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Hawking Radiation

  1. Aug 11, 2011 #1
    Hi PF

    I was wondering, if Hawking radiation could have a different source than black-body radiation. I was wondering, if quantum tunneling could be a possible way, for energy to slip out of the grasp of the black holes gravitational attraction and slip through the event horizon. Would this be reasonable and is evidence of this seen in any QG theory or in strings?

  2. jcsd
  3. Aug 11, 2011 #2
  4. Aug 11, 2011 #3
    We don't really know but there are speculations about it. One thing I do want to say is that when we are talking about any such cases, there are many different proposals as to what happens to "stuff" as it falls into a black-hole, for instance the holographic principal states that it gets encoded into the event horizon and then you can deal with what happens there on. So just to make things more specific, let's say we start with a black hole that encodes information as stated, now the question becomes:

    Can information (in the form of radiation) tunnel through the horizon?

    It must be noted that this process being discussed is separate from Hawking radiation.
  5. Aug 12, 2011 #4
    If something radiates as a black body, that's just a statement about the spectrum of radiation it gives off, not about the physical mechanism. It's very standard to think of Hawking radiation as a quantum tunneling event, and you can derive it in http://arxiv.org/abs/hep-th/9907001" [Broken] which doesn't go into the details of quantum gravity.

    What's difficult is to describe black hole evaporation as a unitary process. If the black hole does radiate as a black body, then different initial states are ending in the same way, which is impossible for standard quantum mechanics. Hawking himself was the first to notice this. Twenty years later he changed his mind because of AdS/CFT - the CFT on the boundary is unitary, and it's equivalent to gravity in AdS, and the latter contains black holes, so their dynamics "must" be unitary - but exact models are still lacking. Everything written about black hole evaporation, even in string theory, seems to involve approximations and guesses.

    Black hole thermodynamics - all the stuff about how a black hole has an entropy proportional to its area, and a temperature expressed in the Hawking radiation - was discovered somewhat indirectly, by studying quantum field theory in black hole geometries. The question immediately arose: what are the microscopic degrees of freedom responsible for this entropy? After many years, http://arxiv.org/abs/hep-th/9607235" [Broken].

    Some other recent papers on this topic: http://arxiv.org/abs/1108.2015" [Broken].
    Last edited by a moderator: May 5, 2017
  6. Aug 12, 2011 #5
    The way I originally learned about Hawking radiation was, that it was due to virtual particle pairs being made near the horizon. Then one gets sucked in, while the other escapes... The black hole looses mass, because it lends its energy (thereby mass) for a short amount of time to the particle pair, and when it annihilates, the black hole gets its mass back. But when one particle gets sucked in and the other escapes, the black hole losses energy and thereby mass.

    Therefore I thought tunneling might be a different approach than the usual? Or is my way of looking at Hawking radiation the same as tunneling? :)
  7. Aug 12, 2011 #6
    Hawking suggested the virtual particle pair description...but it was apparently not directly associated with his mathematical description. Either "virtual particles" or "tunneling" are possible descriptions....Both these and the overall effect can be associated with horizons: If you take a look at ADS/CFT as Porter posted and/or Unruh Effect, say in Wikipedia, you'll get a flavor for related descriptions.
  8. Aug 13, 2011 #7
    Vacuum fluctuations happen in flat space, without any black holes around. If you want to think of them as drawing on energy from somewhere, you should think of the zero-point energy of the quantum fields involved, as the source of the fluctuation. What's different, when you have a black hole, is that part of the field fluctuation can cross the event horizon and fall into the black hole, rather than recombining with the other part of the fluctuation.

    But http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad". Here we have a black hole which looks like a point in the large dimensions of space, but it actually consists of a stack of branes wrapped around the extra compact dimensions at that point (e.g. these might be a Calabi-Yau space), and then there is a gas of open strings stretched between the branes that are perpetually racing round and round one of the compact dimensions. These are the degrees of freedom which give the black hole its entropy! And then sometimes these open strings, when they are moving in opposite directions, collide and form a closed string which can escape as Hawking radiation - that's what page 63 depicts.

    But because we don't know the micro-geometry of our universe, we can't say what sort of bound state of branes might define a black hole here, and so we don't have an exact microphysical model of Hawking radiation for the real world.
    Last edited by a moderator: Apr 26, 2017
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