A time crystal resembles an eternal clock: its atoms spontaneously perform rhythmic movements. In ordinary clocks, a pendulum is driven by a spring or battery, but here the motion arises without an energy supply — the system simply returns to its initial state at regular intervals. This idea was proposed by Frank Wilczek in 2012. While in a spatial crystal atoms are frozen in a regular lattice, here it’s not the position that repeats in time, but the pattern of motion.
In the experiment, scientists placed a cloud of atoms, cooled to ultralow temperatures, between two mirrors. Under these conditions, the atoms merge into a single quantum object, like one huge particle. Light trapped in the mirror cavity interacted with this cloud, and it spontaneously began to 'tick'—alternately contracting and expanding. The researchers detected photons escaping from the trap and, using photometry and spectroscopy, determined the rhythm of oscillations. Each type of time crystal had its own characteristic light signature, as if different clocks keep time in their own way. Although this resembles a perpetual motion machine, the crystal doesn’t violate physical laws: it doesn’t generate energy but simply exists in an unchanging pulsing state.
Theorists described time crystals in 2012, but the scientific community initially rejected the idea, thinking it violated the laws of thermodynamics.
The idea of structures living in time cycles echoes the plot of the movie 'Groundhog Day'—only here it’s not a day that repeats, but a quantum state.