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Charged black hole

  1. Jun 17, 2008 #1

    ehj

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    I was wondering about something.
    If a black hole has charge, would it be measureable? Does the electric field escape the black hole even when light cannot?
    Or if the field does not escape the black hole, how do we know that it's a charged black hole?
     
  2. jcsd
  3. Jun 17, 2008 #2
    Thats a really interesting question. We can definitely predict that black-holes have charge, based on conservation of charge - i.e. for that conservation to hold, BH's would have to have some net charge.
    I really don't think that electric fields can emanate from the interior of a black-hole (not only because E&M forces are conveyed by photons which can't escape; but also because those fields have mass - and would be pulled back in). But i'm curious to hear from others. Could/would E&M fields exist from the event horizon? - i.e. before particles pass the event horizon?
     
  4. Jun 17, 2008 #3
    Just because a black hole is charged doesn't mean that the charge resides within the event horizon. If it does then I don't see a problem with the electric field since the field is not moving and thinking of it as "escaping" seems erroneous to me.

    Pete
     
  5. Jun 17, 2008 #4

    ehj

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    Well if the charge isn't within the event horizon that would imply that the charge has been separated from the matter from which it originates? That sound's odd.

    Well I just wan't to know if there's any way that an observer would know if the black hole is charged or not, without knowing what went into it in the past.
     
  6. Jun 17, 2008 #5

    JesseM

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    Apparently in quantum field theory virtual photons (associated with electromagnetic forces) can escape the black hole even if measurable non-virtual photons (associated exclusively with electromagnetic waves) cannot--see the discussion from How does the gravity get out of the black hole?:
     
  7. Jun 17, 2008 #6

    ehj

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    So the EM field has mass, but is according to JesseM unaffected by gravity? Isn't that a contradiction? Do these two statements come from different theories, or maybe one of the statements doesn't come from any theory ;P?
     
    Last edited: Jun 17, 2008
  8. Jun 17, 2008 #7
    That wasn't what I meant. I said that a black hole can be charged but have the charges piled up just outside its event horizin. The matter is not seperated from the charge. The charged matter is located where the charge is. Think of a uniform charge density sitting just outside the event horizon like a bunch of electrons spread out just outside of the event horizon.
    Yes. Measure the electric field around the black hole and that will tell you whether the black hole is charged or not.

    Pete
     
  9. Jun 17, 2008 #8
    Since when? An EM field is both affected by gravity and can generate a gravitational field.

    Pete
     
  10. Jun 18, 2008 #9

    JesseM

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    The article I quoted didn't say virtual photons were "unaffected by gravity", just that they can escape the event horizon (the virtual particle FAQ mentions that when summing over all possible virtual particle paths, one includes FTL paths). Quantum field theory on a curved spacetime background does give different results than in flat spacetime, I believe.
     
  11. Jun 18, 2008 #10
    I imagine that any E-field or B-field lines originating inside the event horizon do not cross it, because they are drawn in space which is curved to infinity in that region.
     
  12. Jun 18, 2008 #11
    The spacetime curvature inside the event horizon is finite everywhere except the origin. Other infinities are coordinate infinities and do not represent a physical infinity.

    Pete
     
  13. Jun 18, 2008 #12

    ehj

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    I might not have been clear on this, but what I was interested in knowing was what would happen to the field if the matter the black hole is made of, the core of the black hole, is made up of charged matter (not purely). It is obvious that charge outside the event horizon would generate a measureable electric field, so I didn't think this was what you meant.
     
  14. Jun 18, 2008 #13
    It is my understanding that it is impossible for an outside observer to tell the difference. It was for that reason that I used that as an example.

    Pete
     
  15. Jun 18, 2008 #14

    ehj

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    ah :)
     
  16. Jun 18, 2008 #15
    Oh, you're right! Thanks for catching my mistake.
    In that case, how might the E-field lines best be drawn?
     
  17. Jun 19, 2008 #16
    i have a somewhat stupid question but how do E&M fields have mass if they are made up of photons and photons are considered to be massless or is it that photons have an indeterminant relativistic mass that is non zero. Can someone elaborate for me?
     
