A magnetic monopole is a (possibly hypothetical) elementary particle in the form of an isolated magnet with only one magnetic pole (a North pole without a South pole or vice versa). In other words, a particle with a 'magnetic charge' somewhat analogous to an electron with its 'electric charge'.

In 1931 Paul Dirac showed (in :Proc. Roy. Soc. A123, 60) that if even one single magnetic monopole existed in the universe (and the accepted form of Maxwell's equations is valid) then all electric charges (in the universe) would have to be quantised.

Note: We now know that all electric charges are quantised, but a magnetic monopole has never been observed in any experiment to date.

Further technical info. : here (arXiv.org)

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