PAUL
LANGEVIN

PAUL LANGEVIN

(1872—1946)

G. S. Landsberg

Paul Langevin has died. This news deeply saddens scientists throughout the world and is felt with particular acuteness by the scientists of the Soviet Union.

A profound and creative mind has passed from life—a scientist who enjoyed international authority. From the ranks of humanity there has disappeared a humanist who boldly raised his voice against social and political injustice. A sincere friend of the Soviet Union and of Soviet science has passed away, one who followed the successes of our state with constant and active sympathy and who found in himself the courage to speak of this aloud.

Paul Langevin was a son of Paris. He was born on January 23, 1872, into the family of a Parisian worker, one of those French workers who unite love for their craft with an interest in knowledge. Not being in a position to continue his own education, Paul Langevin’s father strove to instill in his son «le desir de savoir»—the desire to know.

From his parents, eyewitnesses of the bloody suppression of the Paris Commune, Paul Langevin also received his first lessons in civic duty: “... by their stories,” he says, “they instilled in my heart an aversion to violence and a passionate desire for social justice.”

The city of Paris took care of the education of the young Langevin. The first three schools that he attended were schools of the Paris municipality, and in all three Langevin was exempted from tuition fees. These were: an elementary school, then the Lavoisier School and, finally, the School of Physics and Chemistry—a distinctive educational institution, a technical school in the best sense of the word, organized with funds from the city of Paris on the initiative of the well-known French chemist Schützenberger.

Paul Langevin entered this school as a sixteen-year-old youth (in 1888), and it had an enormous influence on him. This is not difficult to understand if one takes into account that Langevin’s first teacher was Pierre Curie, at that time still a young, twenty-eight-year-old scientist. Of great importance was the system of instruction that prevailed in the school, where the ideas of Schutzenberger were being put into practice; he believed that the best preparation of young people for practical activity in industry could be achieved through serious study of physics and chemistry on the basis of the broad use of experimental work in laboratories.

Langevin not only preserved a feeling of ardent gratitude toward this school, where the beginning was laid for his education and scientific interests, but also devoted to it a considerable part of his time and labor throughout his entire life. From 1903 he replaced Pierre Curie in the post of professor at the school; from 1905 he became director of all its academic life (directeur des études), and from 1925 until his death he remained director of the school. Under Langevin’s leadership the School of Physics and Chemistry was in 1926 transformed into an institution of higher education and came to occupy an outstanding place in the system of France’s higher schools.

It was precisely in the post of head of this school, as indeed in all his professorial activity, that Langevin in practice defended his favorite idea that “to understand is more important than to know” (comprendre vaut mieux, que connaitre). Therefore he strove to ensure that the school should produce not simply connoisseurs of contemporary technology, which progresses rapidly, but people capable of mastering the technology of the future—people who can develop as technical progress develops and even themselves participate in creating this progress, thanks to the solid education they have received.

As a result of this system, young engineers graduating from the School of Physics and Chemistry are, to the highest degree, readily drawn into responsible posts in the corresponding industrial enterprises. Not a few of the school’s pupils take the path of purely scientific research. Langevin himself followed this path. Having graduated from the School of Physics and Chemistry, he decided, on the advice of his teachers, to enter the École Normale Supérieure, the best-known institution of higher education in France, having a humanities and a scientific section, where the mathematical, physical, and chemical sciences are studied.

The examination requirements for admission to the École Normale Supérieure are unusually high. On the other hand, its pupils are fully provided for during their stay at the school, and the diploma of graduation opens a broad road. For Langevin, who had not received a systematic education in secondary school—in particular, had never studied Latin, knowledge of which was necessary for a successful examination—admission to the École Normale Supérieure…

school must have been no easy task. But Langevin’s brilliant abilities helped him overcome all obstacles, and in July 1893 he was admitted to the “Normale,” having passed the competitive entrance examinations first, and in 1897 he graduated from it likewise first in achievement.

