Dirac (1931): If a magnetic monopole
exists with charge g, then electric
charges must satisfy:
eg = n·ℏc/2, n ∈ ℤ
This explains charge quantization!
Field: B = g/r² (radial)
Dirac string: unobservable
solenoid carrying flux back
(gauge artifact, not physical)
∇·B = 4πg·δ³(r) (vs 0 in Maxwell)
∮ B·dA = 4πg