Time crystals make their appearance, no blue police box reported | Ars Technica: "A time crystal is almost analogous to the crystals that you may be more familiar with, like salt and sugar. Salt consists of two atoms (sodium and chlorine) that are arranged in a fixed order in space. In any given direction, there is a characteristic length over which the crystal repeats itself. If I were located somewhere in the middle of a crystal and moved in any direction by the characteristic length (or a multiple of it), I would not actually be able to tell that I had moved. That is translational symmetry in space. This is more important than it sounds. The properties of matter are often dominated by the spatial order of the crystal. When you break that order, cool things happen (melting is an example of breaking spatial order). Rather than order in space, a time crystal has to have order in time. And we all know things that have order in time: the hands on a clock or a playground swing. These all repeat themselves in time. Do they count? No. And the reason is a bit technical."
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