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Charged Black Holes

  1. May 3, 2012 #1
    Hi, I've been thinking about the concept of charged black holes. What I'm wondering is this; if light cannot escape a black hole, and photons are the bosons of the EM force, then is it meaningful to describe a black hole as having a 'charge'? If the photons cant escape the black hole, then no information about the state of charge of the system will be detectable to an outside observer right? I can imagine there being charged clouds around a black hole, but I don't see how the interior of an event horizon can be considered charged. Thoughts?
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  3. May 4, 2012 #2


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    Well, I may not be able to answer to the full extent, but since a black hole is a place of singularity, I don't think charge can exist beyond the event horizon.
  4. May 4, 2012 #3


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    Photons carry information about changes in the electromagnetic field, which includes changes in charge. If the matter forming the black hole was charged before the event horizon formed, then after the event horizon forms we still see that charge, because for us not to would require the information of the change in charge to escape the event horizon.

    Put another way, the electrostatic field surrounding the black hole is already there when the Black hole formed. After the Black hole forms, no information can leave the black hole to tell it to change.
  5. May 4, 2012 #4


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    But I am thinking how the mass about to become a black hole will be charged. That is it has to be a charged neutron star or charged core of a star, which I think may not be possible. Am I right??
  6. May 4, 2012 #5
    Charged black holes are probably not realistic in most astrophysical contexts, but it is a valid solution to the Einstein equation and has theoretical interest.
  7. May 6, 2012 #6
    I see where you're going with that but the thing is the part of the field that 'stopped' at the horizon would dissipate over time. See, the only way that the hole can be charged in the way you say, is if the horizon boundary holds all the information about the state of the charge of the collapsing object right at the moment it passed the boundary. Such a situation generates two scenarios:

    1) the boundary will hold the charge state of the collapsing matter at the boundary forever. this would be the equivalent of a super sized electron(or proton).

    2) the boundary dissipates the charge, in which case the black hole would only have a temporary(probably very short) charged lifetime.

    I think we might have observed the first possibility though, would be a pretty clear signal.
  8. May 7, 2012 #7
    But in this case a sufficiently charged black hole would stop being "black", right? Imaging you keep feeding it with electrons, shouldn't it reach a point where the electrons would scape the black hole?
  9. May 7, 2012 #8
    Yes in the first case that would be true, though adding a like charge subsequent to boundary formation would require that it have an energy density greater than the formation energy density of the boundary. Seeing as the energy would have been extraordinarily high at the time of formation, most external particles would probably not be able to enter the boundary.

    Expanding on this topic, one can say that ANY information about the collapsing system would have to be held at the boundary, foregoing the influence of tachyons. This means for example that all the momentum energy information would have to be held at the boundary indefinitely, and so for more mass to enter the black hole subsequent to its formation it must have an extremely high momentum. Maybe this is why black holes don't just suck everything in and swallow the universe. Thoughts?
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