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Hawking radiation, Is it possible?

  1. Jan 25, 2006 #1
    According Hawking radiation theory the black holes eventually will evaporate. This process gives energy. The process of black hole creation also gives energy. The accelerating particles emit x-rays while falling into a black hole.

    Now we can construct perpetuum mobile. We create a black hole, getting energy. We wait until it evaporates, also getting energy. We get evaporated matter and start from the beginning.

    Where does all that energy come from? :eek:
    Is the “Conservation of energy” law violated? :surprised
     
  2. jcsd
  3. Jan 25, 2006 #2

    Garth

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    The evaporated BH does not spew out matter but radiation.

    There are just two sources of energy in your scenario, the gravitational binding energy of the mass as it forms into a BH and the conversion of that mass into energy via Hawking radiation and the final evaporation.

    After that game over - nothing else.
    No perpetual motion machine, sorry.

    Garth
     
  4. Jan 25, 2006 #3
    All you're talking about is the conversion of matter to energy. There is no conservation violated, because matter is lost in the evaporation of the black hole--that is where the energy of hawking radiation comes from.

    So no.
     
  5. Jan 25, 2006 #4

    George Jones

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    Hawking radiation consists of both matter and radiation. As the black hole beomes very small, matter is increasingly important.

    Regards,
    George
     
  6. Jan 25, 2006 #5
    OK, this sounds realistic, if we’re speaking about the energy. Now I have a problem to understand the process that creates photons out of quarks and leptons. And when this is supposed to happen? Immediately after crossing the event horizon or after some time, perhaps after hitting the singularity? :bugeye:
     
  7. Jan 25, 2006 #6

    In normal space there are constantly quantum fluctuations in both the electromagnetic fields, and the ambient energy density. These fluctuations at any given time cancel each other out, more or less. We can visualize these fluctuations as the creation of virtual particle pairs-a particle, and an anti particle-that seperate, and then recombine, annihilating each other. In the presence of a black hole, it is possible that the anti particle may fall into the event horizon, reducing the mass of the black hole, while the particle is now free to escape (since it was always outside the event horizon), and decay into other particles.

    This is my hand-waving explanation, as I don't know enough QM at that level to explain in any further detail.
     
  8. Jan 25, 2006 #7

    Chronos

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    I agree with George that Hawking radiation models include a matter component. I think he is also right about it becoming more important as black holes shrink, but, am admittedly a bit fuzzy about the details.
     
  9. Jan 26, 2006 #8

    Garth

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    I stand corrected :blushing: However the overall effect is to convert mass into energy.

    Edit: Afterthought: Are not both particles and anti-particles radiated away from the BH? And if so would not these then self-annihilate leaving just radiation? However, I do understand that neutrinos are also radiated away.

    Garth
     
    Last edited: Jan 26, 2006
  10. Jan 26, 2006 #9
    This explanation is also problematic. Let’s suppose that virtual pair “neutron – antineutron” is created. Antineutron falls in the black hole and annihilates with neutron found there. This process should yield energy equivalent of twice the original black hole neutron mass. The other virtual neutron is outside the black hole. What is its speed? Obviously it has to move faster enough to overcome the gravity field of the black hole. This speed has to be near c, because otherwise it will eventually fall back into the black hole. So, we have 2 n mass equivalent inside the black hole and 1+ n mass equivalent outside the black hole. All we had before the process took place was 1 n inside the black hole.

    :confused: :confused: :confused:
     
  11. Jan 26, 2006 #10

    i think the particle-antiparticle pair that gets created out of vacuum has one of them at a positive energy and the other at negative so that the net energy is zero.
    the antiparticle which falls into the black hole has the negative energy, and if it will encounter a particle with positive enrgy they will just cancel out with no photons emmited, so your antimatter would actually only take the mass while anihilating inside the black hole, and the net result is just a reduction of the BH's mass and a particle emmited away from the event horizon.

    what i dont understand is why is it always the negative energy one that is falling into the BH?
    the person who lectured on this radiation only said that the virtual particles are in a state of superposition, and when one of them falls into the BH it must be the negative enrgy one... but thats no explanation.... i dont think he knows the answer.
     
    Last edited: Jan 26, 2006
  12. Jan 26, 2006 #11

    George Jones

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    .
    Steve Carlip, a physicist at the University of California Davis, has written the best non-technical explanation that I have seen.

    Regards,
    George
     
  13. Jan 26, 2006 #12

    As I said, it was a handwaving explanation, based on what I've been told. You can't do much with handwaving explanations with this kind of thing. And my knowledge of QM is not firm enough (basic wave mechanics only, definitely not QFT) to give any more detail.
     
  14. Jan 27, 2006 #13
    well, i guess id have to learn more GR and QED to understand it without just hearing - negative energy cant exist so thats the one whod fall into the black hole, that explanation doesnt satisfy me.

    i got a friend who knows this stuff, and he said its just a fency mathematical trick, he wouldnt explain though, i dont have the propper knowledge yet.
     
  15. Jan 27, 2006 #14
    Hiya

    Hi guys,

    Pardon me for being dense but as I understand it, energy and matter never go into the black hole. If they did then both would be destroyed by the singularity, that creates a paradox as 'matter & energy are not created or destroyed they are mearly transformed'.

    This means that all matter & energy stay at the event horizon until the black hole evaporates...When this happens all the information that's been stored at the event horizon is emitted back into space.

    Please correct me if Im wrong...It's been years since I dabbled last.

    Frizz
     
  16. Jan 27, 2006 #15
    Thats absolutely not true. Matter and energy falls into a black hole and joins the singularity and it is NOT destroyed. If this was true, and the mass/energy of a black hole was at the horizon, there would be no singularity as such at the center of the black hole, because from classical mechanics we know that inside a hallow sphere the gravitational field is zero. So this theory in itself creates a logical paradox.
     
  17. Jan 27, 2006 #16
    Ooopz

    Just made a terrible error. I just looked it up, you're quite right on most things...Only thing I would say is that no-one really knows what happens inside a black hole as the laws of physics break down. At the moment it's all speculation...That is until someone has the guts to venture inside:eek:

    Not me though...:smile:

    Thanks for correcting me.
     
  18. Jan 27, 2006 #17

    SpaceTiger

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    We can't see inside a black hole, but that doesn't mean the laws of physics break down. We expect general relativity to be valid inside black holes (except perhaps in very strong fields), but I don't think we could ever make an observation to prove or disprove our models of the interior of a black hole.


    But even then, only that person would know (if they survived). They couldn't communicate their observations to the outside world.
     
  19. Jan 27, 2006 #18
    Yes I quite agree. Although there was a scientist some years ago (Cant remember his name) who sent a signal 4.2 times the speed of light accross a small distance, I dont know if it was verified by other scientists...There are also jet streams emitted from some black holes that exceed the velocity of light...If this is true then communication would be possible inside a black hole, assuming you survived the tidal forces and being turned into spaghetti on the way through.

    Frizz
     
  20. Jan 27, 2006 #19

    SpaceTiger

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    You're probably talking about "superluminal motion", an effect in extragalactic radio jets that makes it appear as if they have transverse velocities faster than the speed of light. In reality, it's just another relativistic effect -- the jets aren't actually moving faster than the speed of light.
     
  21. Jan 27, 2006 #20
    Yes, that's what Im talking about...For example, in M87 galaxy, the hubble space telescope detected the jets moving at 6 times the velocity of light.

    You're saying it's a reletavistic illusion yes??? Can you explain how this works?
     
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