Every magnet that has ever been broken reveals the same stubborn truth: cut a bar magnet in half, and you do not obtain an isolated north pole and an isolated south pole. You obtain two complete magnets, each with both poles intact.
Yet electricity obeys no such restriction. Positive and negative electric charges move freely through the world, unbound to their opposites.
In 1931, Paul Dirac asked what would follow if magnetic monopoles existed. The answer: a single monopole anywhere in the universe would explain why all charges are exact integer multiples of the electron's charge.
On 14 February 1982, Blas Cabrera's superconducting detector recorded a single event consistent with the passage of a Dirac monopole through the superconducting loop. The event was never repeated.
Grand unified theories predict that monopoles were produced copiously in the early universe. These primordial monopoles would be immensely massive. Alan Guth's theory of cosmic inflation was motivated in part by the need to dilute the monopole density.
The magnetic monopole remains the most beautiful prediction that nature has not yet confirmed. It stands at the intersection of quantum mechanics and classical field theory. To search for the monopole is to believe that the universe is symmetric.