Evidence of Magnetic Monopoles Found?

TheMatt writes "As reported on PhysicsWeb and published in Science (subscription required), researchers at AIST and co-workers believe they have found evidence of magnetic monopoles. They observed an anomalous Hall effect in a ferromagnetic crystal that they say can only be explained via magnetic monopoles. To refresh your memory, magnetic monopoles are the magnetic analogue of electrons and other charged particles--a "north" or "south" pole only. Dirac in 1931 showed that the existence of a magnetic monopole naturally leads to the quantization of electric and magnetic charge. Thus, showing the existence of just one magnetic monopole would be quite profound for physics, but their mass (> 10^16 GeV) has made searches for them difficult."

Comments

For non-physics geeks...

[rjh]

This is really fundamental and amazing stuff, if it turns out to be true. (Every decade or so someone discovers monopoles, then six months later, oops!, guess we didn't.) The reason why it's fundamental and amazing is the conflict between relativity and quantum mechanics.

Relativity and quantum mechanics currently give physicists nightmares. As near as we can tell, both are fantastically accurate descriptions of the world, and both are fundamentally at odds with each other. If we could find a flaw in one or the other, that would potentially open the door to a new and better theory, one which might allow us to reconcile these differences without resorting to theology [*].

Now, when Einstein devised relativity, he based it very heavily on Maxwell's Laws. The Laws are a set of four equations which describe pretty much all electromagnetic phenomena out there. It was the world's first Grand Unified Theory (GUT), in that it unified electricity and magnetism into one package. And one of Maxwell's Laws ("the divergence of the magnetic field equals zero") has, as a direct consequence, an absolute law: NO MAGNETIC MONOPOLES EXIST. NO OPEN MAGNETIC FIELD LINES EXIST.

So if Maxwell's Laws are wrong and relativity is built heavily on Maxwell's Laws, then there's a tantalizing chance relativity is wrong.

Or if Maxwell's Laws are right and monopoles are conclusively proven not to exist, then there's a tantalizing chance quantum mechanics is wrong.

Either way, physics wins: no matter what happens, we get to see a flaw in our current theories. And seeing that flaw is the first step to coming up with better, more accurate theories.

What's the worst-case scenario? The worst-case scenario is these guys are wrong, just like every other monopole researcher before, and the "do magnetic monopoles exist?" question remains unresolved for the next few hundred years.

Warning: I'm not a professional physics geek. In fact, I may be stark barking mad wrong here. :)

[*] Theology, aka string theory. Sorry, but any theory which literally cannot be experimentally tested at any realistic energy level isn't a theory at all. It's an article of faith.

Re:For non-physics geeks...

[Bootsy Collins]

And one of Maxwell's Laws ("the divergence of the magnetic field equals zero") has, as a direct consequence, an absolute law: NO MAGNETIC MONOPOLES EXIST. NO OPEN MAGNETIC FIELD LINES EXIST.

So if Maxwell's Laws are wrong and relativity is built heavily on Maxwell's Laws, then there's a tantalizing chance relativity is wrong.

Or if Maxwell's Laws are right and monopoles are conclusively proven not to exist, then there's a tantalizing chance quantum mechanics is wrong.

You have to be careful here. Your first paragraph is a correct statement of what's implied by one of the four Maxwell Equations. It's a bit misleading, though, to say that "relativity is built heavily on Maxwell's Laws." That seems to imply that relativity somehow depends upon Maxwell's Equations being true. That isn't correct. A better way of putting it is that Maxwell's Equations can be shown to incorporate special relativity within them, in the sense that the laws of electromagnetism stay the same ("Lorentz covariance") after changing relativistic reference frames (through a Lorentz transformation).

In fact, we already know that the Maxwell Equations are wrong. We know that they're wrong because they fail to explain behavior on the quantum scale. For that, we turn to quantum electrodynamics (QED), which, again, incorporates special relativity. Maxwell's Equations are thus seen as an excellent approximation on macroscopic scales. The failure of such an approximation to allow for magnetic monopoles doesn't seem like much of a failure.

