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…]

When the reverse of reverse isn’t forward: weird symmetry in uranium compound