Magnetic monopoles and a cheap detector

The letters from Ken Frankel and Christopher Harrison (Physics Today, June 2017, page 13 ) in response to Arttu Rajantie’s article on the history of searches for magnetic monopoles (Physics Today, October 2016, page 40 ) brought back a memory. After Blas Cabrera’s 1982 publication of a candidate monopole event detected with a superconducting loop, ¹ three groups ² —a University of Chicago, Fermilab, and University of Michigan collaboration; IBM; and Imperial College London—built Faraday induction detectors with larger areas. Using a coincidence technique of two gradiometer detectors in a nonzero but pinned magnetic field, Joe Incandela and coworkers showed that a likely explanation of the candidate event was a flux jump rather than the transit of a monopole. ³ I was invited by the organizers of the First Aspen Winter Conference to give a review talk of the hot though cryogenic topic. ⁴

I had been a graduate student at the University of California, Berkeley, and had great admiration for Luis Alvarez, so I sent him a draft asking for comments. I had leaned over backwards to give him credit for inventing the Faraday induction technique, as my looking in depth at monopole detection by ionization brought home that there was no way to calibrate the ionization detector, and hence a nondetection could never be definitive. Faraday detection, however, can be calibrated with a “pseudopole,” a very long but small-diameter, tightly wound, magnetic solenoid much akin to a Dirac string.

I was working at my desk when my phone rang, with a furious Luis on the other end. Without any introduction, he barked, “Henry, this was my idea, and I should be the first reference.” I was stunned, as I thought I had done him proud, but I managed to say, “Luis, the guys ahead of you are not to be sneezed at—Faraday, Maxwell, Dirac… .” Still angry, he said, “Yes, but who are these other guys?” Luis later sent a nice note praising the review, and all was well.

A brief addendum: Sunil Somalwar’s PhD thesis followed up on Incandela’s superconducting gradiometers by showing that using just copper wire and a field-effect transistor operating at liquid nitrogen temperature, one could build an inexpensive detector, capable of covering large areas and sensitive to a single Dirac charge. ⁵