Do semiclassical GR and charge quantisation imply magnetic monopoles?
Answers
Yes, magnetic monopoles are theoretically expected to exist and to be quantized due to the electro-magnetic duality, which is an example of a strong-weak or S-duality....
The relationship between magnetic an electric charges, discovered by Dirac, is me=2πnme=2πn....
This means that if electric charges exist, magnetic charges m are quantized in units of 2π/e2π/e.....
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Assuming charge quantisation and semiclassical gravity, would the absence of magnetically charged black holes lead to a violation of locality, or some other inconsistency? If so, how? (I am not asking about how magnetic monopoles explain charge quantisation.) I am looking to fill in the blanks in the claims found in this answer to "Is it possible to separate the poles of a magnet?" according to which "magnetic monopoles certainly exist [assuming] charge is quantized. This follows only from the semiclassical behavior of black hole decay, and so does not require unknown physics ... you can't forbid them, at least not for macroscopically sized black holes, without ruining the theory [of General Relativity]." Related claims are found in this answer to "Why do physicists believe that there exist magnetic monopoles?" according to which, if a black hole cannot carry magnetic charge, it would result in a "violation of the laws of locality". (This refers to "any consistent theory of quantum gravity", which may or may not require more assumptions than the other claim.)