Walther Bothe invented the coincidence method, a technique for detecting subatomic particles with unprecedented precision, and in doing so helped build the experimental foundation for quantum mechanics and nuclear physics. The method was elegant: if two detectors registered a signal at virtually the same instant, the events were likely connected, caused by the same particle or the same interaction. If the signals didn't coincide, they were random background noise. Born in Oranienburg, near Berlin, on January 8, 1891, Bothe studied physics at the University of Berlin under Max Planck. His early career was interrupted by World War I; he spent five years as a prisoner of war in Russia, where he taught himself the Russian language and pursued theoretical physics on his own. He returned to Germany in 1920 and began the experimental work that would define his career. In the 1920s and early 1930s, Bothe and his colleague Hans Geiger used the coincidence method to study Compton scattering, the interaction between photons and electrons. Their experiments provided key evidence that individual photons carry both energy and momentum, supporting the particle nature of light at a time when the wave-particle duality was still deeply contested. Bothe's precise measurements helped settle one of quantum physics' foundational debates. He built and directed Germany's first operational cyclotron at the Kaiser Wilhelm Institute in Heidelberg during World War II. His measurements of the neutron absorption cross-section of graphite contained an error that led the German nuclear program to abandon graphite as a moderator in favor of heavy water, a decision that significantly slowed their progress toward a nuclear reactor. Whether this error was accidental or deliberate has been debated by historians for decades. Bothe shared the 1954 Nobel Prize in Physics with Max Born. By then he was in declining health, suffering from the effects of circulatory disease. He died on February 8, 1957, at 66, in Heidelberg.