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B Adding a huge electric charge to a black hole?

  1. Apr 12, 2017 #1
    I wonder, what would happen if you started dropping lots of electrons into an existing black hole which otherwise doesn't accrete any new matter?

    The charge of the black hole would build up much faster than the mass. Wouldn't its charge become so high at one point that further electrons would no longer fall through the event horizon due to the electromagnetic repulsion?

    Can this process be used to even "deconstruct" a black hole, by charging it to a point where it would simply explode?
     
  2. jcsd
  3. Apr 12, 2017 #2

    Grinkle

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    Since its a B thread, I will hazard a response. Electric force is transmitted by particles. No particles can escape the black hole, so it does not accumulate negative electric charge that is felt outside the event horizon as you are supposing.
     
  4. Apr 12, 2017 #3

    Drakkith

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    From the perspective of an outside observer, the electrons never pass the event horizon because of time dilation (or something like that. It's complicated once you factor in Hawking radiation and evaporating black holes). This means that their charge does build up over time.

    I believe that's correct.

    It cannot.
     
  5. Apr 12, 2017 #4

    Grinkle

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    I retract my response.

    @Drakkith

    Since this is true -

    for what observer does this statement apply?

     
  6. Apr 12, 2017 #5

    Drakkith

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    Ignoring evaporation, any observer that never passes the event horizon. For example, an observer many light-years away, or even one hovering several radii outside the event horizon. But it wouldn't be the same for an in-falling observer, one who is in free fall towards the event horizon. As far as I know at least. I'm not an expert on black holes so I may have some details wrong.
     
  7. Apr 14, 2017 #6
    "dropping" an electron would not work if the repulsion from charge exceeds the attraction from gravity. You would have to accelerate the electrons and shoot them at the black hole. The accelerated electrons have more energy than a non-accelerated electron. Adding energy to a black hole is equivalent to adding mass to the black hole.

    I believe time dilation prevents them from reaching the singularity. Since we are pumping "enough" energy into the black hole the event horizon will expand and electrons will be inside of it.
     
  8. Apr 14, 2017 #7

    Drakkith

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    Not as far as I understand things. To the best of my knowledge time dilation and redshift approach infinity as you near the event horizon, as seen from the reference frame of an observer who is well away from the black hole.
     
  9. Apr 14, 2017 #8
    Take for example a type II supernova event. It looked like there was a big star there yesterday, now it looks like there is a black hole. Yesterday there was no event horizon, now there is one and it is outside of/around the particles. Light emitted by a former-inner-star particle will never get here. A photon emitted by a particle slightly outside the event horizon today will not get here for a long time and it will be red shifted if it does. The core collapse did happen this morning. Background radiation orbited a black hole and is on its way here. Yesterday background radiation bent but it could not orbit. There is "stuff" inside an event horizon today and that "stuff" was not inside an event horizon yesterday.
     
  10. Apr 14, 2017 #9

    Drakkith

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    According to Paul Townsend there is an infinite amount of time dilation at the horizon, not the singularity. From page 125 at this link: http://xxx.lanl.gov/pdf/gr-qc/9707012v1

     
  11. Apr 14, 2017 #10
    Great link. thanks.
    I do not see a contradiction.
    Suppose we threw a very bright clock into a black hole. We knew where the black hole was and we knew the velocity and acceleration on the clock. We tossed at 12:00 and it should be in the hole at 12:10. Later (hours, or years) we look through a powerful telescope and detect photons radiating from just outside the event horizon. The clock display will show less than 12:10. These photons were emitted before the clock went into the hole. This observation does not change the location of the clock. Instead this is evidence that the clock was on the way in. It has arrived there. The clock is inside the event horizon.
     
  12. Apr 14, 2017 #11

    Drakkith

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    I don't think so. If there is indeed an infinite amount of time dilation at the event horizon, then the clock is not inside the event horizon yet. And never will be from our frame of reference.
     
  13. Apr 15, 2017 #12
    Things in space tend to be electrically neutral and there is a good reason for that. If you dumped electrons in a black hole to give it a large negative charge, you might be able to repel other electrons but positively charged particles (e.g. protons) would experience an even greater attractive force towards the black hole and the hole would quickly become electrically neutral as it pulled in surrounding positively charged matter.
     
  14. Apr 16, 2017 #13
    How much charge can a Nordström black hole have?
     
  15. Apr 16, 2017 #14
    In many paper the authors say that max Q=M, however they don't bother to specify units. When I asked about it earlier, @PeterDonis said to use Geometrized Units. Unless I made a mistake, a solar mass black hole would hold some 1.15 10^17 C.
     
  16. Apr 16, 2017 #15
    How does the mass/charge ratio of a Nordström hole compare against an electron?
     
  17. Apr 16, 2017 #16
    That is something you can find out.
     
  18. Apr 16, 2017 #17

    phinds

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    That assumes that there IS "surrounding positively charged matter". Why would you make such an assumption?
     
  19. Apr 16, 2017 #18
    A mole of electrons weighs about 0,55 mg, and has a charge of 96 485 coulombs.
    The amount of 1,15*1017 C should be something like 1,2*1012 moles of electrons... weighing a mere 650 tons.
     
  20. Apr 21, 2017 at 7:33 AM #19
    Actually nothing would happen at all.
    First you need to consider the origin of an electron and how it gets its mass, where it gets its mass from. Also it is important to remember that mass equal energy.
    So electron gets its mass from the Higgs field and for that to happen something called "spontaneous symmetry" has to break down. However we know that it can be restored in high energy fields which means that once the electron passes through the Event Horizon and heads down towards the singularity it loses its mass and essentially seizes to exist as an electron....there is no charge transfer at all....
     
  21. Apr 21, 2017 at 8:04 AM #20

    phinds

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    Then how do you explain that all the black hole experts say that charge is one of the fundamental properties of a black hole?
     
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