(1872-1946) French Theoretician, Experimentalist (Acoustics, Condensed Matter), Mathematical Physicist
Paul Langevin, the foremost mathematical physicist of his time in France, is most renowned for his invention of a method for generating ultrasonic waves. Langevin’s invention became the basis of modern techniques of sonar: a method of finding the range and bearing of an object (target) by transmitting high-frequency sounds and detecting their echoes on their return. He also performed important work on paramagnetic and diamagnetic forces.
Langevin was born in Paris on January 23, 1872. He attended the Ecole Lavoisier and the Ecole de Physiques et de Chimie Industrielles, where Pierre Curie was his laboratory supervisor. He entered the Sorbonne in 1891, and took a one-year leave, in 1893, to serve in the military. In 1894, he entered the Ecole Normale Superieure, where he studied under jean-baptiste perrin. In 1897, he won an award that allowed him to spend a year at the Cavendish Laboratory in Cambridge, England, then under the direction of the great atomic physicist ernest rutherford; there he worked under joseph john (j. j.) thomson, the discoverer of the electron.
Langevin received a Ph.D., in 1902, for work on gaseous ionization done partly at Cambridge and partly under Pierre Curie. In Paris, he spent a lot of time in Perrin’s lab and was swept up in the excitement of the early years of the study of radioactivity and ionizing radiation. He joined the faculty of the College de France in 1902 and, two years later, was made professor of physics. He remained there until 1909, when the Sorbonne offered him a similar position.
Langevin’s early work at the Cavendish and the Sorbonne on the analysis of secondary emission of X rays from metals exposed to radiation resulted in his discovery of secondary electrons from irradiated metals. He was also interested in the dynamics of ionized gases, particularly the mobility of positive and negative ions; in 1903, he published a theory for their recombination at different pressures.
In 1905, albert einstein published his groundbreaking paper on special relativity. Langevin would become deeply interested in Einstein’s work on space and time and a firm believer in the theory of the equivalence of mass and energy. However, it was another paper Einstein published that year, on Brownian motion, the incessant random movement of microscopic particles in a liquid, that would influence Langevin’s own seminal work on paramagnetic (weak attractive) and diamagnetic (weak repulsive) phenomena in gases. In 1895, Pierre Curie had shown experimentally that the susceptibility of a paramagnetic substance to an external magnetic field varies inversely with temperature. Ten years later, in 1905, when Langevin produced a model based on statistical mechanics to explain this phenomenon, he was influenced by Einstein’s proposal that Brownian motion was due to imbalances in the forces on a particle resulting from molecular impacts from the liquid. Einstein’s explanation led Langevin to hypothesize that when an externally applied magnetic field was absent, the alignment of molecular moments in a paramagnetic substance would be random; conversely, when such a field was present, the alignment would be nonrandom. The greater the temperature, however, the greater the thermal motion of the molecules, and thus the greater the disturbance to their alignment by the magnetic field. This theory was extremely useful in describing molecular fluctuations in other systems, including in nonequilibrium thermodynamics.
Langevin went on to propose that the magnetic properties of a substance are determined by the valence electrons, the specific orbital electrons that atomic elements have available to share when forming into molecular compounds. Langevin extended his description of magnetism in terms of electron theory to account for dia-magnetism. He showed how a magnetic field would affect the motion of electrons in the molecules to produce a moment that is opposed to the field. This enabled predictions to be made concerning the temperature-independence of this phenomenon and allowed estimates to be made of the size of electron orbits.
In 1911, Langevin, whose work had been instrumental in verifying Einstein’s atomic theories, published one of the earliest popular accounts of relativity, devolution de l’espace et du temps (The evolution of space and time). His scientific career was flourishing, but his personal life was in disarray. Unhappily married and the father of four children, he became romantically involved with marie curie, who had been widowed in 1906, when Pierre Curie was run over and killed by a carriage in the street. Langevin’s wife, Jeanne, arranged for their love letters to be pilfered and published in the French press. Scandal broke out on the eve of the First Solvay Conference in Brussels, which both physicists were attending, nearly overwhelming the burning issues of the new quantum mechanics the conference was called to address. Although his affair with Curie did not last, Langevin obtained a divorce. Later, in 1921, he would become a member of the Solvay International Physics Institute and, in 1928, be elected its president.
Pan-European scientific gatherings such as the Solvay Conference were interrupted with the outbreak of World War I, in 1914. But the war years, when he worked on military technologies, led Langevin to his seminal discoveries on piezoelectricity, the electric current produced by some crystals and ceramic materials when they are subjected to mechanical pressure. Building on the research of other physicists, which showed that the reflection of ultrasonic waves from objects could be used to locate them, he developed an improved technique for accurate detection and location of submarines. His technique was based on the use of high-frequency radio circuitry to oscillate piezoelectric crystals and thus obtain ultrasonic waves at high intensity. Within a few years, this approach led him to a practical system for the echolocation of submarines, which became the basis of modern sonar and is used for scientific as well as military purposes. In 1917, he pioneered the use of the piezoelectric effect, that is, the generation of a small potential difference across certain materials when they are subjected to a stress, as well as vacuum tube amplifiers in underwater sounding equipment, the first use of electronics in this way. In 1918, this new technology enabled him to receive echoes from a submarine as deep as 1800 meters. This work continued after the war, leading to the development of sonar transducers, circuits, systems, and materials.
In 1940, after the German occupation of France, Langevin became director of the Ecole Municipale de Physique et de Chimie Indus-trielle, where he had been teaching in 1902. However, the Nazis soon arrested him for his outspoken antifascist views. He was first imprisoned in Fresnes and later placed under house arrest in Troyes. After the execution of his son-in-law and deportation of his daughter to Auschwitz (which she survived), he was forced to escape to Switzerland in 1944. Langevin returned to Paris later that year and resumed his directorship of his old school. He died soon after in Paris on December 19, 1946. The Institute Max von Laue-Paul Langevin was established in Grenoble, France, in 1967, in honor of him and German physicist max theodor felix von laue, as a symbol of postwar cooperation between France and Germany.
Langevin’s genius as a theoretical physicist was recognized by Einstein, who wrote that Langevin had all the tools for the development of the special theory of relativity at his disposal, and that if he had not proposed the theory himself, Langevin would have done so. Equally talented as an experimentalist, Langevin, through his research on the piezoelectric effect, initiated the modern ultrasonic era.
