Sheldon is a string theorist, and string theory also predicts the existence of magnetic monopoles. So their discovery would provide a key piece of experimental evidence for this contender for a Grand Unified Theory meshing general relativity with quantum mechanics.
So where are these mysterious particles? Scientists thought they'd caught a glimpse on Valentine's Day in 1982, but that experimental result has never been reproduced.
There was tons of excitement in 1975 when a team announced it had detected a moving magnetic monopole in cosmic rays. Alas, that turned out not to be real, either. It was just a garden variety platinum nucleus pretending to be a monopole by cleverly decaying into osmium and tantalum. That platinum is such a trickster.
Physicists have succeeded in creating analogs of magnetic monopoles, however, in exotic materials known as "spin ice." Back in September 2009, a team of physicists took a single crystal of dysprosium titanate, chilled to sub-Kelvin temperatures (-270 degrees F), and used neutron scattering to create an image showing that, inside this crystal, the magnetic moments self-organized into a kind of "spin-spaghetti" that looked an awful lot like Dirac strings (see image above).