A hot spoon in a cup of tea — heat runs along the metal from the hot end to the cold one. Textbooks describe this process as atoms jiggling and passing on their jolts to each other. But atoms don't just jiggle: they also spin, like tiny tops. Until now, physicists had no common language to describe both motions at once.
The authors of the work built such a language. For each atom, they introduced not only the usual displacement (how much it sways sideways) but also a rotation, as if they attached an invisible arrow to the atom showing which way it's facing. When atoms vibrate, these arrows also rotate. Together, they create a wave in which heat moves not only forward but also swirls, like water in a vortex.
The most unexpected conclusion concerns symmetry. If a crystal resembles a spiral staircase, twisted by nature, then waves running clockwise and counterclockwise will have different strengths. One will run faster, the other slower. This is similar to how the right and left hands feel the rotation of a doorknob differently.
In the limiting case, when atoms are rigidly coupled, their rotation reduces to the usual swirling of the lattice. Then the theory simplifies and reduces to the known equations of micropolar elasticity. But the main point is that in the general case, it predicts how much rotational momentum a thermal wave carries, and this can be measured.
In a quartz crystal, atoms are arranged in a spiral, so heat propagates with a twist, like a corkscrew entering a cork.
In Peter Watts's novel 'Blindsight', alien creatures use rotational vibrations to communicate — almost like phonons with angular momentum in the new theory.