A quantum particle that crosses a ‘duality defect’ – a boundary between order and disorder – is never reflected. It always passes through, but it comes out changed: not a particle, but a spread-out, thread-like excitation that researchers compare to a wisp of fog.
The findings, published in the journal Nature Physics, may help solve a 40-year-old puzzle in physics known as the magnetic monopole paradox.
The researchers, from the University of Cambridge, Ghent University and the University of Oxford, borrowed the wisp of fog image from Goethe’s poem Erlkönig, made famous by the composer Franz Schubert in his 1815 setting. In the poem, a father tells his frightened son that the ghostly figure he sees is only ‘ein Nebelstreif’ – a wisp of fog. But the father turns out to be tragically wrong.
Since the 1980s, physicists have theorised what happens when a charged particle scatters off a magnetic monopole: an isolated magnetic charge. Some scientists showed that the particle’s outgoing state seemed to disappear, while later work suggested the missing states were hidden in ‘twisted’ sections of the monopole.
Now, the team behind the current work say they have shown where these particles go. Using a model called a spin chain, they fired quantum wave packets – localised bundles of quantum waves that help pinpoint a particle’s probable location and motion – at a duality defect. The particle always crossed the boundary, and re-emerged transformed: faint, spread out, and trailing a thread back to the boundary, like Goethe’s Nebelstreif.
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