A simple pile of sand can actually record and play back sounds like a mechanical tape recorder.
arXiv · March 17, 2026 · 2603.14595
The Takeaway
By compressing a random pack of grains while applying a signal, researchers found the 'memory' of that wave becomes embedded in the friction between the particles. When the pile is later decompressed, it plays the signal back in reverse, effectively turning a jar of sand into a data storage device.
From the abstract
Using numerical simulations it is shown that a random, athermal pack of soft frictional grains will store an arbitrary waveform that is applied as a small time-dependent shear while the system is slowly compressed. When the system is decompressed at a later time, an approximation of the input waveform is recalled in time-reversed order as shear stresses on the system boundaries. It is shown that this effect depends on friction between the grains, and is independent of some aspects of the frictio