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Time crystals: I’d hoped I’d never write about them again.

A long time ago, there was a theory paper that proposed a new idea—time crystals.
I was intrigued, but I was also confused.

This either seemed like a trivial idea, or one so deep I didn’t fully grasp it. Now, it seems I might not have had a clue, as two research groups have reported producing actual time crystals.
Time crystals are not made of time
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).
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