If you push heat through a piece of quartz, it usually goes straight, like water through a pipe. But switch on a magnetic field, and the flow veers slightly sideways, as if the pipe suddenly grew a side branch. This effect is called the thermal Hall effect, and until now it was unclear what exactly makes heat deflect: disorder in the crystal or something more fundamental.

Spectroscopy has nothing to do with it, but comparing crystal and glass gave the answer. The researchers took two samples of the same substance — silicon dioxide: one as an ordered quartz crystal, the other as amorphous glass where atoms are arranged randomly. They pushed heat through both and measured temperature with identical thermometers. The crystal showed a transverse signal; the glass did not, even though the instruments' resolution could have detected it.

The authors build their explanation on the idea that heat in a solid is carried not by a single flow but by two channels, like two traffic lanes. If these channels respond differently to the magnetic field, the total heat flow no longer has to go strictly along the temperature gradient. A sideways component emerges. A similar effect is known for gases, there it is called the Senftleben–Beenakker effect, and now it has been seen in a crystal.

The magnitude of the deflection turned out to be surprisingly simple to estimate. The heat flow makes atomic nuclei drift slightly; the magnetic field acts on this drift with a sideways force; and the system is brought back to equilibrium by a force related to entropy. Three forces, and the transverse thermal resistance comes out the same for both crystals, about one millionth of a meter-kelvin per watt at a field of one tesla.