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A Charged black holes and charge balance in galaxies

  1. Jun 19, 2016 #1

    There seems to be a supposition in astronomy that charged black holes are uncommon. Is this assertion well supported?

    According to a paper I found by Briet and Hobill, if you look at gravitational lensing alone charged black holes look just like regular black holes, but with a mass error. It seems likely that the gravitational dynamics of the whole galaxy around a supermassive black hole will also show a mass error if there is a charge gradient there.

    There is an equilibrium argument for charge balance rooted in statistical mechanics, but that argument is tuned for gas densities like the ones we have on the surface of the Earth. Does this argument not break down as the mean free path becomes excessively long?

    Is it true that we have clear evidence for a charge imbalance of some sort near black holes, in the form of charged jets of ionized gas that have been observed?

    A galaxy-scale charge gradient might be created and maintained by some unappreciated dynamics without resorting to any new physical laws or particles. For example there could be an imbalanced ratio of protons vs electrons (for example) taken up as they pass near a black hole event horizon, either alone or in pairs. Or any perturbation of the charge balance of a supermassive black hole at a galaxy's center might tend to increase to some nonzero value based on changing the orbital dynamics for particles with different charge/mass ratios.

    Without trying to assert that charge disequilibrium in galaxies exists, I think it should not be excluded without clear theoretical justification. The reason for this claim is that charge equilibrium is not a fundamental law, and on both large and small scales charge disequilibrium has profound effects in nature. Since the 1960s very complex explanations have been put forward for the mass error in classical continuum models for galactic structure. To feel confident in any new explanation for these mass deficits we should first account for any unmodeled effects of electrical and magnetic effects.

    Could someone knowledgeable about the theory of galactic structure formation please help me by providing a reference to a clear justification for charge balance on the galactic scale? Alternatively, if no such justification has been made I would love to hear that opinion from someone in the field.

    Last edited: Jun 19, 2016
  2. jcsd
  3. Jun 20, 2016 #2

    Jonathan Scott

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    Gold Member

    I can't give an expert answer, but I can repeat what I've heard about the black hole case.

    Electromagnetic forces are naturally many orders of magnitude stronger than gravitational ones. On that basis, I believe it is normally simply assumed that any significant charge excess of either sign on a black hole would tend to be strongly self-limiting because electrostatic effects would selectively repel the same charge and attract the opposite charge. This is also the basis of the assumption that a black hole could not have a significant magnetic field, as the "no-hair" theorem means that any magnetic field can only be due to the rotation of the overall charge.

    Similar arguments about electrostatic charge apply to other cases, in that even if there are effects which create a charge imbalance, the imbalance tends to be limited by the way in which it attracts or repels other charged particles.

    But of course "significant" is undefined in this context.
  4. Jun 20, 2016 #3
    Thanks for your reply Jonathan.

    I guess that like electric charges oppose gravity at the rate of 8.61E-11 C/kg. That is, if this charge density is present then the gravitational force is exactly cancelled out at any distance. That is about a half million electrons per gram. So in one sense the electrostatic force is strong, but in another it is just the same as gravity, since they both decay as 1/r^2. That seems to be what you are saying as well, and I'm just repeating this so we know we are in agreement.

    I also agree with you that there is a tendency of a system to restore itself to a balance of charges, and that the question is how quickly this occurs. If we have some systematic source of imbalance and some process of restoring balance then we can reasonably expect that there will be a steady state and/or an oscillation about some nonzero level of imbalance.

    The mass error for galactic structure formation became a big deal in physics at least by the mid 1960s. I trust that the process of theoretical physics would have have sorted any electric field effects in galactic structure formation by now, except for one fact. It's only been relatively recently that we understood that there is a super-massive black hole at the center of every galaxy. Black holes look like a possible source of systematic disturbance to the charge balance of the surrounding space. We have a little bit of evidence in the form of strong magnetic fields around black holes. And the extremely low density, with massive particles and long mean path between collisions in galaxies makes it seem that once a charge imbalance happens it might take a long time to restore. Then we have the mass deficit problem that has been open for so long and the impossible rate of rotation for the arms of spiral galaxies. This is why I'd like to know if the potential effect of electric fields have been taken into account in the theory of galactic structure formation.

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