Alfred Kastler developed optical pumping, a method for manipulating the energy states of atoms using carefully tuned light, and in doing so laid the essential groundwork for two technologies that now underpin modern civilization: the laser and the atomic clock. Born in Guebwiller, Alsace in 1902, Kastler grew up in a region that had been annexed by Germany after the Franco-Prussian War and returned to France after World War I. He studied physics at the Ecole Normale Superieure in Paris and spent his career at the University of Paris. His key insight, developed through the 1940s and 1950s with his student Jean Brossel, was that you could use polarized light to selectively excite atoms into specific quantum states, then detect what happened when they relaxed back down. This technique, optical pumping, gave physicists precise control over atomic behavior for the first time. Before Kastler's work, studying atomic energy levels required indirect methods with limited resolution. Optical pumping made it possible to prepare atoms in known quantum states and observe their transitions with extraordinary precision. The method directly enabled the development of the maser and then the laser, which amplifies light using the same principle of stimulated emission that Kastler's technique could now control. Atomic clocks, which measure time by tracking the precise frequency of microwave radiation emitted by cesium atoms in specific quantum states, rely on optical pumping to prepare those atoms. The GPS satellites orbiting Earth carry atomic clocks accurate to within a billionth of a second; without that precision, your phone's map would be off by miles. Kastler won the 1966 Nobel Prize in Physics for this work. He was also an outspoken pacifist who opposed French nuclear weapons testing and protested the Vietnam War, unusual stances for a senior French physicist during the Cold War. He died on January 7, 1984, at 81.