To be clear, I don’t favor any explanation at all. I know that something happened; I don’t pretend to know what. I’d like to believe, of course, that it was a real monopole, but there are too many possible horses or zebras for me to actually think the hoofbeats are unicorn.
There would be no need for monopoles in a Maxwell-level explanation of electromagnetism (though Maxwell’s equations can be modified to accommodate them without too much difficulty). We have now gone past that level of understanding, though, and there are again sound theoretical reasons to consider them.
Another question about monopoles. The Dirac quantization is 2q[Sub]e[/Sub]q[Sub]m[/Sub]/(hbarc) = an integer. So the minimal monopole charge is hbarc/(2q[Sub]e[/Sub]). Since the minimal electric charge is now apparently 1/3 of the electron’s charge, does this mean the minimal monopole charge is three times what we thought it was previously? I realize a quark cannot be isolated, and the Dirac reasoning (as I understand it) seems to be given in terms of an isolated electron, but I do not know if that’s necessary nor if the fact that monopoles were apparently created shortly after the Big Bang maybe before quarks formed into hadrons out of the quark-gluon soup, would still let us use the argument.
Well, for one thing, most of the models which unify the strong and weak forces predict that the proton should, on extremely long timescales, decay… But they also predict that this decay should occur much, much more rapidly in the presence of a monopole. Catalyzed proton decay would, in practice, be effectively a way of entirely converting matter to usable energy, rather than the less than 1% conversion you get from things like fusion reactions. Such an energy source would doubtless have a great many practical applications.
Aside from the basic physical implications, the practical applications would be huge. Many of the problems of magnetic confinement for fusion, for instance, are due to the complexity and self-interaction of magnetic fields; the flux lines form surfaces that particles you are trying to contain naturally want to flow along, and tend to fluctuate due to the movement of charged particles within them. Using an array of discrete magnetic “charges” (which is effectively what magnetic monopoles are) would allow very fine control of magnetic fields at quite reasonable energies. As an analogy, imagine trying to pick up a marble with oil-coated chopsticks, and then picking it up with rubber-tipped forceps. Other applications, like compact linear motors, directed magnetic force (think “tractor beams”) and the ability to shield sensitive electronics from EM interactions without thick diamagnetic insulators. Unlike, say, finding the Higgs boson or demonstrating Bekenstein-Hawking radiation at the event horizon of a black hole, discovering and having the ability to produce magnetic monopoles would have an immediate and revolutionary impact upon technology.
Stranger
When Buford Price’s monopole was “discovered” in the 1970’s, I remember the newspaper was filled with speculation about how useful monopoles would be. My favorite for silliness was cargo ships that would take on a load of south poles when they wanted to go north and vice versa.
“Price and his associates speculate that the discovery could some day lead to “new medical therapies in the fight against cancer, new sources of energy, extremely small and efficient motors and generators and new particle accelerators of much higher energy than any yet built.” At a Berkeley press conference last week, there was even far-out talk of equipping a great ship with a few monopoles and having the earth’s magnetic field tug it across the ocean. But any such achievements require the locating and controlling of at least one monopole, which could be used, says Price, to create others by “banging it against matter” in a particle accelerator. Before that happens, scientists will need more than a photographic trace. “The goal,” says Price, “is to capture a monopole and bring it back alive.””
What are monopoles (theoretically) made of? Are we talking about some kind of exotic particle here, or (as with conventional magnets) ordinary matter, with magnetic properties?
Magnetic monopoles are particles that exhibits discrete units of magnetic charge, similar to how the electron exhibits an electric charge. Julian Schwinger, who shared his Nobel Prize in Physics with Richard Feynman and Shinichiro Tomonaga for their work in quantum field theory, proposed a particle called the dyon, which bears both unitary electric (e) and magnetic (g) charges; it is a magnetic monopole when the electric charge (which is always an integer) is zero. In SU(5) and SU(10) proposals for extending the Standard Model of Particle Physics to a grand unified theory, the magnetic monopole is a topological defect in U(1). (The capital letters with a number in brackets are just topological groups in Lie algebra, which is basically a way of representing the interactions between particles in the theory. How it works isn’t important; it is just necessary to understand that in quantum field theory particles are considered to be “defects” or blivits in a vacuum state that are only allowed to interact or convert in specific and discrete ways that are conceptually not unlike how a Rubic’s Cube operates.)
Either way, the photon serves as the bosonic force carrier for magnetic charge just as it does for electrical, which is allowed both in quantum field theory and classical electrodynamics as defined by James Clerk Maxwell, although it has to “move” and interact in a different way to carry pure magnetic charge that is totally non-intuitive. From Anthony Zee’s excellent Quantum Field Theory in a Nutshell:*That a duality may exist between electric and magnetic fields has tantalized theoretical physicists for a century and a half. By the way, if you read Maxwell, you will discover that he often talked about magnetic charges. You can check that Maxwell’s equations are invariant under the elegant transformation (E + iB -> e[sup]iq[/sup]E + iB) if magnetic charges exist.
One intriguing feature of [Equation] (10) is that if e is small, then g is large, and vice versa. What would magnetic charges look like if hey exist? They wouldn’t look any different from electric charges: They too interact with a 1/r potential, with likes repelling and opposites attracting. In principle, we could have perfectly formulated electromagnetism in terms of magnetic charges, with magnetic and electric fields exchanging their roles, but the theory would be strongly coupled, with the coupling g rather than e.*
So to answer the question of the o.p. to the best we are able, magnetic monopoles are little knots in vacuum, just like any other fundamental particles. There is no reason that monopoles shouldn’t exist, and in fact symmetry in both classical and (many) gauge theories allow for and appear to indicate that they can and should exist. However, observationally they don’t appear to exist as stable isolates, and as Chronos’s advisor says, the actual number of magnetic monopoles in the universe may be a very small number, quite possibly zero.
As the person who made the original comment - and, without claiming it as an excuse, it’s sobering to realise that I made it nearly ten years ago - I’ll agree that I sloppily worded that sentence and hence it 's a fair cop on the narrow interpretation.
But has any physicist actually really factored that “observation” into the conclusion of a paper in, say, the last 20 years? (Thereby narrowly setting myself up as the fall guy - someone surely has.) I’m not even sure that Blas Cabrera himself has in any way pushed that conclusion in that period. It certainly stands as the classic experimental anomaly that can’t quite be explained away and so just tantalisingly sits there - and hence still strictly counts as “observational evidence” - but that in practice nobody actually believes in it.