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

Charged black hole

  1. Jun 17, 2008 #1

    ehj

    User Avatar

    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

    User Avatar

    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

    User Avatar
    Science Advisor

    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

    User Avatar

    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

    User Avatar
    Science Advisor

    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

    User Avatar

    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

    User Avatar

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Charged black hole
  1. Charged Black Holes (Replies: 7)

Loading...