The Dirac monopole

While preparing some educational material for the MoEDAL experiment, I found I needed to illustrate what Dirac's hypothesised magnetic monopole looks like. The monopole has magnetic charge, and so one can draw magnetic field lines emerging from the monopole just like you can for an isolated electric charge. Unable to find a good quality, rights-free image that would do the job, and inspired by Figure 2 of (Preskill, 1984), I made the figure below:

A schematic representation of Dirac’s magnetic monopole, inspired by Figure 2 of (Preskill, 1984).

Normally when you draw magnetic field lines you can only draw complete loops, as (experimentally, at least) there are no isolated magnetic charges. Dirac's monopole circumvents this problem by imagining an infinitely long and infinitely thin "string" that carry the looping field lines away, making the monopole appear to behave like an isolated magnetic charge. You can read more about it in Dirac's original paper (Dirac, 1931) or in MoEDAL Collaboration theorist Arttu Ranjantie's review paper (Rajantie, 2012).

If an infinitely long, infinitely thin string sounds a bit odd, well, yes. Yes it is. But that's almost the point; in order to make such a string undetectable in an experiment (which, after all, is all that matters) electric charge must be quantised - i.e. free charged particles that we observe have to have whole numbers of electric charge. Of course, we know that this is the case experimentally - which is why, when writing his paper, Dirac himself remarked:

...one would be surprised if Nature had made no use of it.
— Paul A. M. Dirac, 1931 (http://dx.doi.org/10.1098/rspa.1931.0130)

That's certainly what those of us working on the MoEDAL experiment are hoping, anyway!

If you would like to use this image, it has been published on Zenodo and released under a Creative Commons 4.0 license. Enjoy!