Paul Langevin the Scientist

“Summarizing Paul Langevin’s work implies revising the whole history of physics (…), this is a difficult but exciting task” Louis de Broglie said during Langevin’s funeral. Paul Langevin was a universal mind, and he had worked in all branches of physics: gaseous ions, Brownian motion, electromagnetism, dia and para magnetism, birefringence, special relativity, ultrasonic waves, and neutron collisions. Very rare are those who can pretend to such a scientific impact. Yet at the same time, during a time span that included two world wars, Paul Langevin directed an engineering school ESPCI (Ecole Supérieure de Physique et Chimie Industrielle), gave courses at collège de France, ENS (Ecole Normale Supérieure), and was actively involved in the Solvay Conferences that he organized when Hendrick Lorentz passed away. The last part of the presentation is devoted to the present Paul Langevin legacy. As illustrations, a membrane wave experiment that revisits the 1919 Eddington observation of Albert Einstein prediction is followed by a walking liquid droplet set-up by Yves Couder. Open questions about the unfinished work that Paul Langevin and his scientific Solvay group left us will conclude this presentation.