hypothetical particle with a magnetic charge, a property analogous to an electric charge. As implied by its name, the magnetic monopole consists of a single pole, as opposed to the dipole, which is comprised of two magnetic poles. As yet there is no evidence for the existence of magnetic monopoles, but they are interesting theoretically. In 1931 the English physicist P.A.M. Dirac proposed that the existence of even a single magnetic monopole in the universe would explain why electric charge comes only in multiples of the electron charge. Since the quantization of electric charge remains a great theoretical mystery, physicists have repeatedly renewed their search for monopoles whenever particle accelerators attain a new energy level or when a new source of matter is discovered. Lunar rock samples brought back by U.S. astronauts in 1969, for example, were extensively studied because it was thought that monopoles might be trapped in the surface material of the Moon. Research showed, however, that such was not the case.