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Hawking's radiation

  1. Aug 21, 2009 #1
    as we know that nothing can escape from the black hole. but theory given by hawking for his radiation is fool proof or not. i mean he say that collision b/w particle and anti particle at the horizon emit the rays called radiation? then how we can say that these radiation are emitted from black hole?
  2. jcsd
  3. Aug 21, 2009 #2


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    It isn't the collision of between particles, but a creation of a pair of particles. Quantum fluctuations create a matter/antimatter pair of particles at the event horizon. Occasionally, one of the two falls into the event horizon before they can recombine. The other particle leaves.
    Since the creation of the pair came from "borrowed" energy, the energy needed to maintain the existence of this particle must have come from somewhere. That somewhere is the mass of the black hole. To balance the energy books, the particle that falls into the black hole has a negative mass, and actually reduces the mass of the black hole. The result is that you see particles leaving the vicinity of the black hole while the black hole loses mass.
  4. Aug 25, 2009 #3
    but particle pair creation relies on the idea that they can only exist for less than the plank time before recombining into nothing. i think that implies that any gravitational influence experienced by one of the particles would also be experienced by the other particle, since the two cannot be physically very far apart - so the odds of one being sucked into, while the other escapes from, a BH seem extremely unlikely - any particle anywhere near the EH of a BH is oging to wind up inside the BH, unless it is somehow travleing at very very near C. i, and many others have long been skeptical of hawking radiation.
  5. Aug 25, 2009 #4
    not the plank time but time defined by HUP. Much, much longer!
  6. Aug 25, 2009 #5
    excuse my lack of understanding, I'm not an educated physicist. Isn't there such a thing as quantum vacum energy? like in the casimir experiment? don't these pairs of particles pop into existence everywhere, even very far away from any black hole
    or other mass? so where does that energy come from? or do the equasions still balance out because the particle anihilate and go back to nothing? and if it is a particle/antiparticle pair then would they not emit some energy gama ray or something when they anihilate?
  7. Aug 26, 2009 #6
    1 Yes, Casimir effect is explained by the virtual particles
    2 Yes
    3 From nowhere, virtual particles dont carry any free energy. They violate energy conservation law for small periods of time (obeying HUP)
    4 yes, they go back to nothing because they dont carry free energy
    5 and for that very reason they dont radiate - they dont have energy

    The only difference near the horizon is horizon breaks the pairs, preventing the annihilation.
  8. Aug 26, 2009 #7
    how can being near the EH of BH break up the pairs? if they do not come into existence with an enormous amount of velocity (near C), neither particle could ever escape from the gravity well of the BH.
  9. Aug 26, 2009 #8
    so they don't have mass or energy unless a black hole pulls one of them in and then they do have mass and energy? one positive and one negative? That's strange, but then so many things are.

    if a pair pops into existence near a black hole why would one get pulled in and not both considering how close together they must be to each other? there is such a fine balance in the gravity that it can pull one in and not the other? and why would the free one go flying off ? when they come into existance they don't have any speed or velocity relative to the black hole do they? or maybe they do? So why would the free one spontaneously go flying off away from the blackhole? Well I'm sure Hawking has a complete mathmatical framwork describing the process so I accept it at face value, but it's just hard to imagine them not both being pulled in.
  10. Aug 26, 2009 #9
  11. Aug 26, 2009 #10
    talon - your questions at the end of your post are pretty much the same questions i am asking. while hawking may have laid out a mathematical possibility for something to occur, does not necessarily mean that it will actually happen.
  12. Aug 26, 2009 #11
    Perhaps not but that method is the basis for theoretical physics.
  13. Aug 27, 2009 #12
    funky - i am a pretty firm believer in the idea that if anything can possibly happen in this universe, it will happen. however, i just cannot see any possible way a particle created near the EH of a BH could ever escape the gravity well - can you?

    and no, i cannot understand hawking's math. in fact, hawking is pretty much completely non-understandable to me with his more technical writings, eg "the nature of space and time" with penrose - penrose passages are clear, concise and understandable, while the chapters by hawking are fairly TOTALLY obtuse, confusing, and give me a headache trying to figure out quite what he is trying to say. how about you?
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