Perpetual motion ‘time crystals’ may really exist, physicist says:

Remarkable research is out on structures that repeat periodically in the fourth dimension.

The characteristic trait of a time crystal, Wilczek says, is that it moves without consuming or shedding any energy. Instead, it is in a stable, minimum-energy state, just as diamonds and other conventional crystals are. Even so, it is also in a state of perpetual motion.