Proposed four-dimensional crystal will keep perfect time forever

Schematic of creating a space-time crystal. In a googol of years or so, scientists expect the universe to enter a state known as “heat-death,” where there's no thermodynamic free energy remaining and everything's stopped moving. A clock that could keep time even after the heat-death would require a crystal that has periodic structure in time as well as space, which would mean it is a four-dimensional crystal.

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