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Black hole question

  1. May 12, 2008 #1
    could u help me in the following question:
    Ilike to know does a blck hole distinguish between a charged or an unchared particle when that particle fall into the black hole? If yes, how?
  2. jcsd
  3. May 12, 2008 #2


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    A "black hole" is purely a gravity phenomenon. No, a black hole does not distinguish charges.
  4. May 12, 2008 #3

    I basically agree with what you are saying, but it should also be noted that a black hole can accumulate charge and the nature of the black hole would change if it only accumulated particles of one type of charge, eg electrons. We know this because there are black hole solutions for charged black holes that differ from uncharged ones. It is interesting that a charged particle that is crushed to zero volume still retains its charge property in the singularity. I think it would also retain its angular momentum (Kerr solution) which is interesting for a particle which has truly become a point particle in the singularity. So despite the "no hair" theorem, a blackhole can still have bad hair days due to static electricity :P
  5. May 12, 2008 #4
    in classical electromagnetic a charge can make energy. Now consider Howking radiation effect: if an anti-particle fall into the BH the volume of BH will reduce. But if that particle be a charged particle, this charge can make positive energy and increase the volume. Is that right?
  6. May 14, 2008 #5
    You'rre correct, the angular momentum and the charge are retained by the balck hole. THe mass is also retained, to be slowly leaked out in another form later, not in the costituent atoms that enters, so in effect, black holes are just universal recycle bins. =)
  7. May 17, 2008 #6
    Can this effect help us for underestanding the information paradox?
  8. May 17, 2008 #7


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    Hawking radiation

    Hi zarei! :smile:

    You're talking about Hawking radiation, in which a particle and anti-particle are simultaneously created just outside an event horizon, and one falls in and the other escapes, thereby reducing the mass of the black hole.

    But you're wrong about the distinction between the particle falling in or the anti-particle falling in.

    The result is the same, whichever of the two falls in.

    They have the same mass, not opposite mass! :smile:

    They only have opposite charge.
  9. May 17, 2008 #8

    The trouble is that Hawking suggests the black hole will lose mass and eventually evaporate away completely. That suggests that the addition of an antiparticle to a black hole somehow subtracts mass or energy from the black hole. The trouble with that is it is equally likey that the normal particle of the particle/anti particle pair will fall in and the nett effect of particle/anti particle formation just outside the event horizon on the black hole is zero. Add to that that anti particles have positive mass just like a normal particle and that photons created inside the black hole by anhilation of the antiparticles have energy that adds to the gravitational mass of the black hole (and can not escape) , it then becomes clear that mechanism proposed by Hawking for the evaporation of black holes is completely flawed. That is not to say that black holes do not evaporate, just that they do not evaporate by the method suggested by Hawking.
  10. May 17, 2008 #9
    One solution to the information paradox is that a true singularity of infinite density never actually forms and mass never penetrates beyond the event horizon. See the discussion and links in post~32 of this thread https://www.physicsforums.com/showthread.php?t=234292&page=3
  11. May 17, 2008 #10


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    kev, I'm pretty sure that Hawking knew that anti particles have positive mass. :rolleyes:
    No … Hawking's idea is that the particle-antiparticle pair is created just outside the event horizon …

    … something to do with the energy of the black hole being an uncertain amount, and just when it's a bit on the low side, an equally uncertain amount of vacuum energy "takes advantage", creates a pair, and one of them escapes.

    (… cue world war two music …)

    Everything happens outside … and one of the pair escapes! :smile:
  12. May 17, 2008 #11
    Yep. I've been reading around the subject a bit more. Apparently he meant virtual particle antiparticle pairs. Unlike real particles, virtual particles are allowed to have negative energy.

    I agree that the particle antiparticle pair is created outside the event horizon. I was referring to the photons that are normally created when a particle and an antiparticle anhilate. Since the negative energy virtual particle that falls into the black hole subtracts energy and mass from the black hole I can only assume the annihilation of a real particle by a virtual particle (inside the event horizon) does not result in photon production unlike when a real particle anhilates with a real antiparticle.

    Hawking states that the black hole evaporates away to nothing. Therefore you can not say it all happens outside the black hole. The process has to explain how how mass and energy is removed from inside the black hole.

    It seems that when a negative energy virtual particle falls into a black hole the reversed roles of time and space below the horizon allow the virtual particle to become a real particle and yet retain its negative energy/mass quality which is not normally allowed for real particles. Makes you wonder why real particles are not turned into virtual particles by the reversed roles of time and space below the the horizon?

    According to an explanation by Steve Carlip, when a positive energy virtual particle falls into the black hole and the negative energy virtual particle escapes, the negative particle ceases to exist after a short lifetime and the positive energy virtual particle that fell in does not become real. This implies some sort of communication between the negative and positive energy particles a bit like with entangled photons. It also begs the question as to how a positive energy virtual particle going away from the black hole gets the information that its negative twin below the event horizon has become real so that it too can become real and maintain the energy/mass/momentum of the universe. Entangled photons exhibit properties that require realism or local (light speed) interaction to be violated and maybe the interaction between a virtual particle pair is not only non local but has no problem interacting from inside an event horizon to outside an event horizon. Anyway, the full explanation by Steve Carlip who knows a lot more about virtual particles than I do is here: http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad

    Personally I am not convinced and tend to agree with John Baez when he said:
    How does this work? Well, you'll find Hawking radiation explained this way in a lot of "pop-science" treatments:

    Virtual particle pairs are constantly being created near the horizon of the black hole, as they are everywhere. Normally, they are created as a particle-antiparticle pair and they quickly annihilate each other. But near the horizon of a black hole, it's possible for one to fall in before the annihilation can happen, in which case the other one escapes as Hawking radiation.