His stay at the École Normale Supérieure not only gave Langevin an exceptionally solid and broad education, but also determined his scientific interests. At the same time his friendships were formed with Aimé Cotton, Jean Perrin, and Pierre Weiss, which lasted throughout his life.

He also took part in the experimental work of Jean Perrin, at that time still a young teacher at the school, on the study of the ionization of gases by X-rays. Apparently, from these works arose Langevin’s own later investigations concerning the nature of gaseous ions. The friendly and serious atmosphere of the École Normale Supérieure played no small role in the formation of the young scientist.

Later Cotton recalled with affection “that milieu where the critical mind was held in esteem, where friendly disputes among the students, their joint work, often meant more than classroom teaching.” This atmosphere was preserved also in the intimate meetings of the school’s students with young scientists, who readily gathered in the hospitable home of Pierre Curie and his wife Marie Skłodowska-Curie.

After graduating from the École Normale (in 1897), Paris again came to Paul Langevin’s aid. Langevin was awarded a “scholarship of the City of Paris,” enabling him to spend a year in England, at Cambridge, in the Cavendish Laboratory, then headed by J. J. Thomson.

Recalling with gratitude this new assistance from Paris, Langevin says: “This year was decisive for the orientation of my scientific work.” From this trip Langevin brought back, together with an expansion of his scientific horizon, friendly ties with Rutherford, Wilson, and Thomson himself, whom he regarded as one of his teachers, alongside Pierre Curie and Marcel Brillouin.

After returning from England, Langevin received a teaching post in the department of physics at the Sorbonne, University of Paris. There he prepared his doctoral dissertation, after defending which in 1902 he was invited by Mascart to the Collège de France, and in 1909, after Mascart’s death, was elected professor at the Collège de France, one of the highest scholarly institutions of France.

The Collège de France, founded in 1530 by Francis I as a counterweight to the Sorbonne, which at that time had been a center for the study of theology, is a distinctive institution. The scientific activity of its professors is combined with their teaching activity, which is free in the highest degree, or, better said, educational activity. The professors of the Collège de France have no compulsory courses, no permanent

listeners. They give lectures or entire courses that have the character of public readings, and a free audience gathers for them. Sometimes these are lectures or courses devoted to special questions and drawing a small and select circle of listeners. Sometimes they are readings on general questions—social, political, or philosophical—attracting broad circles and not infrequently turning into “fashionable readings,” attendance at which is a matter of “good tone.”

Of course, Langevin’s lectures belonged to the first type and played an outstanding role for French science. Over the forty years during which all of the new physics was being created and developed, including quantum theory, the theory of relativity, and new ideas in the theory of the atom and nuclear physics, Paul Langevin, with extraordinary talent and brilliance, propagated and explained the flow of new ideas, commenting on them and often supplementing them with the fruits of his own reflections.

These lectures were not published, but their trace remained in the scientific works that appeared under the names of his listeners. All of Langevin’s French contemporaries, from people of his own age (such as Jean Perrin, Cotton, Weiss) to the younger generation, unanimously acknowledged the enormous influence that P. Langevin and his lectures had on them.

The brilliant representative of contemporary French science, Frédéric Joliot-Curie, in his speech addressed to Langevin on the occasion of the celebration of his seventy-third birthday, said: “... Your teaching at the School of Physics and Chemistry and your lectures at the Collège de France had a decisive influence on me and on many of my comrades.” And the ninety-year-old Marcel Brillouin, whom Langevin himself considers his teacher, who drew him to theory, wrote to him on the same occasion of the celebration: “How much you have sown, without suspecting it yourself, by your remarks, which prompted reflection or commentary among some of your listeners, and moreover by no means among beginners! ..”

Summing up his activity, Paul Langevin said with full justification: “My pupils at the Collège de France and at the School of Physics and Chemistry have included a large part of the physicists of whom French science is now proud.”

