Imagine a book with latches on its pages: you can only flip a page if you know how to click its edge. That is roughly how the mechanical metamaterial we are about to discuss works. Each cell is a small mass inside a larger one, connected by an elastic beam. The beam can sit in two stable positions, like a switch. In one position, the inner mass is almost decoupled from the outer one, and the whole structure responds to a push in one way. In the other, a magnet presses against the inner mass, and the response changes dramatically: the resonant frequency jumps from 26 to 280 hertz. It is as if the same string could sound like a double bass or a violin.
In the experiment, a chain of three cells was assembled. Each cell was switched to one state or the other, and then a short probing pulse was sent along the boundary—a pulse that does not switch anything itself, only gives the system a gentle shake. An accelerometer on the last cell recorded acceleration over time. It turned out that for all eight possible combinations of states, the response is unique, like a fingerprint. Even if the cells are not identical—with different masses and energy barrier heights—the method works. Tests on a chain of eight cells showed that the stronger the input pulse, the more cells switch, and by counting the switches you can measure the amplitude with an accuracy of three micrometers.
Before this work, mechanical memory was like a book you could write in but not read without opening it. Now there is a way to read it by the sound of flipping pages, without touching the pages themselves. Moreover, the same mechanism works as a force sensor and as a wave filter. A mechanical computer without a single transistor comes a step closer: it no longer needs an outside observer with a magnifying glass.
So far, these are chains of a few cells, but the principle is easy to scale. You could line up dozens of cells, and then move to two-dimensional lattices to control waves in space. Questions of durability remain: how many switching cycles can the beam withstand, and will the magnet tire? But it is already clear that mechanical memory read by a wave is not a theoretical toy but a working structure assembled on a lab bench.
The difference in resonance frequencies between states 0 and 1 is 254 Hz, comparable to the difference between the low note of a double bass and the high note of a violin.
The idea of mechanical memory without electronics echoes the computers from Frank Herbert's "Dune," where the ban on thinking machines forced people to create alternative information-processing technologies.