French physicist best known for establishing the modern theory of magnetism. Langevin explained paramagnetism and diamagnetism in terms of interactions between magnetic fields induced by orbiting electrons and external magnetic forces. He showed how to slow down neutrons, which proved essential for later work on atomic reactors. He is important for introducing and explaining Albert Einstein's theory of relativity to the French scientific community. His echolocation technique for detecting submarines was the basis for sonar.