P. Langevin also enjoyed recognized authority beyond the borders of France. He was an honorary doctor of many foreign universities, a foreign member of the Royal Society in London, and an honorary member of the Academy of Sciences of the USSR. The most striking recognition of his authority on the part of the best representatives of world science was his election in 1928 to the post of president of the Scientific Council of the Solvay International Institute of Physics, in place of the late H. A. Lorentz, who had been the permanent president of the Institute’s Council from the moment of its establishment in 1911.

This institute was a free association of a comparatively small number of the greatest physicists of all countries. Suffice it to say that, together with Lorentz, its membership included: N. Bohr,

M. Curie, P. Langevin, Lord Rayleigh, E. Rutherford, A. Einstein, P. Dirac, P. Ehrenfest, A. Poincaré, W. Heisenberg, L. de Broglie, P. Debye, A. Compton, W. L. Bragg, M. Planck, M. Born, J. Perrin, L. Brillouin, and W. Pauli. The significance of the Solvay Institute for the development of physics was very great. Organized in 1911 for the discussion of the most important and most difficult questions of modern physics, the Institute held several congresses, the proceedings of which contain an exceptionally rich body of material in the form of reports by first-rate scientists and substantive discussions. And it was this brilliant assembly which, after the death of the acknowledged head of theoretical physics at the beginning of our century, H. A. Lorentz, unanimously elected Paul Langevin as its president.

Langevin owed his outstanding position in the eyes of the entire scientific world both to his reputation as one of the best and most brilliant connoisseurs of modern physics and to his major scientific investigations.

First place among these investigations is occupied by his works on the theory of magnetism. There is no doubt that the starting point of these investigations was Langevin’s interest in the question, aroused by the works of Pierre Curie, who had established the fundamental laws of diamagnetism and paramagnetism.

Langevin gave a clear electronic picture of these phenomena and a complete thermodynamic and statistical theory of them. To him belongs the proof of the universality of diamagnetism (even in ferromagnetic bodies) and of its connection with the Zeeman effect; he showed that the numerical smallness of the diamagnetic coefficient is a consequence of the smallness of the electronic orbits in the atom, which, generally speaking, do not change under the action of a magnetic field; he gave a theoretical interpretation of the independence of diamagnetism from temperature, established experimentally by Pierre Curie. Langevin further developed a thermodynamic proof of Curie’s second law, according to which magnetic susceptibility is inversely proportional to the absolute temperature

\[ \left( \chi \sim \frac{1}{T} \right). \]

Of greatest interest is Langevin’s statistical theory of paramagnetism, which gives a clear molecular picture of the phenomenon and makes it possible to derive the value of the paramagnetic moment as a function of absolute temperature, the intensity of the external magnetic field, and the value of the magnetic moment of the atoms or molecules of the substance under investigation (in the vapor phase). Despite a number of objections expressed later, Langevin’s theory retains its significance to this day. The statistical theory of paramagnetism served as a prototype for the application of the methods of statistical mechanics to the study of the properties of matter. Over the 40 years that have passed since the publication of Langevin’s fundamental work (1905), the statistical method developed by him has found numerous and highly fruitful applications. By Langevin himself

It was later used to create the theory of the Kerr effect, subsequently supplemented by M. Born; in the hands of P. Debye it led to the theory of dielectrics; W. Heisenberg applied it in the theory of ferromagnetism, and in our time it is widely used in nuclear physics. The formulas obtained by Langevin for paramagnets, and the analogous formulas for dielectrics (Debye), form the basis for the possibility of the experimental determination of the magnetic and electric moments of molecules, which play so important a role in modern theory. The results theoretically obtained by Langevin in the field of paramagnetism became the starting point for extensive experimental investigations undertaken by P. Weiss and the school he founded, and contributed to the success of Weiss’s work in the field of ferromagnetism.

Thus the great contribution made by French science to the doctrine of magnetism is adorned by three names—Pierre Curie, Paul Langevin, and Pierre Weiss—scientists linked by personal friendship and mutual influence.

The second significant direction whose beginning was laid by Langevin consists in the development of methods for obtaining ultrashort elastic waves with the aid of piezoelectric quartz.