  18. Jun 19, 2008 #17
    The Kerr-Newman metric describes a charged black hole, in the sense that the electric flux through a surface surrounding the black hole is nonzero (i.e at large distances the electric field lines point radially away from the black hole). I'm not sure what the electric field looks like near the horizon.

    http://en.wikipedia.org/wiki/Kerr_metric

    Dave
     
  19. Jun 19, 2008 #18
    Photons have zero rest mass, and zero gravitational field.
    From E=mc2 we can find a equivalent mass of any photon.
    Since photons have energy, they have an equivalent mass.
    This equivalent mass is responsible for their momentum.

    Further reading:
    http://www.newton.dep.anl.gov/askasci/phy00/phy00332.htm
    http://en.wikipedia.org/wiki/Mass-energy_equivalence
     
    Last edited: Jun 19, 2008
  20. Jun 19, 2008 #19
    Photons do have mass, just not rest mass (and the mass is easily determinable depending on the frequency - as i recall).

    The concepts of fields and of the gauge bosons that convey the forces are 2 somewhat different things; its my understanding that they don't coexist in a single interpretation of what used to be "action-at-a-distance"... what i mean is, either you can look at it as fields, or you can look at it as photons - not positive about that - but i'm sure any observable (like mass) could be consolidated between the two.

    Fields have mass because they have to have momentum - i can't remember why/how to prove that (sorry). And this is consolidated with gauge because the photons conveying the forces also have mass.
     
  21. Jun 19, 2008 #20
    Black holes do have charge because the angular momentum, mass, and charge of any particles that are sucked in are retained by the black hole itself.
     
  22. Jun 19, 2008 #21
    That is incorrect. Light most certainly does generate a gravitational field. Take a look at the second link that you posted because it says
    Why would you believe otherwise??
    What you call an equivalent mass is what is known as inertial mass aka relativistic mass.
    When you state it that way it gives the impression that "equivalenc" mass is somehow different that the "real" mass (whatever "real" means)

    Notice that your second link contains the following comment.
    Before one addresses the question regarding whether a photon/light has mass or not you really must state what definition you are using. A photon has an inertial mass (since it has momentum) a passive gravitational mass (since it is affected by gravity) and an active gravitational mass (since it can generate a gravtiational field).

    Pete
     
    Last edited: Jun 19, 2008
  23. Jun 20, 2008 #22

    ehj

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    I still don't think I've found a satisfying answer as to why the EM field can escape from the interior of a black hole, passing the event horizon. Why is it your understanding that it is impossible for an outside observer to tell the difference whether the charge is located in the core of the black hole, or on the event horizon? The quantum explanation (what you linked) seems to be saying, in short, that "the EM field can pass the event horizon because the virtual particles conveying the force, can pass the event horizon" which doesn't really solve the problem, just introduces some new particles with peculiar properties.
     
  24. Jun 20, 2008 #23
    I don't understand your concern? You are almost speaking of the E-field as if it were in motion when you say something like why the EM field can escape. Why does that bother you and not the fact that the gravitational field itself can "escape" from the black hole?

    Pete
     
  25. Jun 20, 2008 #24

    ehj

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    The gravitational "field" is the curvature of spacetime and not an actual field like the EM field, that's how it "escapes" the black hole. My concern is the fact that from within the event horizon of a black hole there exists no path through spacetime leading out of the black hole - atleast not for anything with mass or energy. But then someone says that the EM field has a mass, and if nothing with mass can leave the black hole, then how can the charge me measured from the outside? The field having mass makes me think of it as consisting of something that can be attracted or bent by gravity - is this not correct?
     
  26. Jun 20, 2008 #25
    That is incorrect. The presence of a gravitational field does not require the presence of spacetime curvature. The term spacetime curvature is merely a term which refers to tidal forces. Its use does not mean that there is no field. In any case what makes you think that the spacetime outside the event horizon can know that there is mass inside the event horizon and "know" to curve spacetime?
    All that means is that there can be nothing which moves from the inside to the outside the event horizon. The electric field doesn't move nor is there a flow of enery associated with it.
    Yes. It does.
    Once again, there is nothing leaving the black hole. The field is static as is the distribution of matter. There is no flow of energy either into or out from the event horizon.
    Fields contain not only energy but also stress and tension. The tension acts to reduce the mass in certain instances. Perhaps this is why the matter of the field is not attracted by the field. In a certain sense the mass density of the field is zero, even though the energy density isn't. This is one of those less known facts about relativity.

    Pete
     
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