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Black hole plus charge

  1. Jul 20, 2010 #1
    So black holes, nothing can escape.. not even light.

    Now let us imagine a black hole of arbritary size, and we throw a heape of charged particles (electrons say) into the black hole (negative net charge). Laws of physics state that charge cannot be created nor destroyed. So when this electron goes past the event horizon of the black hole it is gone to the universe(?) along with its' charge.. Black has now negative net charge.

    Now if an object has charge, then something of the same charge must be repled by it. So, just because we're curious, we bring another electron close to this black hole we're playing with. What happens next?

    For the black hole and the electron to know that they should be repled by each other information would have to travel from within’ the event horizon telling the electron,on the outside, that the black hole has a net negative charge. But, by the very definition of a black hole nothing can escape( no information can be transmitted from within' the black hole, so what ever carries the "information of charge" can't escape either(?) ).. So the electron on the outside (and indeed the rest of the universe) can no longer tell that the black hole has negative charge.

    Has the black hole destroyed the charge? Physics says no. If not, where has it gone? Does the electron get repled away by the black hole?

    Does the answer lie in the fact that the effect of gravity (space-time curvature) around a black hole just dominates the electromagnetic force between the black hole and the electron? But why should there be any electromagnetic force at all, the black hole cannot be observed as charged.

    Or does the answer lie in some physics i've not learnt yet, highly likely possibility.
    Can someone help me? I need some sleep haha

  2. jcsd
  3. Jul 20, 2010 #2


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    The black hole retains a charge for the same reason that it retains a gravitational field. Photons cannot escape the event horizon, and photons are what carry information about changes in the electromagnetic field.

    So imagine your electron falling into the black hole. The instant it crosses the event horizon, we lose contact with any further changes in its state. That includes the cessation of its electromagnetic field. The last information we can know about it is what came from it just before it crossed the event horizon. As far as the outside universe is concerned, it is as if the electron is stuck at the event horizon. (This is also true from a time dilation aspect. For an outside observer, time slows to a stop at the event horizon, and the electron never crosses it.)
  4. Jul 21, 2010 #3


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    All black holes are believed to possess, to some degree, 3 properties - mass, spin and charge. I wouldnt worry much about charge, no evidence to date suggests it is nearly as important than mass - the big player. Charge is probably pretty much like spin, which is pretty much irrelevant save for very near the event horizon - where spacetime distortion starts to play a major role.
  5. Jul 21, 2010 #4
    So we (the universe) observe no charged particles enter a black hole. All the "information of charge" or photons (the gauge boson of the electromagnetic force?) are trapped on the event horizon.

    So is the definition of the net charge of a black hole

    The net charge of mass trapped just outside the event horizon of the black hole?

    This would make more sense to me, as then it would not be destroyed and it would also have the potential to act upon other charges if it were not for the mass curvature of space-time making the time taken for it to act on other charges tend toward infinity. However I also understand that whether or not it makes sense to me is meaningless toward the truth of the matter.

    Even though you said it is insignificant Chronos, i'm still curious as I can't help but see the conservation of charge law be broken here.. Which can't be right.

    Interesting stuff though :)
    Last edited: Jul 21, 2010
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