Coming Unstrung over Monopoles

Abstract

To many physicists, not to mention other people, magnetic monopoles seem as weird as the winged pigs of Lewis Carroll's original poem (...And why the sea is boiling hot/And whether pigs have wings). They are certainly from the other side of the looking glass -the mirror that reflects every electrical phenomenon into an equal and reciprocal magnetic one. The original reason for believing in magnetic monopoles is that monopolar electric charges exist. For reasons of symmetry it has seemed for a long time that magnetic ones should be discoverable, too. Lately physicists have an even more compelling reason: The existence of magnetic monopoles is demanded by the new Grand Unification Theories (GUTs) that physicists are developing in the hope of tying up all of physics into one comprehensive, connected theoretical package. The time had come to talk of monopoles and of many things about and connected to them. Not only was there the theoretical imperative, there was an experimental result too. In a deceptively simple experiment Blas Cabrera, a physicist at Stanford University in Palo Alto, Calif., had made a measurement that he considers a candidate event for a monopole (SN: 5/15/82, p. 323). This convergence of impulses brought about 90 physicists to a Magnetic Monopole Workshop at Racine, Wis., in a house called Wingspread, which is maintained as a conference center by the Johnson Foundation. There, under the subtle symmetries and balances of Frank Lloyd Wright's architecture, they discussed the symmetries, asymmetries and antisymmetries of monopoles. The main thing to emerge from three days of intense conversation is that there is no complete consensus about the details of what a monopole is nor about the best way or even possible ways to hunt them. For example, some speakers assumed that magnetic monopoles have electric charge as well as magnetic charge. Others assumed that they do not. Still others were vague on the point. Faced with the necessity of putting precise properties on such a never-never object so as to give experimenters phenomena to look for, some theorists seemed to grope a little. The whole question begins with a curious asymmetry of the observed, practical physics of the nineteenth century. Electric charges, the sources of electric fields, are objects. They are microscopic things that sit in space, emanating electric fields and exerting electric forces. They come in positive and negative varieties. Electric field lines, invented by Michael Faraday to describe the phenomenon graphically, start at a positive charge and end at a negative one, or vice versa. Ordinary observation finds no corresponding magnetic charge. Magnetic fields are generated by electric charges moving in circuits (or rotating on their axes). Magnetic field lines neither start nor end; they are closed loops threading the electric circuit. The convention of speaking of the north and south poles of a solid magnet is a concession to the hardness of such things as the ends of an iron bar. The field lines continue right through, and so would magnetically attracted objects if they could. Cutting a bar magnet in half yields two new magnets; the field-line loops reestablish themselves for each piece. There is no magnetic charge point at which they can stop. This basic asymmetry in an otherwise symmetric theory it makes magnetism derivative -was accepted by most physicists. If nature is not symmetrical, nature is not symmetrical, and that's all there is to it. There were some, however, who hankered after an independent status for magnetism and complete reciprocity with electricity. In the early 1930s physicist P A. M. Dirac found a better reason for the existence of independent magnetic charge than such aesthetic wishfulness: the quantization of electric charge. Studies of atomic physics had shown that electric charge is quantized, that is, all electric charges in nature were seen to be integral multiples of a basic unit, the charge of the electron (or of the proton, the difference is a matter of sign). Classic electromagnetics as it came from the hands of Faraday and James Clerk Maxwell, and even as modified by Albert Einstein, makes no provision for this quantization. Dirac found that if a magnetic monopole existed (it takes only one in the whole universe), the mathematical relationship between magnetic charge and electric charge quantizes both.