Monopoles and Magnetricity 67
szotz writes "Although there was once a hint from a cosmic ray experiment (on Valentine's Day, no less), no one's found any solid evidence of monopoles (unpaired north and south magnets) flying around the cosmos. But physicists did find monopole-like quasiparticles in some exotic crystals in 2009. One of the discoverers has an article this month in IEEE Spectrum that looks at how the particles were found and what's happened since. They might seem like a wacky curiosity, but the author says we shouldn't write them off — they might one day make useful new 'magnetronic' devices."
Magnetism = relativistic electricity? (Score:5, Interesting)
Question for any physicists in the audience: I have long heard that magnetic forces can be described as relativistic effects of classical electricity (here, for instance [wikipedia.org]). How do magnetic monopoles fit into this? Are they are purely quantum mechanics/QFT concept, or is there some way to describe them classically that makes it clear why so many people are expecting to find them?
Re:Magnetism = relativistic electricity? (Score:5, Interesting)
According to special relativity, magnetic and electric fields can be "mixed" by Lorentz boosts (i.e. moving from one inertial reference frame to another that is moving with respect to the first one.) As a result, some say that a magnetic field is nothing more than an electric field as viewed by a moving observer. That isn't quite correct. The quantity E^2-B^2 (the square of the electric field minus the square of the magnetic field) is a Lorentz invariant. It is the same according to all observers. That means that if the magnitude of the electric (or magnetic) field is greater than the magnitude of the magnetic (or electric) field in one frame of reference, it is in all frames of reference. So a purely magnetic field can never be described as a purely electric field as seen by a moving observer.
In particular, Maxwell's equations, if valid in one inertial frame of reference, are valid in all frames. One of Maxwell's equations is div B = 0, which says there are no magnetic monopoles. A violation of this equation in one reference frame would be seen as a violation in all frames. That is a magnetic monopole in one frame of reference would be seen as a magnetic monopole in all frames of reference, despite the fact that in frames in which it is moving, it will generate an electric, as well as magnetic field, just as an electric charge, when moving, produces a magnetic field as well as an electric field.
Monopoles and quantized charge (Score:4, Interesting)
As a followup to the excellent relativistic answer above...
As one of my professors put it: "if there is one monopole anywhere, then charge is quantized everywhere". This was at the end of a fairly straightforward derivation, first done by Paul Dirac IIRC.
We see by experiment that charge is quantized everywhere, but this doesn't prove the existence of monopoles. It's a tantalizing clue to why things may be the way they are in our universe.
Monopoles are predicted by some of the unified theories, so if they exist how come we don't see any?