Paul Langevin

Paul Langevin was a French physicist known for his theoretical explanation of the magnetic behavior of materials, developing a mathematical theory of paramagnetism, in which a material becomes weakly magnetized in the direction of an applied magnetic field, and diamagnetism, in which a material becomes weakly magnetized in the opposite direction, based on the behavior of the magnetic moments of individual atoms. During the First World War, Langevin pioneered the practical use of ultrasound, high-frequency sound waves beyond the range of human hearing, for detecting submarines underwater, developing early piezoelectric transducer techniques that became the basis for sonar systems used ever since. Langevin also introduced what is now called the Langevin equation, a way of mathematically describing the erratic motion of a particle subject to both a steady force and a rapidly fluctuating random force, an approach that became a standard tool for modeling systems affected by random noise across physics and other fields. He was a close friend and scientific collaborator of Marie Curie and remained active in French scientific institutions and left-wing political causes throughout his career, including opposing the rise of fascism in the 1930s.