Parent is probably referencing https://en.wikipedia.org/wiki/Time_crystal Because a time crystal is a driven (i.e. open) quantum system that is in perpetual motion, it does not violate the laws of thermodynamics... does not produce work...does not spontaneously convert thermal energy into mechanical work...cannot serve as a perpetual store of work... A time crystal has been said to be a perpetuum mobile of the fourth kind: it does not produce work and it cannot serve as a perpetual energy storage. But it rotates perpetually.