MAGNETIC MONOPOLES

Before 1974, speculations concerning the existence of pure magnetic charges had been diverse. Many experimental searches had been carried out, and up to today, no single magnetic charge has ever been isolated, apart from some indirect evidence [1] that slowly evaporated when it appeared to be impossible to reproduce it. The theoretical situation was also somewhat confused. Dirac [2] had written a brilliant paper on the subject, showing the Dirac quantization rule. But then Julian Schwinger came with an argument that a factor 2 should be added to this quantization rule — this would be falsified by our later results; presently, we know that if the Dirac quantum is odd, there will be a violation of the spin-statistics addition theorem: fermions can be made out of bosons. Many researchers tried to devise a perturbative scheme to handle monopoles in field theory — in vain, because, if the electric charge unit e is small enough for perturbation theory to make sense, then the magnetic charge unit g = 2πn/e will be far too big. In particular, the use of a separate ‘dual vector potential’ for magnetic charges is doomed to lead to inconsistencies if also electric charges occur. Either g or e is small, but never both…