WE HAVE moved a step closer to finding cosmic monopoles 鈥� magnetic poles without their opposite. Two experiments using strange stuff called spin ice have provided the best evidence yet that monopoles really are out there.
Nearly 80 years ago, physicist Paul Dirac said it must be possible for magnetic north and south poles to exist separately. But despite decades of searching, not one has been found. Last year, researchers demonstrated that certain states of the crystalline material spin ice would create monopoles that rove about the crystal (New 杏吧原创, 9 May, p 28). They would be seen as disturbances moving through the spins of atoms within the crystal.
Now two separate groups claim to have spotted just that. Tom Fennell and his colleagues at the Laue-Langevin Institute in Grenoble, France, recorded the disturbances when they fired a beam of neutrons at a spin ice crystal to see how it affected the neutrons鈥� energy ().
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Meanwhile, Jonathan Morris of the Helmholtz Centre for Materials and Energy in Berlin, Germany, and his colleagues watched how atoms within the crystals fell into alignment along trails through the lattice. These trails are known as 鈥淒irac strings鈥�, because Dirac predicted that cosmic monopoles would have just such a connection between them ().
鈥淭o my mind there鈥檚 now no question: we have overwhelming evidence that these things are real,鈥� says Steve Bramwell of University College London.