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How does black hole evaporation end?

  1. Feb 23, 2008 #1
    Someone asked me what happens when micro-black holes evaporate, and I have no idea, not really knowing anything about the theory of black holes, other than what you learn in a first course in GR. What he was asking was whether the black hole just keeps emitting energy (in the form of Hawking radiation, I presume?) until there is nothing left, or at some point is it light enough that it's no longer a black hole at all, but is just a lump of stuff?

    My guess was that there is some critical value of the event horizon radius below which it no longer makes sense to talk about something as a black hole anymore, but I don't know what that would be - maybe some kind of effective size of the particle(s) involved?

    Is this even a meaningful question? I hope so, but I'm a virtual layman when it comes to this stuff.
  2. jcsd
  3. Feb 23, 2008 #2
    I, as Rioghio extreme, also known as Quantum Leap, think it is the most viable question ever asked.
  4. Feb 23, 2008 #3
    Don't I know you from somewhere?? ;-)
  5. Feb 23, 2008 #4
    If there do exist micro-black holes at all, then that is your answer, they evaporate and yet continues to exist as black holes even to the micro stage.
  6. Feb 24, 2008 #5
    From what I recall the black hole continues to shrink until it reaches an effective radius on par with the planck length, at which point, as the name evaporation implies, it just 'explodes' into a lot of energy via photons and leptons, and it does this because I suppose as you said, it becomes too 'light' to be a black hole still and also has then an enormous temperature.
  7. Feb 24, 2008 #6
    Ah, yes, the temperature does do up, doesn't it? I was thinking of it as just becoming more benign, in terms of energy density, but I guess it energy goes up as its mass decreases.

    I guess if any of this is real, we might find out this Summer at the LHC ... should be fun.
  8. Feb 24, 2008 #7
    Might being the operative word. I'm a bit skeptical the energies will be sufficiently high enough to test black hole evaporation.
  9. Feb 25, 2008 #8
    My understanding is that it's mostly a matter of whether certain extra-dimension theories are true. If specific large-extra-dimension theories are true, then maybe it will be possible to make mini black holes at LHC energies. If different extra-dimension theories are true, or if there are only 4 dimensions, then the LHC (or even anything even close to the order of magnitude of the LHC's energy level) could never possibly make any black holes.

    So it seems pretty unlikely indeed that the LHC will see mini black holes! It's not just a matter of "are the energies high enough?" but also "do we happen to live in a universe which meets this specific set of slightly contrived assumptions about spacetime?"

    But, maybe there are other ways to experiment on black hole evaporation. There is for example I am told an ongoing radio astronomy project which could among other things detect the explosions of primordial black holes which formed and died in the chaotic conditions of the early universe. (I got that link from here.)
    Last edited: Feb 25, 2008
  10. Feb 25, 2008 #9


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    The odds of producing a mini black hole in the LHC are virtually zero. Cosmic rays regularly produce much higher energy collisions in earth's atmosphere, but, there is no evidence we are besieged by radiation emanating from mini black holes evaporating in our atmosphere.
  11. Feb 27, 2008 #10
    That is why I am skeptical there will be enough energy!
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