Hawking Radiation

Discussion in 'Physics & Math' started by The God, Mar 10, 2017.

  1. Fednis48 Registered Senior Member

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    725
    Just saw this on the arXiv:

    https://arxiv.org/pdf/1703.09369.pdf

    RJBeery, I think you'll find it vindicating.

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    Basically, the authors calculate gravitational collapse with Hawking radiation (or "pre-Hawking" radiation) for a spherical dust shell and two simple cases of spherical dust ball that admit analytic solutions. For all three cases, they find that the collapsing dust approaches the Schwarzchild radius, then stalls out under time dilation as it evaporates; an event horizon never forms. Not published yet, but based on the format it looks like they're going for Physical Review Letters.
     
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  3. The God Valued Senior Member

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    I wonder why that requires so much agony. The maths says time inside EH is hugely dilated as compared observer at infinity. So technically as soon as the dust is at EH or EH-, it cannot move further down till our eternity.
     
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  5. Q-reeus Valued Senior Member

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    2,497
    Of course. As I have pointed out many times before, and particularly in a thread WRONGLY banished from Physics & Maths:
    http://www.sciforums.com/threads/fr...-is-bs-or-dont-just-trust-authorities.142870/

    Every so often a newbie at Physics Forums or even here brings up the issue of whether anyone or anything can cross a classical GR BH so-called EH and reach the dreaded singularity in finite and typically short proper time. The Gatekeeper GR buffs inevitably respond by claiming infinite external time is an 'optical illusion' and merely the result of a poor coordinate choice i.e. standard Schwarzschild chart. Pick say Kruskal-Szekeres chart and all is resolved with no conundrum to worry about.
    Which however is easily shown to be false and ignores that proper time to reach the EH for an infaller corresponds to an infinitely old rest of universe. We expect something to intervene in the interim - quite apart from any appeal to so-called HR!

    As I have further pointed out many times here at SF and elsewhere, introduce HR, then at best an infaller simply rides the EH down to zero radius and never encounters a singularity. The inconsistency in all of this is that on the one hand free fall should mean no experience of HR by analogy with Unruh Radiation which requires proper acceleration to be experienced. On the other hand, since the entire BH is supposed to be eventually consumed as HR, at some point (which should be early since the outermost matter will be first to 'evaporate') the infaller must likewise be 'evaporated' thus will feel the HR.

    And so 'firewalls' were cooked up to solve such matters. No matter it DRASTICALLY overturns the classical picture of a BH or even the semi-classical 1974 Hawking one.
    GR's BH's are simply a mess of inconsistencies, but the herd will follow regardless, because a long maintained consensus of experts and dutiful media promoters sets the tune.
     
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  7. The God Valued Senior Member

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    You, say we, forget that the golden egg giving hen or milk giving cow cannot be killed. Only when the desired funding is blocked in this direction, the physics will improve...otherwise some GR guy will always come up and say look rulers and clocks are different and so...
     
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  8. Q-reeus Valued Senior Member

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    2,497
    Well it certainly is an industry in it's own right, with substantial funding for those willing to stick within the GR BH paradigm. The article cited in #81 is typical of attempts to tweak things semi-classically but never question whether GR has the basic classical aspect of it right in the first place. GR's reign will likely end as a very slow and painful process and only after - perhaps - something like decisive data from the EH Telescope starts to show up departures from the expected result according to GR.
     
  9. The God Valued Senior Member

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    Poor guys cannot start with an anti GR premises, it is as sime as that. Do that and forget publication in any worthy journal, even their Arxiv affiliation may be blocked....

    There is already great deal of data not in sync with GR, but it is either brushed aside or...

    Take for example Lensing, enough data is there which do not match GR. List is endless.

    No one disputes that delta_t inside EH correspondence to infinity on this side...so where is the question of any time elapse once inside EH. And of course none stops these GR guys to calculate the proper time from EH to r =0, even though it is senseless.
     
  10. Q-reeus Valued Senior Member

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    2,497
    Are you sure? That's hard to believe because lensing is merely a 1st order prediction compatible with virtually all alternative gravity theories. Even Newtonian gravity predicts lensing (but at half the level of GR and any viable alternative theory).
    Actually, believe it or not, there are a number of experts who claim that time 'starts again' wrt an exterior observer, once anything gets to be inside the EH. By slavishly following the formal maths when extending Schwarzschild coordinates to the interior where common sense says it will only give nonsense. Evidently getting past the EH at all is a 'non issue'.
     
  11. rpenner Fully Wired Staff Member

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    4,833
    It's not clear to me that the gluing together of flat spacetime and a non-zero-mass Vaidya solution is a solution to GR which renders the infalling dust as normal matter. Likewise, when they generalize to spherical metrics of all shapes and sizes, they never review what form the matter (by which I mean the stress-energy tensor) takes.
    I am somewhat troubled by their reliance on the outgoing Vaidya metric when 1) it describes outgoing radiation and 2) it ignores the GR dynamics of the postulated infalling sphere of non-null dust. It's almost like they want to skip over the gravitational parts of GR.
    Maybe these concerns are addressed in the multiple references I didn't have time or access to follow.

    It is also completely unclear how pre-Hawking radiation would evaporate the (material) dust, unless the proposed sub-Planckian skin is a cosmological mixmaster undoing the primordial bias against anti-matter and resulting in total matter-to-energy conversion. I don't see a mechanism for the infalling shell to see or interact with its pre-Hawking radiation.
    Finally, stability versus classical perturbations from sphericity or quantum tunneling to the interior are not considered.

    As a nit-pick, I didn't care how they introduced notation conventions after use with dummy variable A.

    http://www.blau.itp.unibe.ch/newlecturesGR.pdf (huge!) covers the Vaidya metric in some detail in sections 39-41.
     
    Last edited: Apr 2, 2017
  12. Fednis48 Registered Senior Member

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    725
    Thanks for the feedback, Q-reeus and rpenner. I only understand GR well enough to really understand the paper's problem statement and conclusion, so it's good to hear what other, better-versed posters think of it.
     
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