[grey_whiskers]

Dirac played around until he got something interesting, then was able to predict some spectral deviations that non-relativistic QM couldn't predict...and he also got the prediction of anti-matter, too, which was a neat plus (er, minus?).

IIRC, the Dirac equation allowed for the existence of magnetic monopoles as well.

IIRC (again), Van Vleck and others were insistent that there were no such beasts...

[snarks_when_bored]

Magnetic monopoles aren't predicted by the Dirac equation, g_w. But a couple of years after he published his equation, Dirac did point out that the existence of magnetic monopoles could account for the quantization of electric charge. To date, no magnetic monopoles have been discovered.

[Doctor Stochastic]

I think that Maxwell's equations allow for magnetic monopols to exist. If "all that is not forbidden is mandatory" then where are they?