  It is proven that if more than a single magnetic charge exists it is
impossible to define a proper quantum mechanical angular momentum operator for
an electrically charged particle in the field of the magnetic charges. Assuming
that quantum mechanics is correct we conclude that free magnetic charges (i.e.
magnetic charges with a Coulomb-like magnetic field) can not exist. The only
apparent way to avoid this conclusion is if magnetic charges do exist, they
must be permanently confined in monopole anti-monopole pairs, much in the same
way quarks are thought to be confined.
