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Hawking radiation

  1. May 23, 2005 #1


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    In cosmology Hawking radiation, is often quoted as a way Black Holes evaporate.
    AFAIK there is no way this can be tested other than getting up close to a BH,
    so is HR accepted in the quantum phys world, as a good model?
  2. jcsd
  3. May 23, 2005 #2
    What we need to do to observe HR is, as you say, to watch black holes evaporate. How can we do this? As the Wikipedia article says, large ones have a very low temperature, so have a negative net energy output.

    We need small black holes.

    Luckily enough there's going to be a new particle accelerator turned on in a couple of years - LHC (the Large Hadron Collider) at CERN. This will accelerate protons towards each other at 7TeV a piece - 14TeV centre of mass energy.

    Unfortunately the modeling of black hole creation is pretty much a hand waving exercise as quantum gravity isn't at all well formed. However, there are simulations that can be run using latest theory that model black hole production at an accelerator. The main one used is Charibdis - http://www.ippp.dur.ac.uk/montecarlo/leshouches/generators/charybdis/

    Whether or not a black hole is produced depends on the Planck Scale, something we don't know. The Planck Scale is in turn reliant on the number of hidden dimensions, which we don't know.

    So, if there's a low Planck scale, many black holes will be produced at LHC, and they will (judging by a friend's study that has just been completed) light up the detector like a christmas tree.

    However, if the Planck scale is large, there will be very few black holes produced (if it means anything to you, in some cases they were seeing production cross sections of order 10^-9pb - that's not very much... A Higgs Self coupling decay, by comparison, has cross section of order 20fb for higgs mass 120GeV, and that's a pretty rare decay), like one per two universe time scales or something silly like that.

    So, we might see black holes at LHC, then again we might not. Depends on some fundamental knowledge we don't possess!

    Edit: Original Question

    Hawking Radion makes sense mathematically. If you look at particle pairs being created and annihilated from the vacuum, the maths works in terms of one pair from the particle being 'sucked in' to the black hole, where the massive gravitational forces involve essentially relativistically reverse the energy of the particle, so that energy is conserved. Otherwise you'd wouldn't have conservation of energy.

    Hope this helps.
    Last edited: May 23, 2005
  4. May 23, 2005 #3


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    Thanks for that JAMES.

    Luckily enough there's going to be a new particle accelerator turned on in a couple of years - LHC (the Large Hadron Collider) at CERN. This will accelerate protons towards each other at 7TeV a piece - 14TeV centre of mass energy.

    So a proof of QG may come from CERN? Seems that HR is dependant on
    many things so far unknown.

    sorry i do not know the units at bottom of post.
  5. May 23, 2005 #4


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    "picobarn".[itex] 1pb=10^{-40}m^{2}[/itex].

  6. May 23, 2005 #5


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    A barn is a unit of area, abbreviated mostly as "bn", equal to 10−28 m2. Although not an official SI unit, it is widely used by nuclear physicists, since it is convenient for expressing the cross sectional area of nuclei and nuclear reactions. A barn is approximately equal to the area of a uranium nucleus. The etymology is clearly whimsical -- the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions.
  7. May 23, 2005 #6
    Yes, a barn can (very roughly) be taken as a representation of the probability of a certain decay process occuring. It's historically based in scattering and fixed target experiments.

    I wouldn't say a proof of QG will come from LHC, just that we might see what looks like black holes evaporating, dependant on lots of things (backgrounds, relative intensities, cross sections etc etc). How this links to QG I'm not entierly sure, not being an expert on it... My work recently has been looking at techniques for more accurate event reconstruction in linear collider vertex detectors, but as I say I've picked up a fair bit of info on black holes at LHC from a friend who's been looking into them.
  8. May 23, 2005 #7


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    James may i give you this,
    http://arxiv.org/PS_cache/gr-qc/pdf/9603/9603064.pdf [Broken]
    Last edited by a moderator: May 2, 2017
  9. May 23, 2005 #8
    You may. Interesting paper - would you like me to comment on it in some way?
  10. May 24, 2005 #9


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    Please, i am interested in the possibility that HR will give a signature of
    QG in future cern experiments. I only know what i have read an could
    have the wrong end of the stick.
  11. May 24, 2005 #10
    What the paper seems to be saying is that if loop gravity is a correct theory (and I know nothing about it...) then macroscopic black holes will emit thermal Hawking Radiation, and not some horrifically over quantised spectrum that had been previously derived using a niaeve method.

    As a sidenote, the curve you get from Hawking radiation resembles a black body curve very closely, except there are small pertubations due to things called grey body factors.

    So... If we see black hole evaporation at the LHC, and we can separate it from backgrounds etc, we should be able to get a curve representing the Hawking Radiation, in which case, following that paper, it looks like a point for the predictions of quantum loop gravity.

    That's my take on it anyway, it's not an overly complex paper at all, so I think I've got the right end of the stick.
  12. May 24, 2005 #11


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    Thankyou JAMES

    Some thing to look for in future papers i hope.
  13. May 24, 2005 #12
    Got to get the accelerator and detectors working first... They've started lowering the accelerator magnets now, which is nice.
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