Like vinyl records, the strange concept of a time crystal is spinning back into fashion. In 2012, a Nobel Prize–winning physicist proposed that the properties of a system of quantum particles might cycle in time much as a crystal’s pattern of atoms repeats in space, even without the addition of energy, making it a bit like perpetual motion machine. But others soon proved a “no-go theorem” that said such a thing was impossible—and replaced it with a less fantastical definition of a time crystal that researchers soon demonstrated in the lab. But now, two physicists have shown that the original notion of a time crystal is possible after all—at least in theory.

“I think it’s right,” says Frank Wilczek, a theoretical physicist at the Massachusetts Institute of Technology in Cambridge, who dreamed up time crystals but who was not involved with the new work. The new scheme is “one way of getting around the ‘no-go.’” But realizing the system experimentally may be exceedingly difficult, other physicists say.

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