On the Trail of Magnetic Monopoles

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Fans of CBS’ The Big Bang Theory might recall the Season 2 season finale, “The Monopolar Expedition,” in which everyone’s favorite socially challenged physicist, Sheldon, accepts an invitation to spend three months at the North Pole searching for magnetic monopoles.

He figures finding a magnetic monopole would put him on the fast track for a Nobel Prize. And he would be right. But he shouldn’t count on finding one right away; magnetic monopoles have eluded our best scientists for centuries. (Episode highlights can be viewed here.)

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Perhaps it’s ironic that the #1 sitcom in America also has some of the most accurate cutting-edge physics on television, but it’s not surprising — not if you know that the show has a regular technical consultant, UCLA physicist David Saltzberg.

Saltzberg even maintains a blog, The Big Blog Theory, where he comments on each new episode. In the entry for “The Monopolar Expedition,” he explains the basics of magnetic monopoles — magnets with only one magnetic pole — and why they’re so weird and significant to particle physics and cosmology:

As Saltzberg says, scientists have been tantalized by the prospect of magnetic monopoles since James Clerk Maxwell first formulated his famous four equations describing electromagnetism. Maxwell’s equations are “asymmetrical”: electricity has an electron monopole and a proton monopole, each with opposite charge, but magnetism does not.

Pierre Curie hypothesized that magnetic monopoles might exist in 1894, but the real mathematical heft for the idea came from Paul Dirac in 1931: he showed that if one quantized electric charges, then the existence of monopoles was consistent with Maxwell’s equations. Specifically, monopoles could exit at the ends of long tubes carrying magnetic fields called “Dirac strings.”

And the search was on! As it happens, the best places to look for these exotic particles is the North and South Poles, since scientists can use the Earth’s magnetic field as “a funnel for magnetic monopoles.” That, says Saltzberg, is why Sheldon is sent to the Arctic for his expedition.

Sheldon is a string theorist, and string theory also predicts the existence of magnetic monopoles. So their discovery would provide a key piece of experimental evidence for this contender for a Grand Unified Theory meshing general relativity with quantum mechanics.

So where are these mysterious particles? Scientists thought they’d caught a glimpse on Valentine’s Day in 1982, but that experimental result has never been reproduced.

There was tons of excitement in 1975 when a team announced it had detected a moving magnetic monopole in cosmic rays. Alas, that turned out not to be real, either. It was just a garden variety platinum nucleus pretending to be a monopole by cleverly decaying into osmium and tantalum. That platinum is such a trickster.

Physicists have succeeded in creating analogs of magnetic monopoles, however, in exotic materials known as “spin ice.” Back in September 2009, a team of physicists took a single crystal of dysprosium titanate, chilled to sub-Kelvin temperatures (-270 degrees F), and used neutron scattering to create an image showing that, inside this crystal, the magnetic moments self-organized into a kind of “spin-spaghetti” that looked an awful lot like Dirac strings (see image above).

A year later, in October 2010, physicists were able to directly image these analogue monopoles using high-intensity x-rays from the Paul Sherrer Institute’s Swiss Light Source — this time at room temperatures. (You can view a video explaining this achievement here, courtesy of Imperial College in London.)

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Analogues are not, ultimately, a substitute for the real thing. Perhaps a planned experiment at CERN’s Large Hadron Collider will find evidence for magnetic monopoles in the collisions of ultra-high-energy protons.

Real-life string theorists like Joseph Polchinski of the Kavli Institute of Theoretical Physics remain hopeful about future success. Polchinski has famously declared that the existence of magnetic monopoles is “one of the safest bets that one can make about physics not yet seen.” Less frequently quoted is his qualifier: “But we must continue to hope that we will be lucky, or unexpectedly clever, some day.”

Incidentally, the National Science Foundation allowed The Big Bang Theory to use their official logo on all the shipping crates, and in the final scene, the cast members all don bright red parkas worn by real polar scientists on such expeditions. As for the white board equations in the background that have become a staple of each weekly episode, Saltzberg provided classic diagrams and equations describing magnetic monopoles.

Who says there’s no good physics on television?