    In fact this argument also does not correspond in any clear way to the actual computation. Or at least I've never seen how the standard computation can be transmuted into one involving virtual particles sneaking over the horizon, and in the last talk I was at on this it was emphasized that nobody has ever worked out a "local" description of Hawking radiation in terms of stuff like this happening at the horizon. I'd gladly be corrected by any experts out there... http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

    Now while it is reasonable to assume black holes should have entropy and temperature and thermodynamics suggest anything with a temperature should radiate, I find the proposal involving virtual particles to transport mass from inside the event horizon to outside clever but dubious. It still leaves the information loss problem and a Google search of the "trans-Planckian problem" will show there are other serious objections to how Hawking proposes black hole radiate.
    Last edited: May 17, 2008
  13. May 17, 2008 #12


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    Both cases are possible, the Hawking particle may also originate from inside the horizon.

    A derivation closer to this heuristic picture was presented by M. K. Parikh, F. Wilczek, "Hawking Radiation As Tunneling", Physical Review Letters, 85:24, 5042-5045 (2000).
  14. May 18, 2008 #13
    I like how you think.
    Don't believe everything you ead on the internet.
    The last statement "Both cases are possible, the Hawking particle may also originate from inside the horizon."


    I too have a problem with Hawking Radiation. First to really define a virtual particle as it relates to quantum mechanics. A virtual particle as it relates to quantum mecanics is a borrowing of energy mathematically to properly show the events that have occured. No one has or ever will see a virtual particle. The quantum mechanics that we know will be displaced by quantum gravity the problem is at the level of energy that we normally observe quantum interactions does not equal the energy density that we see at an event horizon. Hawking radiation is based on the heisneburg uncertainty principle af energy levels relative to a flat spacetime not those of a singularity. You are talking probabilities and I don't think Hawking really pushed at the energy levels as to what the real probabilities were. From the zero point energy aspect I would be inclined to think that all real particles created in the region near the event horizon would eventually be captured in that no additional energy input i.e. particle collisions that would keep the particle part of our observable universe.
  15. May 18, 2008 #14
    The problems with BH is that GR breaks down and goes to infinities where G exceeds C. Remember nothing exceeds C. Any theory no matter how good that at some point gives an infintie solution has failed at that point. A good example of defining what we know and great at telling us we don't know everything when we derive infinities.
    It should make us rethink what we know and Hawking radiation was one such attempt. He was going in the right direction and my instincts tell me that he was trying to make us think past the infinities. I think that Hawking knows he doesn't have all the answers and incites people that can think to think.
  16. May 19, 2008 #15
    Does (or can) G ever exceed C? Or is the case that G = C?
    If G can exceed C, then is this akin to Inflation Theory where the Universe is thought to have expanded fater than the speed of light? I was under the impression that the size of the Event Horizon is dependent on the point where the G of the BH equals C, meaning that G still cannot exceed C.

    On a related note, since the gravity of BH can obviously be felt by regions outside of its Event Horizon, wouldn't this imply that Gravitons cannot exist? If one assumes that nothing can travel faster than C, then the Gravitons, which should be emitted by matter, should not be allowed to escape from within the Event Horizon, unless the entire mass of the BH is contained on the fringe of the Event Horizon itself (even that has problems since the mass residing on the other side of Event Horizon should not be allowed to penetrate the BH to make itself felt -- again due to FTL violation). If so, I would assume that Gravity truly is a feature of Space/Time as opposed to being explainable via the quantum particle Graviton.

    I'm just musing, so please point out all the faults in my logic.
  17. May 20, 2008 #16
    If you think about it.......... The event horizon does not represent the surface of a BH except where C=GM if C<GM the surface area of the matter contained in a bh while the event horizon follows Bekensteins BH entropy formula. G exceeds C and that is how a BH is created. Assuming that then you would be correct regarding gravitons would violate the c speed limit but again think! As a BH becomes more massive the distance between the event horizon and the actual surface of a bh increases. Below the event horizon you would have the inverse of e=mc2 where energy is converted to matter effectively cooling the matter to a bose einstein condensate analog. With QED it would be a long process but with QCD relating to an initial collapse to a QG plasma and that quarks and anti quarks don't self anniliate you would have the equivalent of BH rain. Cool and an equivalent atmoshere for BH.

    Below the event horizon to the surface of the matter contained in a BH the acceleration increases on the square until the accelaration =c2 where it should exceed the photon degeneracy pressure and convert to E.

    I think I am a heretic with that statement! Alas my mother called me a savant but my wife just calls me an idiot!
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