With nothing more than beads in a glass box, physicists have revealed yet another mysterious property of granular solids, now recognized by scientists as a unique state of matter, like solids or gases.

When the box was filled to the brim and rotated, the beads moved in patterns known from convection clouds — another system whose basic physical dynamics are only dimly understood.

The experiment, displayed in a video posted Monday to arXiv, was a variation on one performed 70 years ago by Japanese physicist Yositsi Oyama, who observed that beads of different sizes placed in a rotating circular drum would eventually self-sort by size.

That intriguing result set in motion the study of granular solids, which behave in ways that can’t be predicted with known physical laws. And though research has accelerated in the last decade, scientific understanding of granularity is roughly akin to that of fluid dynamics in the 18th century.

In the latest experiment, conducted by Otto von Guericke University physicists Ralf Stannarius and Frank Rietz, a partially full rotating box of beads displayed the self-sorting patterns seen by Oyama. But when they filled the box nearly to the top — which, they expected, would cause the beads to clog — the beads instead moved in graceful, swirling currents.

Why this should happen is unclear. No equations exist to describe why such a slight change in packing density should produce such different system-wide behavior. “Known mechanisms for granular convection could not be applied,” wrote Rietz and Stannarius.

Intriguingly, similar currents can be seen in clouds, or ocean currents. In a paper published last February in Physical Review Letters, the pair described the mysterious movements of beads in their box as suggesting “the existence of comparable phenomena in situations where so far no systematic search for dynamic patterns has been performed.”