B = g·r̂/r² radiating from point source; Dirac string carries return flux
Dirac (1931) showed a magnetic monopole is consistent with quantum mechanics if charge is quantized:
e·g = nħc/2. The field B = g·r̂/r² is identical to an electric Coulomb field. However, the vector potential
A is singular along a "Dirac string" — a semi-infinite solenoid carrying the return flux. The string is
unobservable (pure gauge singularity) if Dirac's quantization condition holds. Toggle string to see its effect.