Magnetic monopole

Magnets exert forces on one another; similarly to electric charges, like poles will repel each other and unlike poles will attract. Experimentally, magnetic poles have so far been found only in inseparable north-south pairs. (If a magnet cut in half, one does not wind up with just a north pole and just a south pole; one winds up with two smaller magnets, each with its own north and south poles.)

An isolated magnetic pole is called a magnetic monopole; it has been theorized that such things might exist in the form of tiny particles similar to electrons or protons, forming from topological defects[?] in a similar manner to cosmic strings, but no such particles have ever been found. Many cosmological theories suggest that they may be so rare that only one would exist in the entire visible universe.

The existence of a magnetic monopole automatically leads to the conclusion that electric charge must be quantized (a result due to Paul Dirac).