Inside Black hole

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Saint, Feb 12, 2020.

  1. Saint Valued Senior Member

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    Inside black hole, is it a singularity?
    Does black hole annihilate information?
    Assuming your spacecraft is being absorbed into a black hole, is it moving faster and faster towards the center of the black hole? Will it be faster than the speed of light?
     
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  3. paddoboy Valued Senior Member

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    Scientists and Cosmologists generally now reject any singularity as defined by infinite spacetime curvature and density. The singularity though as defined by where our laws and GR fail us, exists at and below the quantum/Planck level.
    Still debatable I think, although Hawking Radiation has now been shown to be probable.
    No, Inside the EH, the curvature of spacetime is such that there is no world line or path, other then towards the centre. Light itself simply loses energy and is blue shifted from the PoV of someone further up the gravitational well. The escape velocity at the EH is "c" or 300,000 kms/sec....Beyond that the escape velocity increases in line with the critical curvature, and light/photons as always, follows the geodesics of that curvature towards the singularity.
     
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  5. paddoboy Valued Senior Member

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    And just preempting the next question, no gravity does not need to escape a BH, as it is simply the fossil field curvature as was defined before it became a BH.
     
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  7. Schmelzer Valued Senior Member

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    No. There is no derivation of it which does not have a trans-Planckian problem.
     
  8. paddoboy Valued Senior Member

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    As you would know, no scientific theory is ever 100% proof, but evidence for Hawking Radiation has certainly increased in recent time. Even the BB has some problems, still the overwhelming evidence supporting it still prevails and it remains overwhelmingly supported in line with that evidence....
    Hawking Radiation at least to me, sounds a reasonable logical scenario as to what can happen this side of the EH.
    https://www.sciencealert.com/a-blac...d-new-evidence-that-hawking-radiation-is-real

    https://www.nature.com/articles/d41586-019-01592-x

    https://www.sciencealert.com/a-lab-made-black-hole-might-have-finally-proved-stephen-hawking-right
    The results have been published in Nature Physics.

    https://arxiv.org/pdf/1809.10412.pdf
    27 Sep 2018
    Evading the Trans-Planckian problem with Vaidya spacetimes:

    Abstract:

    Hawking radiation, when treated in the ray optics limit, exhibits the unfortunate trans-Planckian problem — a Hawking photon near spatial infinity, if backtracked to the immediate vicinity of the horizon is hugely blue-shifted and found to have had trans-Planckian energy. (And if back-tracked all the way to the horizon, the photon is formally infinitely blue-shifted, and formally acquires infinite energy.) Unruh has forcefully argued that this implies that the Hawking flux represents a vacuum instability in the presence of a horizon, and that the Hawking photons are actually emitted from some region exterior to the horizon. We seek to make this idea more precise and somewhat explicit by building a purely kinematical model for Hawking evaporation based on two Vaidya spacetimes (outer and inner) joined across a thin time-like boundary layer. The kinematics of this model is already quite rich, and we shall defer consideration of the dynamics for subsequent work. In particular we shall present an explicit calculation of the the 4-acceleration of the shell (including the effects of gravity, motion, and the outgoing null flux) and relate this 4-acceleration to the Unruh temperature.
    ::::::::::::::::::::::::::::::::::::::::::::::::::

    While having no barrow to push either way, and as I did say earlier, Hawking Radiation seems to me to be a reasonably logical outcome.
     
  9. James R Just this guy, you know? Staff Member

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    According to the general relativistic model, yes.

    According to Hawking, no.

    Yes, if it's in free fall towards the hole.

    No.
     
  10. Schmelzer Valued Senior Member

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    There is no such a thing like evidence for Hawking radiation. Those experiments are about something completely different, namely about particular flows in condensed matter theory. These experiments are fine, but they simply have nothing to do with Hawking radiation.
    Making something inherently wrong more precise does not help much.
    The analogy with Unruh radiation fails because stable stars don't Hawking-radiate. Despite the fact that observers at rest relative to them would be accelerated. As long as a star collapses, there will be radiation, when it stops, the radiation stops. This allows to identify the region which causes the radiation uniquely. You cannot explain this using a model which is based on what happens far away, because far away there is no difference between a collapsing star and a stable star.
     
  11. paddoboy Valued Senior Member

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    There's plenty that would disagree with you....and of course as I did say, while having no barrow to push either way, Hawking Radiation seems to me to be a reasonably logical outcome.
     
  12. paddoboy Valued Senior Member

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    https://www.nature.com/articles/s41586-019-1241-0

    Observation of thermal Hawking radiation and its temperature in an analogue black hole:

    Abstract
    The entropy of a black hole and Hawking radiation should have the same temperature given by the surface gravity, within a numerical factor of the order of unity. In addition, Hawking radiation should have a thermal spectrum, which creates an information paradox. However, the thermality should be limited by greybody factors, at the very least. It has been proposed that the physics of Hawking radiation could be verified in an analogue system, an idea that has been carefully studied and developed theoretically. Classical white-hole analogues have been investigated experimentally, and other analogue systems have been presented. The theoretical works and our long-term study of this subject enabled us to observe spontaneous Hawking radiation in an analogue black hole. The observed correlation spectrum showed thermality at the lowest and highest energies, but the overall spectrum was not of the thermal form, and no temperature could be ascribed to it. Theoretical studies of our observation made predictions about the thermality and Hawking temperature. Here we construct an analogue black hole with improvements compared with our previous setup, such as reduced magnetic field noise, enhanced mechanical and thermal stability and redesigned optics. We find that the correlation spectrum of Hawking radiation agrees well with a thermal spectrum, and its temperature is given by the surface gravity, confirming the predictions of Hawking’s theory. The Hawking radiation observed is in the regime of linear dispersion, in analogy with a real black hole, and the radiation inside the black hole is composed of negative-energy partner modes only, as predicted.

