Magnetic monopol-y: has it passed Go?

Magnetic monopl-y: Has it passed Go?

Finding a magnetic monopole seems more difficult than getting rich enough to build a hotel on the Boardwalk. Since Feb. 14, 1982, there has been on record one event that seems to have been the passage of a magnetic monopole -- a single magnetic north or south pole -- recorded by Blas Cabrera of Stanford University. Now there is a second "candidate event" recorded at 7:06 a.m., British Summer Time, Aug. 11, 1985, in the Blackett Laboratory of Imperial College in London. A.D. Caplin and M. Koratzinos of Imperial College, M. Hardiman of the University of Sussex in Brighton and J.C. Schouten of Oxford Instruments Ltd. in Oxford present their candidate in the May 22 NATURE.

Magnetic monopoles, whose existence was first postulated by P.A.M. Dirac more than 50 years ago, would round off the symmetry between electricity and magnetism. Monopoles should have been produced in the early stages of the development of the cosmos. Exactly how many remain depends on the details of what happened in those early moments (SN: 11/27/82, p. 348; 12/4/82, p. 364).

The detectors used at Stanford and London are loops of superconducting metal through which electric currents flow. Magnetic flux -- that is, a magnetic field -- is trapped inside these rings and persists at a constant level for a very long time. If a magnetic monopole passes through one of these rings, it will increase the flux in a sudden, stepwise way, and the increase will likewise persist.

Cabrera's original experiment had been turned on only a short time when he recorded his one event. That seemed to indicate that magnetic monopoles were more abundant than they ought to be, in view of the continuing existence of the magnetic field of our galaxy. The galactic field would be destroyed by a flux above a certain abundance.

As a unit of exposure for these detectors, the Imperial College experimenters use the area of the superconducting ring multiplied by the amount of time it has been turned on. They call the exposure of Cabrera's original experiment before his event "1 cabrera." On that basis, the total exposure of their experiment is 400 cabreras and that of the world's other three such experiments is 800 cabreras. Even with that much exposure, two events still indicate a monopole abundance 200 times too large for the survival of the galactic field -- suggesting either that previous calculations are wrong or that these are not magnetic monopoles.