He and Shapere showed that a material could have zero total energy yet still be in motion. They did so by mathematically reformulating the ordinary definition of kinetic energy (one-half mass times velocity squared) to a different but equally valid value that depends on a velocity in an alternative way (for instance, adding an additional term such as velocity to the fourth power and changing the sign of the usual kinetic energy).
"I'm just very surprised at this," says theoretical physicist Maulik Parikh of Arizona. "Frank found subtle exceptions that link motion and the state of being at minimum energy."
Carroll agrees: "It's amusing to find a system that features motion in its ground state, but it certainly doesn't violate any truly cherished beliefs of physics. I'm ready to believe that such a system could even be constructed in the real world."
The motion of the crystal "'spontaneously breaks' time translation symmetry, even if the theory itself does not contain any preferred time direction," says Cristian Armendáriz Picón of Syracuse University, who has studied the possibility of similar phenomena in cosmology.
Once set in motion, a time crystal could remain in motion forever, with no outside force needed to keep it going. This type of perpetual motion machine would not violate any known physical law because no energy could be extracted from the system without first adding energy. Such systems might even be arranged to convey information that would persist after everything else around them has died.
Carroll cautions, however, that a time crystal may not survive indefinitely. Even if the crystal has the minimum possible energy, it might not have the highest possible entropy, or disorder; a crystal blown up into individual particles and spread across space would have higher entropy. If so, the time crystal would eventually suffer such a fate, because the universe always evolves toward higher entropy. "I don't think it's realistic to expect such a thing to last literally forever—but much longer than anything else is quite conceivable," Carroll says.
The closest that modern technology has come to a time crystal, Wilczek says, is a current-carrying superconductor, a material that carries a moving, persistent current at low temperatures. In an ordinary superconducting cable, the current is constant, and if nothing actually changes with time, the superconductor does not qualify as a true crystal. But if engineers could construct a superconductor with a lumpy rather than uniform distribution of charged particles, then as the current flows, the lumps move, and the persistent current would change with time.
The concept of a time crystal, Armendáriz Picón says, may shed light on how natural phenomena that are asymmetric with time can be described in terms of symmetric theories. It could even apply to the origin and evolution of the universe. "One can think of these [time] crystals as a new form of matter, and this matter may be responsible for ill-understood phenomena such as the current stage of accelerated cosmic expansion," he says.
"It's early days" for the theory, Wilczek cautions. For physicists, time crystals are "like discovering a new continent," he said in a recent talk. But, he adds, whether that new continent is "a New World, or Antarctica, time will tell."