What's the deal with magnetic monopoles?
A monopole is just something with only one pole (enlightening, I know). Normal permanent magnets (or the Earth) are dipoles, they have a North and a South pole. If you cut a magnet in half you don’t get one North and one South pole, you get two smaller and weaker dipole magnets.
Imagine an electron. You probably picture it as a point in space, and you probably picture its electric field as lines pointing radially outward. That’s a monopole, an electric monopole. Now if you switch from it having electric charge to having magnetic change, you’ll have a magnetic monopole.
You might be saying right now, “Well that’s all well and good, but I’ve never seen one of the magnetic monopoles, why should I think they exist?” If you are thinking that, then you’re on to something. No one ever has seen a magnetic monopole. If you look at Maxwell’s Equations, you won’t see any magnetic charge anywhere. But Maxwell noticed that it could easily be added, but the lack of evidence for them dissuaded him from including them.
I just said that no one has seen a magnetic monopole, and this is true, in a sense. Valentine’s Day 1984, Blas Cabrera, my advisor, saw a signature on a detector that is exactly what you’d expect if a monopole had passed through. However, further experiments made it vanishingly unlikely that this was an actual monopole, and Blas will tell you it almost certainly was not. (Personally I’d like to believe it was, and I’m impressed Blas can admit that it wasn’t.)
Now you’re probably thinking, “If we’ve never seen one, why do we care about them?” That’s also a good question. It turns out that if you take quantum mechanical principles and mix them with electrodynamics, you can prove that if there exists one magnetic monopole anywhere in the universe, then both electric and magnetic charge will be quantized. This is called Dirac’s quantization, and is a pretty stunning result. I’ve seen the calculation done and it’s quite beautiful (but too complicated and lengthy for a blog entry). There’s no real reason for the quantization of charge without this (at least that I know of), so the fact that charge is indeed quantized is a good indicator that there is a monopole somewhere out there (maybe it did go through Palo Alto in 1984). Unfortunately for any monopole lovers, there’s probably less than one per cosmic horizon, which means your odds of finding one are just about nil.
Besides that, Grand Unification Theories and other high-level theories, such as String Theory, demand their existence. For a while, theories demanded too damn many of them, and people were concerned about why there were so few. Alan Guth’s Inflationary Cosmology did a fantastic job of explaining the small level of monopoles, which is one of the many reasons it’s so widely accepted.
I hope that now you have a decent grasp of magnetic monopoles, what they are and why they matter.