Electricity and magnetism appear everywhere in the modern world and form the basis of most of our technology. Therefore, it would be natural to assume that they are already fully understood and no longer pose unanswered fundamental physics questions. Indeed, for most practical purposes they are perfectly well described by classical electrodynamics, as formulated by James Clerk Maxwell in 1864. At a deeper level, a consistent quantum mechanical account is given by quantum electrodynamics, part of the standard model of particle physics. The theory works so well that it predicts the magnetic dipole moment of the electron accurately to 10 significant figures. Nevertheless, there is still an elementary aspect of electromagnetism that we do not understand: the question of magnetic monopoles.1
That magnets always have two poles—north and south—seems like an obvious empirical fact. Yet we do not know any theoretical reason why magnetic monopoles, magnets with a single north or south pole, could not exist. Are we still missing some crucial fundamental aspect of the theory? Or do magnetic monopoles exist and we simply have not managed to find them yet?