George de Hevesy

Born György Bischitz in 1885, the Hungarian who'd become George de Hevesy built his career on a simple, radical idea: mark an atom and watch where it goes. He co-discovered hafnium, but the work that earned him the 1943 Nobel in Chemistry was the development of radioactive tracers — a technique that let scientists follow chemical processes inside living animals for the first time. Metabolism, once a black box, could suddenly be mapped step by step. He died in 1966, having turned invisibility into legibility.