Monopoles and Magnetricity

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."

Comments

Magnetism = relativistic electricity?

[AdamHaun]

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). 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?

[BitterOak]

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.

Re:Monopole Magnets

[Roger W Moore]

Pierre Curie pointed out the possibility of magnetic monopoles in 1894 based solely on Maxwell's equations which was close to a century before Superstring theory was on the scene. In fact in 1931 Dirac showed they could be used to quantize charge using nothing but relativistic quantum mechanics and Maxwell's equations. So monopoles actually only require Maxwell's equations and would lead to an explanation of why charge is quantized. It may come as a shock but the real world does not play like Alpha Centauri (and considering what happened to Earth in that game that's a good thing!).

Re:Monopole Magnets

[Noughmad]

So monopoles actually only require Maxwell's equations and would lead to an explanation of why charge is quantized.

Not, not really. Maxwell's equations, in the known form, do not allow for magnetic monopoles. They are explicitly forbidden by "div B = 0". However, they could very easily be changed (symmetrized - so they would look the same for electric and magnetic fields) to account for them.

So just from Maxwell's equation you can't disprove the existence of monopoles, but you can't prove them either.