Thursday was Monopole Day

Here’s an odd coincidence. Thursday, just for fun, I declared it Monopole Day in my facebook status. It was largely because I was prepping a class on Dirac’s ideas about magnetic monopoles (roughly: point sources of magnetic fields analogous to point sources of electric fields), why it seems (from looking at Maxwell’s equations – the defining equations of electricity and magnetism) that Nature might very much like the idea of them (using a symmetry argument – roughly: electric and magnetic fields can turn into each other so if there’s one kind of point source, isn’t it strange that there’s not the other?) but chose to hide them away…

It’s a favourite topic of mine (and many physicists – it is one of the first grown-up tastes of modern ways of thinking about physics that you get as an advanced undergraduate, actually), and so I was excited to lecture about it (as I am every year… I find en excuse to bring monopoles into every course I teach… almost). I even handed out one of my class worksheets, which was all about how to build a monopole (on paper – you take a semi-infinite solenoid (a “Dirac String”) and let the magnetic flux lines spill out of the end) and compute its resulting properties and the consequences of building one (you’d explain the mysterious fact that electric charge is quantized in little discrete packages, for example – you never find particles with say, half the charge of an electron).

Later that day, I went home and checked my facebook chatter and a friend, Philip Shane*, had noticed my status (and/or a followup one on a similar topic) and said:

“…don’t know if this is related, but coincidentally came across this article today: http://physicsworld.com/cws/article/news/40302”

(As usual, beware of the usual amusing and mostly misleading chatter in the comments to the article.)

The title of the article, by Hamish Johnston, is “Magnetic monopoles spotted in spin ices”, based on papers “Dirac Strings and Magnetic Monopoles in Spin Ice Dy2Ti2O7” (Morris, et. al.,) and “Magnetic Coulomb Phase in the Spin Ice Ho2Ti2O7” (Fennell, et. al.).

Note that this came out the very same day I gave the lecture! I love coincidences like that.

A brief word about the physics. No, a “fundamental” monopole has not been spotted. That is not what this is about. In effect, they’ve found an effective monopole configuration. That solenoid configuration I mentioned above? They found that some materials confine flux lines into those very sorts of filaments that Dirac talked about, the endpoints of which then look like monopoles. It is very cute. The difference between these monopoles and a “fundamental” monopole is that in the latter the Dirac strings would be physically undetectable. This is what would then force charge quantization, etc, etc. Not the case here. It is still exciting to see this sort of system create this effective monopole and flux tube configuration. It is an example of one of the kinds of special phases of gauge theories we usually study in work on field theory and string theory, so it is very nice to see it show up in the lab.

-cvj

*Thanks Philip!