     
  13. paddoboy Valued Senior Member

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    Plus of course, I have not said anything about Hawking Radiation certainty, as the following statements shows. By the same token I still see reasonable logic in the scenario and like others, accept its likelyhood.......
     
  14. Schmelzer Valued Senior Member

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    Your subjective likelihood is irrelevant.

    I have tried to clarify the objective parts. The trans-Planckian problem remains, and will remain forever, because no new derivation can change the point that Hawking radiation forever depends on the continuation of the collapse forever, and this leads necessarily into speculations about trans-Planckian theory.

    Analogue black holes can prove nothing. All what they can do is to show that the standard mathematics used in Hawking's derivation are, by themselves, fine, and can be applied somehow in condensed matter theory. But there was never much doubt that this math is fine - the trans-Planckian problem is simply not about this.

    The point is that if something changes in the gravitational field (like during the collapse) then there will be such radiation. The change is essential. Without change, there will be no radiation. The "analogue black holes" are all dynamical configurations, some permanent flow. So, some corresponding radiation has to be expected.

    If the collapse of a star leads in a yet unknown quantum theory of gravity to some stable configuration or some sort of permanent change is a pure guess, about trans-Planckian physics. (I would guess that, as typical for quantum theory, that the collapse ends in some stable state of minimal energy instead of collapsing forever. If this is correct, there will be no Hawking radiation. Of course, that's only a guess. For something better, one needs a theory of quantum gravity.)
     
  15. paddoboy Valued Senior Member

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    *shrug* I have no agenda to push and as I said, plenty of qualified professionals appear to be of the same opinion.
    I have made mention of any singularity as defined by infinite density and curvature, as dismissed by cosmologists, as I have of singularities as defined by the failure of our laws and GR.
    Again other professionals disagree and one could also note that your total dismissal is irrelevant.
    So, you do agree with me re some doubt rather then your previous total dismissal?
     
    Last edited: Feb 13, 2020
  16. paddoboy Valued Senior Member

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    Another paper on an earlier result......
    https://arxiv.org/abs/1009.4634

    Hawking radiation from ultrashort laser pulse filaments:

    Event horizons of astrophysical black holes and gravitational analogues have been predicted to excite the quantum vacuum and give rise to the emission of quanta, known as Hawking radiation. We experimentally create such a gravitational analogue using ultrashort laser pulse filaments and our measurements demonstrate a spontaneous emission of photons that confirms theoretical predictions.

    https://arxiv.org/pdf/1009.4634.pdf
     
  17. paddoboy Valued Senior Member

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    Here is another interesting account that also does not totally dismiss the probability of Hawking Radiation.......
    http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html
    Modified by Ilja Schmelzer 1997.
    Original by John Baez 1994.
     
  18. Saint Valued Senior Member

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    We know that space is 3D, if we view a black hole 360 degree from all directions, then we shall be able to know what is inside or the Inlet and Outlet of a black hole, isn't it?
    Then we answer the question "what is inside a black hole?".
     
  19. paddoboy Valued Senior Member

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    View it from any angle you like. No light is emmited from a BH so all one would see is blackness and any heating of any accretion disk which is outside the BH anyway.
     
  20. Write4U Valued Senior Member

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    I tend to agree with that perspective, but just a thought experiment, if we visualize a toroid universe where the center of the universe is a mega black hole which swallows up the entire universe at one end of the donut hole compresses it into a singularity and spews it out as a inflationary white hole.
    IOW, a recycling universe?
    The Toroidal Universe?
    http://www.greatdreams.com/grace/126/131toroiduni.html

    or more illustrative;
     
  21. James R Just this guy, you know? Staff Member

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    Are you aware that from the outside black holes look approximately like spheres? They aren't like circular plugholes in space.
     
  22. Schmelzer Valued Senior Member

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    Science is, fortunately, not a democracy. Else, there never would have been any progress.
    Fine. Hawking radiation is simply another point where semiclassical gravity fails to predict something reasonable.
    I have never made a total dismissal. All I have said is that there is no derivation which does not have a trans-Planckian problem. So, there is no valid derivation. It is simply pure speculation without any base.
     
  23. Schmelzer Valued Senior Member

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    4,774
    You confuse Hawking radiation with a physical theory. It is none. Physical theories cannot be proven. But that in a given physical theory certain effects happen can be proven. The relevant theory is in this case semiclassical gravity, which is known to be wrong because it can be only an approximation of a quantum theory of gravity.

    In a quantum theory of gravity, it would be possible to prove that there will be Hawking radiation - or not. The proof would be based on the axioms of that theory. With lower requirements for rigidity, one could prove something even using only the approximation of semiclassical gravity. Such a proof would require that the computations made using the approximation rely only on the region where the approximation is assumed to be valid.

    There is nothing of this sort for Hawking radiation.
    For the BB, there is no such trans-Planckian problem. As far as in the very early universe quantum effects for matter are relevant, they can be handled approximately with semiclassical gravity.
    It is, in fact, far from logical - assuming it even leads to serious contradictions, like the information loss problem. And, again, the dumb holes of analog gravity in condensed matter theory have no relevance at all. It is an analogy which fails in essential points. First, the region where the radiation is created in the analog is the one where the analog of relativistic symmetry fails. Then, a stable black hole is something static, while the dumb holes are highly dynamic flows, and it is the change which creates the radiation.
     

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