Time crystals arise from quantum interactions once thought to prevent their formation

Nature has many rhythms: the seasons result from Earth's movement around the sun, the ticking of a pendulum clock results from the oscillation of its pendulum. These phenomena can be understood with very simple equations. However, regular rhythms can also arise in a completely different way-by themselves, without an external clock, through the complex interaction of many particles. Instead of uniform disorder, a fixed rhythm emerges-this is referred to as a "time crystal."