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Single-pole magnet emerges in frozen concoction

Experiment simulates long-sought magnetic particle

NORTH POLE  An artificial monopole (left), whose magnetic field spreads away in all directions, came from researchers’ manipulation of an ultracold gas. In a traditional bar magnet (right), the magnetic field juts out from the north pole and bends toward the south pole.

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Somewhere lurking in the universe, most physicists agree, are minuscule magnets with just one pole — a north or a south, but not both. Scientists haven’t spotted any yet, but a new experiment offers an unprecedented glimpse at what these elusive magnetic particles should look like.

“It provides a window into the physics of the particle without having the particle itself in front of you,” says David Hall, the physicist at Amherst College in Massachusetts who led the research.

Magnets seem to come in only one variety, with two poles like a bar magnet’s. But in 1931, Nobel-prize winning physicist Paul Dirac demonstrated mathematically that single-pole magnets, known as monopoles, could exist. His mathematical reasoning was so strong that most physicists today have little doubt of monopoles’ existence, despite decades of fruitless searches for them at CERN and other leading institutions.

Hall had rarely thought about monopoles until 2009, when he read a paper that proposed a way to simulate one in the lab. The recipe called for a Bose-Einstein condensate, an exotic state of matter produced by cooling a gas to billionths of a degree above absolute zero. At that extreme temperature, hundreds of thousands of atoms can behave collectively like one particle, allowing scientists to simulate quantum processes on a larger scale.

In following the recipe for an artificial monopole, Hall and his team had to manipulate a condensate’s rubidium atoms, each of which acts like a compass needle. The researchers exposed the atoms to a carefully crafted magnetic field, which caused the compass needles to orient themselves toward a single point in space — as if someone had placed an isolated north-pole magnet there. The researchers detail their findings in the Jan. 30 Nature. “It’s a very nice paper,” says Wolfgang Ketterle, an MIT physicist who shared the 2001 Nobel Prize for demonstrating Bose-Einstein condensates.

Hall emphasizes that his creation is a simulation of a monopole: There is no physical particle where the monopole appears to be. But he says the experiment gives physicists a chance to explore a so-called quasiparticle that, at least mathematically, behaves just as Dirac predicted an actual monopole would 83 years ago. “We’ve realized Dirac’s conception of what a magnetic monopole ought to be, and his conception is the gold standard,” Hall says.

Hall expects other physicists to replicate his experiment and simulate how the artificial monopole interacts with other particles. Such experiments could yield clues as to how an actual magnetic monopole might reveal itself in nature. 

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