Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Hawking Radiation: violate laws of conservation?

  1. Nov 19, 2005 #1
    I was thinking, when virtual particles come into exsistance and then one particle go into a blackhole and the other gets emmited as hawking radiation and so the particles are no longer virtual and become "real" particles. Does this violate the laws of thermodynamics, and so is the mass/energy of the entire universe growing over time because of hawking radiation?
  2. jcsd
  3. Nov 19, 2005 #2


    User Avatar

    Don't virtual particles last for an infinitesimally small amount of time?
  4. Nov 20, 2005 #3
    The energy is conserved because the black hole loses mass equal to the particle that escaped. The anti-particle that gets pulled into the black hole annihilates it's opposite particle inside the black hole decreasing the mass there.

    Thats how I understand it, but I'm far from an expert.
  5. Nov 20, 2005 #4
    so does that mean when particles fall into a blackhole they keep their identity as what ever particle they were and wait for an antiparticle? When the particles annihilate inside the blackhole how can the energy escape from the annihilation from within the black hole?
    Last edited: Nov 20, 2005
  6. Nov 20, 2005 #5
    It can't. The energy was released in the form of the real particle that escaped from the pair creation. Any annihilation taking place inside the black hole is un-observable as that energy cannot escape the event horizon.
  7. Nov 20, 2005 #6

    Claude Bile

    User Avatar
    Science Advisor

    On the contrary, without Hawking radiation, the second law of thermodynamics would be violated.

    Consider a scenario whereby a disordered object falls into a black hole. As soon as the object passes the event horizon, the disorder is lost to the universe, causing entropy to decrease (a violation of the 2nd law of thermodynamics). Hawking's theory ensures that the disorder lost when an object falls into a black hole is returned to the universe via Hawking radiation, thus preserving the second law of thermodynamics.

    This is how I understand it from what he has written in some of his literature.

  8. Nov 21, 2005 #7
    but doesn't a black hole have the maximun entropy physically possible
  9. Nov 21, 2005 #8


    User Avatar
    Science Advisor
    Homework Helper

    I think the best answer is that that particular explanation for Hawking radiation involves 'local' violations of the conservation laws. I believe that there actually other explanations for black hole radiation, but I'm not an expert in the field.
    The Uncertainty Principle indicates (among other things) that the so-called conservation laws can be bent locally, for example, that it's possible to have a particle pop into existance spontaneously as long as an equal amount of mass, charge, and spin pops out of existance a short time later. I'm not going to get into interpretation issues, but you might think of it as a particle - un-particle pair, or as a particle skipping forward or backward in time. Now, it's concievable that the vacuum around a black hole borrows some mass from the vacuum, and then ends up taking the mass out of the black hole.
    And, it turns out that according to the best theories we have, there will be a net flow of mass or energy out of the black hole due to this phenomenon. I don't know nearly enough to explain the particulars of the calculation, but it's out there on a variety of web sites.
  10. Nov 24, 2005 #9

    Claude Bile

    User Avatar
    Science Advisor

    It was not the entropy of the black hole that was in question, it was the entropy of the rest of the universe.

  11. Nov 24, 2005 #10
    but won't the entropy of the entire universe including the blackhole have a net rise in entropy when things do fall in
  12. Nov 27, 2005 #11

    Claude Bile

    User Avatar
    Science Advisor

    Yes, but the relationship between the properties of the black hole and its entropy was not well understood before Hawking. The theory of Hawking radiation went some way to resolving the original conundrum.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook