Illumination with green light reveals a time crystal formed in a network of electron spins (red) within the defects of a diamond. [Courtesy: Georg Kucsko].
Time crystals are hypothetical structures that pulse without requiring any energy — like a ticking clock that never needs winding. The pattern repeats in time in much the same way that the atoms of a crystal repeat in space. The idea was so challenging that when Nobel prizewinning physicist Frank Wilczek proposed the provocative concept1 in 2012, other researchers quickly proved there was no way to create time crystals.