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It’s been almost a century since Wolfgang Pauli mooted the idea of “hidden rotation”: a new quantum variable that would double the number of possible electron states. Today, this variable is known as spin angular momentum, and it’s widely accepted as an intrinsic property of fundamental particles. Yet despite the ubiquity of spin, there is still no real consensus about its physical meaning.

For photons, the usual explanation is that spin is related to circular polarization – a state of affairs in which the direction of the electromagnetic field in a beam of light rotates in a plane perpendicular to the direction of propagation, like hands round a clock face. This explanation has a straightforward consequence: no polarization, no spin. Now, however, an international collaboration of researchers has cast doubt on this principle by measuring non-zero transverse spin values in totally unpolarized light. In ruling out this supposedly fundamental requirement, the new observations expand our understanding of what spin angular momentum is not, while raising further questions about what it is.

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