As every young science student knows, moving objects have kinetic energy. But just how much energy does something need to move? In a new study, a pair of physicists has shown that it’s theoretically possible for a system in its lowest energy state, or ground state, to exhibit periodic motion. This periodically moving system can be thought of as the temporal equivalent of a crystal, which is defined by its spatial periodicity. What’s even more intriguing about these “time crystals” is that, by exhibiting motion at their state of lowest energy, they break a fundamental symmetry called time translation symmetry and become “perilously The physicists, Frank Wilczek of MIT (a 2004 Nobel Laureate) and Alfred Shapere of the University of Kentucky, have posted two papers on their novel idea of time crystals at arXiv.org. One paper focuses on classical time crystals, while the other looks at quantum time crystals.

Modern physics deals with many types of symmetry, but one of the most fundamental is time translation symmetry, which basically says that the laws of physics we have today should still be here tomorrow. Likewise, if a system’s features remain constant over time, that system obeys time translation symmetry. On the other hand, a clock, which has hands that are constantly moving, breaks time translation symmetry.

But while a clock requires a power source to break this symmetry, a system in its lowest energy state has no power source by definition. To show that such a system can indeed exhibit motion, Wilczek and Shapere performed some complex mathematical calculations, which revealed that a system in its lowest energy state could move in a periodic motion, such as a loop or orbit. As far as the scientists know, this is the first time that a system in its lowest energy state without any external source of power has been shown to exhibit motion and break time translation symmetry.

via Time crystals could behave almost like perpetual motion machines.