In this week's "Ask a Physicist" we tackle magnetic monopoles, why we love them, why we yearn for them, and why we haven't given up on them yet.
Today's "Ask a Physicist" comes to us from David Sparks and is right to the point:
Whatever happened to magnetic monopoles?
David went straight to the point, so I will too. They might be out there, but if so, they're the angry loners of the universe.
Before I get into why monopoles would be awesome (and they would be), let me say a few words about electricity and magnetism. Take an electron, and it creates an electrical field. Now throw that same electron across the room, and voila! It also generates a magnetic field. One of the major insights from special relativity was that you don't even need to throw your electron to get a magnetic field. If I run past your electron, it looks the same to me as if I'm standing still and your electron is moving, and I see a magnetic field.
Electromagnetic inbreeding comes into play in a lot of ways. You might even say (if you were being all smug) that the two forces are unified. Nevertheless, all of the phenomena that you normally observe as magnetic are actually caused by moving (or spinning) electrical charges. Even bar magnets are ultimately caused by electrons all spinning the same way, and producing a magnetic field. However, if you cut a bar magnet in half, you get two bar magnets, each with a North pole and a South pole. What a bar-gain!
With a magnetic monopole, on the other hand, you have a single particle which is essentially all North pole or all South pole. And yes, if you were to throw a magnetic monopole, you'd generate an electric field. Congratulations. You're very clever.
Now that you know everything you need to about what they are, let's have some:
Magnetic Monopole Fun Facts!
1. They explain why you can't slice an electron in half.
One of the big mysteries of the universe is why electrons and quarks (the major players in the particle physics universe) only come with particular charges, and that the ratios of those charges are such nice round numbers. An electron has 3 times the charge of a down quark, for example, and an up quark has -2 times the charge of a down. P.A.M. Dirac tackled this problem in the 1930's.
He was a physicist's physicist; one of those guys who could basically take a mess of equations and come up with a mathematical certainty which turned out to tell us something amazing about the universe. Doubt me, if you will, but don't doubt Dirac. If I told you how he went about predicting the existence of anti-matter, you would think I was completely nuts, but damned if he wasn't 100% correct.
I'm not going to get all mathemagical with Dirac's derivation, but will instead give you a simpler one that's almost right. You can figure out where I've cheated in the comments section if you dare.
Take a magnetic monopole, and then throw an electron towards it. Magnetism is all curly (you may remember something called the "right hand rule" if you took physics in high school or college), so the electron takes a helical path around the monopole, and gets an angular momentum out of the deal. That, in and of itself, is cool, but there's more! Angular momentum, in case you've forgotten, is a measure of how quickly something is rotating around something else, and quantum mechanics says that all angular momenta that show up are all going to be integer multiples of a number known as the Planck's constant. As a result, the magnetic charge can only be a particular set of values. From that, Dirac found that the magnetic monopoles can only have particular charges, and by extension, so can electrical charges.
2. When they sit around the house, they sit around the house.
The thing about Dirac's trick is that when you work through the numbers, even the smallest possible magnetic charge would have to be huge. As a result, if monopoles are floating around, they should be ridiculously easy to detect. So in partial answer to David's question, monopoles haven't gone anywhere. Lots of people are still looking for them with small superconducting devices called SQUIDs. If a magnetic monopole happens to fly through your device, then bang! You measure it.
But the reason you don't hear so much about it is that, um, we haven't actually seen any. Well, that's not perhaps entirely true, since about 30 years ago there was one possible detection, but nobody's been able to repeat it since. As a result, even if monopoles exist, they are really rare — there are less than 1 for every 1029 protons or neutrons. To give you an idea of how rare that is, if monopoles are distributed more or less evenly in the universe, there should only be a hundred (at most) or so in the entire inner solar system.
3. They'd tell us some amazing things about space, time, and the laws of physics.
I can take a bunch of electrons and bounce them around off one another. If I had mastery of space and time (which would be awesome), I could imagine creating a mirror universe, and in that mirror universe my electron bouncing experiment would look... perfectly normal. The same would hold true if I turned all of my electrons into positrons, or reversed the direction of time.
However, the same can't be said for magnetic monopoles. If I throw an electron at a monopole, it spirals in a particular direction. But in the mirror universe, it spirals in the opposite direction. You could tell that you were in the mirror universe! Electromagnetism isn't supposed to work this way. In fact, except for the weak force (and then, only weakly) the laws of physics aren't supposed to care about either the direction of time or whether we're in a mirror universe. The existence of monopoles would tell us so much more about the true symmetries in the universe.
All of this is nice, but it doesn't explain why there are so few of them (if any), and so:
Why bother talking about them?
The jury's still out on whether monopoles do exist, but there are some good reasons to suppose they might.
1. Pretty much every Grand Unified Theory predicts them.
I mentioned before that electricity and magnetism were unified. Well, once upon a time, the strong and weak nuclear forces were as well. During that period, physics was described by what we call a "Grand Unified Theory," or GUT. There's no consensus on what the true GUT is at the moment, but all of them have some qualities in common, and these include so-called phase transitions.
The fraternal order of physicists demands that when I talk about phase transitions, I mention ice, and so I will. Take water and freeze it, and you get ice. You'll notice, though, that within a small patch of ice, the crystals align with one another. However, if you freeze an entire lake parts of the lake that are far enough apart don't align at all.
The fields in the universe behaved exactly the same way, and these regions where the phases of the fields don't line up are known as "topological defects." For GUTs, the topological defects are magnetic monopoles. What's more, if any were created in the early universe, they'd still be around today. Watching. Waiting.
The good news, though, is that these theories predict that there need to be as few as 1 monopole within the horizon of the universe, and if that were the case, it would be very hard to hunt down. But maybe we got insanely lucky 30 years ago, and already observed it.
2. So does String Theory.
I am not on board the string theory bus, so if you're looking for someone to blame for all of the sins of physics, you'll have to look elsewhere. On the other hand, we can't rule out string theory, either, based on experiment. That's actually the problem that lots of people have with it. However, string theory does make a bunch of predictions, including supersymmetry (which we can perhaps talk about another time), and magnetic monopoles. That's the good news. The bad news is that monopoles would have such a gargantuanly huge mass that there would be almost no chance of making them in an accelerator. Ever.
3. They could be dark matter, but not much of it.
We end, as you knew we might, with Dark Matter. One of the big things people (including and especially io9 readers) complain about is that we don't really know what dark matter particles are. There are some good dark matter candidates out there (we discuss the odds-on favorites in our book), but for a while, I would have given good odds to magnetic monopoles.
I hope you're not relying on them though, because I have some bad news. Remember when I said that monopoles are at best 1029 times rarer than ordinary protons and neutrons? If they are rare, then they must also be massive. Really massive. It turns out that if they were to make up all of the dark matter in the universe, they'd need to be at least 100 kg apiece. You would notice if you were hit by one in the sense that you would notice if you were hit by a sumo wrestler or a tractor. So they probably are a hell of a lot lighter than they need to be to make up the bulk of the dark matter. But they could make up some of it.
After all, there may be only one.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe: Surviving the Perils of Black Holes, Time Paradoxes, and Quantum Uncertainty." (follow us on facebook or twitter.) He is an Associate Professor of Physics at Drexel University. Feel free to send email to firstname.lastname@example.org with any questions about the universe.