The Sound of One Hand Clapping
Posted on | September 8, 2009
Two different European groups are reporting the detection of the elusive magnetic monopole:
Nearly 80 years ago, physicist Paul Dirac said it must be possible to separate the north and south poles of a magnet to give them a separate existence. But despite decades of searching moon dust, the debris from particle collisions and cosmic radiation for traces of a monopole, not one has been found.
Spin ice is a kind of crystalline material with essentially the same atomic arrangements as water ice. Last year, researchers demonstrated that certain states of spin ice would create monopoles that rove about the crystal. The monopoles would be seen as disturbances moving through the spins of atoms within the crystal.
Now two separate groups claim to have seen just that.
Tom Fennell and his colleagues at the Laue-Langevin Institute in Grenoble, France, fired a beam of neutrons at a spin ice crystal to investigate how the crystals affected the neutrons’ energy. They chilled the crystal to near-zero Kelvin – almost as cold as it is theoretically possible to get. The results implied that when the temperature of the crystal rose to around 1 kelvin magnetic monopoles were being formed within it.
Meanwhile, Jonathan Morris of the Helmholtz Centre for Materials and Energy in Berlin, Germany, and his colleagues watched how neutrons scattered off a spin ice crystal in a changing magnetic field. The magnetisation of the particles within the crystals fell into alignment along trails through the crystal, suggesting that the magnetic field was pulling the monopoles apart. These trails are known as “Dirac strings”, because Dirac predicted that cosmic monopoles would have just such a connection between them.