That said, I've forgotten enough of my field theory that I don't remember how monopoles fit into QED. But I do know lots of field theorists, and most of them believe that monopoles are around (they'd be produced by the breaking of certain fundamental symmetries that many theorists expect to have been present in the early universe; although any inflationary epoch in the history of the Universe would be expected to make monopoles very very rare); and they don't seem to be stressing about the implications of monopoles for QED. So my suspicion is that QED is OK.

Oh, and just as a brief defense of string theorists . . .I don't much like string theory myself, and I echo your reluctance to take it seriously on the basis of the difficulty in making comparison to experiment. But I think most string theorists would take exception to your statement that string theories "literally cannot be experimentally tested at any realistic energy level." To we who are not string theorists, that's what it looks like, yes; but a string theorist would simply reply that string theorists haven't yet been clever enough to figure out how to extract observable predictions. In other words, I don't think many string theorists would disagree with the principle that for a theory to be interesting, it must be testable. In fact, they would claim that that's their goal with string theory -- to figure out how to make realistically testable predictions -- and they just haven't yet been successful. The Planck scale is a long way from observable energy scales, to be sure. But people explore ideas that allow the examination of classes of models, e.g. the "one large dimension" stuff from a few years ago that would have had observable consequences.

Re:For non-physics geeks...

[jaakkeli]

Relativity and quantum mechanics currently give physicists nightmares. As near as we can tell, both are fantastically accurate descriptions of the world, and both are fundamentally at odds with each other.

From the later parts of your post it's obvious that you're now talking about *special relativity*. It is not in any way at odds with quantum mechanics; in fact, the fusion of relativity and quantum mechanics (something called "quantum field theory") is *the most succesful theory of physics ever developed* (at least when success is measured by how well the theory fits with experiment).

What we don't have is a quantum theory of gravity. We have a very well working *classical* theory of gravity, called "general relativity", which is obviously as much at odds with quantum mechanics as any classical theory is.

Now, when Einstein devised relativity, he based it very heavily on Maxwell's Laws. The Laws are a set of four equations which describe pretty much all electromagnetic phenomena out there. It was the world's first Grand Unified Theory (GUT), in that it unified electricity and magnetism into one package.

That would not be a _GUT_.

And one of Maxwell's Laws ("the divergence of the magnetic field equals zero") has, as a direct consequence, an absolute law: NO MAGNETIC MONOPOLES EXIST.

This is certainly true, but it is trivial to fix this law to handle magnetic monopoles. Remember, you have one Maxwell equation basically stating that

div E = electric charge density

and another, the one that states that no magnetic monopoles exist,

div B = 0

(for the completely unprepared reader: here E and B are the electric and magnetic fields and "div" is a certain sort of an operator that acts on vectors) If you compare these two equations, you'll see *why* the divergence of B is zero: by analogy, div B should just equal the "magnetic charge density", but since there are no magnetic monopoles, the magnetic "charge" density is always zero and div B = 0. In other words, there is no *theoretical* reason why you couldn't write

div B = magnetic charge density

but since the *experiments* tell us that this is always zero, we don't usually bother talking about magnetic charges at all and just set this to zero. If the experiments ever tell us that magnetic monopoles exist, then we'll just have to include these magnetic charge/current terms (which are normally set to zero) in Maxwells equations as well.

So if Maxwell's Laws are wrong and relativity is built heavily on Maxwell's Laws, then there's a tantalizing chance relativity is wrong.

No, there isn't. First of all, including magnetic monopoles the way I outlined above won't make Maxwells theory of electromagnetism in any way incompatible with special relativity. This would be a very minor modification of electodynamics. Second of all, special relativity isn't *based* on electrodynamics at all - ED was an inspiration for Einstein (basically, Maxwells ED is at odds with Newtonian mechanics; Einstein saw this and decided to seek an alternative theory of mechanics that wouldn't be in conflict with it - and found one). If electodynamics ever turned out to be wrong, it wouldn't yet say anything at all about the validity of special relativity.

Warning: I'm not a professional physics geek.

Well, I am. Trust me, I know what I'm doing. :-)

Re:For non-physics geeks...

[Froze]

Don't forget that you also need to include the magnetic monopole current term, analogous to
curl(B)-(1/c)(partial E/Partial t) = (4pi/c)J_e to get
curl(E)+(1/c)(partial B/partial t) = (4pi/c)J_b rather than 0.

