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What is the electrical charge of a black hole‑ Can they be neutral?

I understand the Reissner-Nordström solution for charged black holes, but every review states astrophysical black holes are neutral. Why is this assumption so robust? Couldn't a black hole form from a highly charged progenitor, or preferentially accrete one charge? What physical processes act as a "charge sink" to neutralize it so effectively?

 

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By Nitin Answered 1 year ago

The neutrality assumption is extremely robust due to the highly efficient neutralizing effect of the surrounding plasma. Even if a black hole formed with a slight charge, it would immediately begin attracting opposite charges from the interstellar medium or its own accretion disk. This selective accretion, or the Schwinger mechanism of pair production near the horizon, acts as a powerful regulator. I've seen simulations where any initial charge is neutralized on timescales far shorter than the black hole's lifetime. While a charged black hole is a beautiful mathematical object, sustaining a significant charge against a universe full of free charges is practically impossible.

 

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