crystals are made of atoms bound together, those atoms dont move or change unless energy is applied to the crystal which alters the crystal making it a different crystal. in a time crystal, over time those atoms move around or change in some way on their own so in theory they do this without having any energy applied. so far they have only been able to make/simulate time crystals which change after energy has been put into them so they arent fully proven yet, but recent study has found maths that seems to suggest they are actually possible.
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