When time crystals were first proposed in 2012 by physicist Frank Wilczek they seemed like an exotic consequence of quantum mechanics in systems of many interacting particles. Wilczek argued that such systems broke symmetry in time, changing so as to return periodically to the same state just as ordinary crystals exhibit periodicity in space.

Subsequent experimental work has found that quantum time crystals can exist in systems maintained out of equilibrium by some driving force. Now Norman Yao of the University of California at Berkeley and colleagues suggest that time crystals can arise without the need for quantum physics at all. They argue that purely classical systems of oscillators such as coupled pendulums could have the same time-crystal order as their quantum counterparts. What is more, time crystals could be made experimentally, and might even exist in nature.

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