Magnetic monopoles outed, powerless to help grand unified theory

In the last year, the scientific community has seen an uptick in papers about magnetic monopoles, an elementary particle that could allow scientists to move forward on their grand unification theories. Last week, a group of researchers published a paper in Nature Physics that detailed their real-space observations of magnetic monopoles in a material called “spin ice.” But since the monopoles are limited to this system, it’s debatable whether these are really the monopoles that theorists are looking for.

Magnetic monopoles were first posited as part of a quantum theory by Paul Dirac in 1931, who thought of them as a magnetic analog to an electron or proton. Instead of carrying a single type of electric charge, a magnetic monopole would have, as the name suggests, only one pole, north or south. This sounded logical, but reality hasn’t been cooperative—while it’s possible to chop a molecule into electron and proton pieces, chopping up a magnetically dipoled object just creates lots of dipole pieces.

The possible existence of magnetic monopoles has frustrated physicists for some time, especially those attempting to develop grand unified theories about how the physical world works. A magnetic analog to an electron would provide the balance necessary to be able to reduce the existing, somewhat-disjointed electric and magnetic equations to one that rules them all.

In the last couple of years, researchers have made some strides with condensed matter systems. Back in September 2009, they were able to create conditions where they knew a monopole would exist, if fleetingly, based on certain magnetic arrangements, but they didn’t actually witness a physical monopole in real space.

In a more recent experiment, a group of scientists have had more luck using the same unusual material, called spin ice. Spin ice is not ice, but is arranged like an ice molecule with rare earth metals where the oxygen should be.

The researchers watched the spin ice arrays with the Swiss Light Source, a photoelectron emission microscope, as they applied a magnetic field to it. This lined its dipole molecules up so all of their fields pointed to the left.

They found that when they rushed a magnetic field oriented in the opposite direction over the ice, certain chains of molecules would flip their magnetic directions the other way, resulting in “dumbbells” with defects at either end of the chain. Each defect had only one magnetic pole.

Together, the two ends of the dumbbell were a pair of magnetic dipoles and opposites of each other, but were separated by a significant distance. The researchers even found that they could move the monopoles independently of one another.

But while it sounds like monopoles are here to stay, these monopoles are not quite what is needed to do any sort of grand unification. This most recent research comes over a year after the first significant monopole evidence and involves observing their actual physical presence, but they still leave us in the same place when it comes to a grand unified theory.

PhysicsWorld notes that, though these are functional monopoles, they still have a very different and more specific origin than the ones Dirac had predicted. Theoretically, magnetic monopoles should be able to fall out of any system, even from materials that don’t have and can’t respond to magnetic fields.

These spin ice monopoles come from playing with magnetically manipulable materials with atoms that arrange themselves at unusual angles. This provides just the right environment to trick them into producing an incongruity, a defect, that is functionally like a monopole. While monopole proponents may argue that monopoles are like sausage—never mind how they are made, and just enjoy the taste—nothing will work, grand-unified-theory-wise, until a monopole can come from anywhere, just like electrons can.

Still, this doesn’t mean that these particular monopoles aren’t a useful discovery. The authors of the Nature Physics paper note that manipulation of their monopoles could lead to “new types of logic and spintronic devices.” The ability to separate magnetic spins could be useful in developing transistors that are operated using magnetic fields instead of electricity.

Nature Physics, 2010. DOI: 10.1038/nphys1794 (About DOIs).