Typically, reversing the direction of time twice is the same as never reversing it at all. Think of running an old-fashioned filmstrip backward, then forward (not an unusual experience for those of us um…of a certain generation): the film will look the same as though you never ran it backward. However, a particular uranium compound, URu₂Si₂, may break that rule. In that sense, it behaves akin to a spinor, the mathematical description of particles like electrons, protons, and so forth. (For more on spinors,  read my earlier post at Galileo’s Pendulum.) This model could explain all the weird properties of the uranium compound, including its strange magnetic behaviors.

A new model may help resolve the confusion by proposing a different form of symmetry breaking. Ordinarily, if you reverse the direction of time (akin to running a movie backward), then reverse it again, everything comes back to normal. For the particular uranium-rubidium-silicon compound at issue, Premala Chandra, Piers Coleman, and Rebecca Flint argued that symmetry is broken: it will not behave normally even under double time reversal. While a literal double reversing of time isn’t possible in the lab, the broken symmetry has a measurable consequence in the distortion of electron orbits in the uranium. If confirmed, this hypothesis could resolve a thirty-year-old mystery. [Read more…]