A Brief History of Time Crystals

A time crystal should break time translation symmetry in the same sense as a spatial crystal breaks spatial translation symmetry. While this is an intuitively appealing idea, it also runs up against the tendency of macroscopic systems to equilibrate which is why perpetual motion machines are not commonly available. I will discuss the history of this idea and how, in the recent efflorescence of non-equilibrium quantum dynamics a phase with a family resemblance to this idea was discovered. It requires working out of equilibrium, generalizing the idea of a phase and breaking a discrete time translation symmetry - but at the end of it all we have a genuinely new macroscopic and surprisingly robust phenomenon. I will also report on the status of its experimental realization.

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