Ralf Röhlsberger

A tuning fork cut from a perfect metal rings at a single frequency. Its sound is as pure as a mathematical formula. But clamp it in a vise or dip it in a viscous liquid, and the tone smears, picks up noise, and loses its sharpness. The nucleus of scandium-45 behaves much the same way when placed in a crystal lattice.

A clock isn't just hands. Every atomic nucleus is a natural metronome: it oscillates with nearly perfect regularity. If you lift one such metronome a couple of meters, its rhythm speeds up ever so slightly — that's time warping, as predicted by Einstein.