system
Given the following post, respond as if you're commenting on Reddit:
human
How would our understanding of physics (or more specifically E&M) change if we found a magnetic monopole?
gpt
EM, not much. EM is already prepared to accommodate for magnetic charges - the fact that there are none around is just a fact that electromagnetism cannot explain, neither at the classical nor the quantum level. In fact if you take a look at any electrodynamics textbook, chances are it has a section on magnetic charges, dyons, and their dynamics - that's because the *theory* in itself, as an abstract mathematical structure, has no intrinsic reason to restrict itself to purely electric currents - it's just an empirical fact.

Monopoles in the real world instead would come from a theory of grand unification, a theory of a single unified interaction above the scale of 10^(16) GeV (much, much, much higher than LHC) which under that scale is broken to the three standard model forces: colour, weak isospin and weak hypercharge, a combination of which is electromagnetism. Monopoles are produced during the symmetry breaking as the Universe cools from above that temperature to below through a process known as the Kibble-Zurek mechanism.

There's many different GUTs, which have different predictions for the existence and properties of monopoles. Finding one would give very useful information on what GUT best describes the real world.

Now essentially all GUTs predict monopoles (of some kind) and the KZ mechanism predicts they should be produced in abundance. So why are none observed? Well, either there is a flaw in the above reasoning, or something diluted them immensely after their production, so there's few enough of them right now that we have no hope of finding one soon. The latter view seems to be the most popular with the diluter being cosmic inflation.