Physics Colloquium

In 1931 Dirac introduced the MM in order to explain the quantization of the electric charge and obtained the basic equation eg=nhc/2 , where n is an integer. The existence of magnetic charges and of magnetic currents would symmetrize in form Maxwell's equations, but there would be a numerical asymmetry since the basic magnetic charge would be much larger than the basic electric charge. Though there was no prediction for the MM mass, it was clear that this mass would be considerably larger than that of the basic electric charge. From 1931 searches for "Classical Dirac Monopoles" were carried out at almost every new accelerator, first using simple setups and, more recently, using parts of large collider detectors.
Electric charge is naturally quantized in Grand Unified Theories (GUT) of the basic interactions; they imply the existence of GUT monopoles with calculable properties. GUT MMs would appear in the Early Universe at the phase transition corresponding to the breaking of the GUT group into subgroups, one of which is U(1). The monopole mass would be mM>1016 GeV: this is an enormous mass and these MMs cannot be produced at any man made accelerator, existing or conceivable. They may have been produced only in the first instants of our Universe and may be searched for as fossil particles in the penetrating cosmic radiation. Large underground detectors have been used for GUT MM searches.
Intermediate mass magnetic monopoles (IMMs) may have been produced in later phase transitions in the Early Universe, when a semi-simple gauge group yields a U(1) group. IMMs with mM=107-1012 GeV may be accelerated to relativistic velocities in one galactic magnetic field domain; very energetic IMMs could yield the highest energy cosmic rays. Searches have been made at high altitude labs.
A large number of theoretical and phenomenological questions about MMs may be asked: I shall try to answer some of them.