The switchable vacuum rotor

Why the library suggests it

Benjamin Spreng, Calum Shelden, Tao Gong and Jeremy Munday show that a modest forward bias across a semiconductor gives its emitted photons a chemical potential, and that adding this push to the ordinary Casimir pull flips the sign — gallium arsenide facing gold turns repulsive beyond a few hundred nanometres, with the crossover moving closer as the voltage rises, and tens to hundreds of piconewtons of repulsion predicted in the sphere-and-plate geometry every Casimir laboratory already owns (Casimir repulsion with biased semiconductors, 2024). Zixuan Dai and Qing-Dong Jiang show that altermagnets — magnets with no net magnetisation whose spin pattern is tied to the crystal's own rotation symmetry — develop a Casimir torque under a field pointing straight through the plates, which picks out no direction in the plane at all; the torque grows as the square of the field and a magnetic underlay chooses which way it turns (Vacuum Torque Without Anisotropy: Switchable Casimir Torque Between Altermagnets, 2026). The torque itself is an established branch of the subject (Recent developments on the Casimir torque, 2022).

The experiment or build

Fabricate a disc of altermagnetic material suspended tens of nanometres above a second one, with a coil supplying the out-of-plane field and a bias line on a semiconductor spacer. Read the rotation optically. Sweep the field and confirm the torque follows the square of it; reverse the underlay and confirm the direction reverses; apply the bias and confirm the levitating branch appears. The settling measurement is the torque in newton-metres as a function of applied field, with the sign reversing on command and the whole curve returning to zero when both the field and the bias are removed. A vacuum device that switches off is a vacuum device you can believe.