itself is not treated quantum mechanically; it's just put in by hand. For example, a typical QM model of a hydrogen atom treats the electron as being in a Coulomb potential well created by the nucleus; the potential well is not a quantum degree of freedom, it's just put into the Hamiltonian by hand. But we know that's not precisely correct: a fully quantum treatment would treat the EM field as quantum mechanical as well, including the part of it that appears as a Coulomb potential in the standard model. And once you do that, you no longer have a "potential" at all in the usual sense, because you aren't using the Schrodinger Equation or anything like it; you're using quantum field theory. And such a theory can also give you states of the quantum field that don't have any interpretation as a "potential well".

    Similar remarks would apply to a model of a system which, classically, involves a gravitational potential well: in a fully quantum model the potential well would no longer be there, it would be replaced by an appropriate quantum field, in this case the quantum field corresponding to gravity. And such a model could give you states that don't have any interpretation as a "gravitational potential well"--including possible "black hole" states. (It is true that such states cannot arise in a quantum field theory of the EM field--that's because such a field is linear. A quantum gravity field is not linear, so it can have "bound" states that don't have a "potential well" interpretation.) The problem is that we know there are issues with the straightforward QFT of a massless, spin-2 field, the main one being that it is not renormalizable; that's why physicists are still searching for a theory of quantum gravity. But there is no reason to suspect that a correct theory of QG will somehow rule out black hole states because they don't have a "potential well" interpretation; in fact, the contrary is true, one of the main models that QG researchers are searching for is a QG model of a black hole, which will show us how to account for things like its entropy.
     
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