What are time crystals?

Ice crystals are more orderly than flowing water – right? Wrong. There is a so-called translational symmetry in the liquid. That is, no matter where we look in the stream, it always looks the same. However, when the water freezes, its molecules arrange themselves in a fixed lattice. Now, if we look at a particular spot, there is either a molecule there (if we are lucky) – or not (if we caught the interstices of the crystal). The translational symmetry of the system is broken, the physicist says,...