Re:For non-physics geeks...

[bcrowell]

The existence or nonexistence of magnetic monopoles has nothing to do with the validity of relativity. Relativity has been thoroughly tested. It's not speculative at all. Here is a book with some information on tests of special relativity (chapter 1). General relativity is also starting to be a well tested theory; the GPS system incorporates general relativistic effects, for instance.

Although people often state the principles of relativity using the word "light," e.g., describing c as the speed of light, actually relativity is about space and time, not electromagnetic radiation. The factor c is better understood as a kind of space-to-time conversion factor. Even if light didn't exist, special relativity would be the same theory; you could then describe c as the speed of X, where X is any massless particle other than the photon.

Theology, aka string theory. Sorry, but any theory which literally cannot be experimentally tested at any realistic energy level isn't a theory at all. It's an article of faith.
I agree that string theory has been oversold, and that it's in danger of becoming theological. However, quantum gravity doesn't have to be entirely voodoo. For one thing, there was this big experiment called the Big Bang, which probed the right conditions to test theories of quantum gravity such as string theory. A really cool book is Three Roads to Quantum Gravity by Lee Smolin; I like it because it focuses on the basic principles that are relatively independent of which approach you use.

Relativity not based on Maxwell's theories

[metamatic]

Second of all, special relativity isn't *based* on electrodynamics at all

Right. Just as a reminder to anyone reading this thread, the three axioms from which special relativity can be derived are:

1. The laws of physics are the same for all non-accelerating observers.
2. The speed of light in vacuum is a constant.
3. Causality always applies.

The third is often not stated, as it's implicit in most of physics anyway. I wish I could remember the entire proof, but it's been a few years. It's not especially arcane or incomprehensible, though, and you don't need a degree in physics to understand it.

The fact that special relativity has so few dependencies, and is relatively simple, is part of its brilliance. It's also why theories that special relativity is flawed tend to be treated with extreme skepticism--it's hard to think of a theory that's more solid.

Maxwell's theory of electrodynamics supports the idea that the speed of light in a vacuum is a constant, but it's not the only evidence; there's the Michelson-Morley experiment, for starters. So Maxwell's theory falling over would not prove that the speed of light wasn't constant, and would not knock down special relativity.

If you want to knock down special relativity in favor of your own masterpiece, though, axiom #2 is certainly the one to go for... Throwing out either of the others tends to be a bit self-defeating. That's why most aether-theory crackpots claim that the Michelson-Morley experiment is flawed in some bizarre way.

their mass; not in "real space"??

[bcrowell]

Thus, showing the existence of just one magnetic monopole would be quite profound for physics, but their mass (> 10^16 GeV) has made searches for them difficult.
This is an inaccurate representation of the article. The article says:

• Magnetic monopoles are also predicted by some theories that seek to unify the electroweak and strong interactions. However, the monopole masses that are predicted by these so-called grand unified theories are much too large - about 10^16 giga-electronvolts - to be detected in experiments.

They probably don't exist at all. Even if they do exist, it's only within the context of certain theoretical frameworks that this mass estimate could apply. If the mass is in the 10^16 GeV range, then there's no hope of creating them artificially with any forseeable technology; the only way to search for them would be to look for ones that occurred naturally soon after the big bang, and happen to cruise through your detector on a certain day.

Instead of searching for magnetic monopoles in real space, Yoshinori Tokura of the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba and co-workers turned to momentum space - the mathematical space in which condensed matter physicists construct Fermi surfaces, Brillouin zones and so on. The team was motivated by recent theoretical work which suggested that the behaviour of magnetic monopoles in momentum space is closely related to the anomalous Hall effect.
I'm not a condensed matter specialist, so I don't really understand what they're saying here, but it sounds like they may be saying they found something mathematically analogous to a magnetic monopole, not a real magnetic monopole. Unfortunately they don't seem to have posted preprints anywhere, but they certainly aren't creating 10^16 GeV particles in a condensed matter lab, nor does it sound like they claim to have captured natural ones.

Re:Well I feel dumb

[Pav]

StarControl II? Classic game! ...now open sourced and available at sc2.sourceforge.net . Binaries for Linux and Win32...