The piezoelectric properties of quartz were discovered by Pierre Curie and his brother Jacques Curie and were used by them to create a method of electrical measurements, which is still widely used at the Curie Institute in Paris. G. Lippmann, on the basis of theoretical considerations, pointed to the possibility of the inverse effect—the change in the dimensions of a quartz crystal under the action of an electric field—and this effect was established by the Curie brothers. P. Langevin applied this property in order to make quartz oscillate at high frequency under the action of an alternating electric field, and obtained short elastic waves (ultrasonic waves). In our time Langevin’s pupils R. Lucas and Biquard (simultaneously with P. Debye and Sears) discovered the diffraction of light by ultrasonic waves, thereby opening the beginning of a large field in which numerous scientists, including Soviet ones, are successfully working, applying the method of ultrasonic waves to important problems of molecular and technical physics.

It is interesting to note that Langevin—this brilliant theorist—not only created an important experimental method, the method of ultrasonic waves, but was also the first to apply it to purely technical questions: underwater signaling, ultrasonic sounding, and the detection of submarines, which apparently played a substantial role also during the World War.

Mention should also be made of the works that arose from the participation of the young Langevin in J. Perrin’s first investigations of the ionization of gases by means of X-rays. Developing these investigations, Langevin discovered the existence of large ions, a thousand times greater in mass than ordinary ions. An important

as a result of these investigations, it was found that precisely two types of ions occur in the atmosphere—ordinary and large—and that intermediate types are not observed. Langevin gave a thermodynamic interpretation of this circumstance and used it to explain an interesting meteorological phenomenon—the existence of two kinds of clouds, separated by a difference in altitude of several kilometers.

The image of Paul Langevin would be incomplete if one did not mention him as a public figure and citizen. Langevin himself said that he considered it “his duty to divide his energies between service to Science and service to Justice.” This awareness of the social obligations of the scientist determined Langevin’s actions and public statements throughout his entire life.

As early as 1900 he joined the best people of France, who had decided to protest against the “Dreyfus affair,” the case of a captain in the French army, a Jew by origin, accused of treason; by this accusation, which subsequently proved false, a tremendous wave of antisemitism was raised in France, against which only a few representatives of the French intelligentsia dared to speak out, among them Paul Langevin.

In 1920, while scientific director of the School of Physics and Chemistry, he published in Humanité an energetic letter of protest against attempts to recruit student youth as strikebreakers. In 1932 he, together with Henri Barbusse and Romain Rolland, took part in organizing the Amsterdam Anti-Fascist Committee. In 1935 he was among the organizers of the Popular Front in France. For more than 20 years he worked in the League of the Rights of Man, and in his final years served as its president.

An active friend of the Soviet Union, Langevin in the last year of his life was president of the “France—USSR” Association. It is interesting to note that Langevin considered one of the causes giving rise to the suffering of humanity to be the circumstance that “the means created by Science are not placed exclusively at the service of Justice.” “That is why,” he says, “from the very beginning I followed the gigantic Soviet experiment with passionate interest, for I felt that the Soviet State was following Justice, relying on Science. As I came to know these guiding ideas better, I gave myself to them with ever greater and greater fullness, culminating in my recent entry into the French Communist Party.”

It should be noted that this public activity took place in a bourgeois state, where every such statement increased the number of Langevin’s embittered and influential enemies.

There is no doubt that precisely this reputation as a “leftist and red” was one of the reasons why Paul Langevin was for a long time not elected to membership in the French Academy of Sciences. He became a member only in 1934. “His foreign friends and ardent admirers …”

physicists,” says E. Cotton, “could in no way understand Langevin’s absence from the French Academy of Sciences”—and he explains it by the old statute of 1795, which limited the number of physicist academicians to six, “as in those times when electric currents were not yet known.” Undoubtedly, the conservative statute of the Academy was a serious obstacle to the election of Paul Langevin, an obstacle that his ill-wishers skillfully used, finding, in the new and relatively rare elections, a more “suitable” candidate.

His deserved reputation as an implacable enemy of every kind of social and political reaction soon brought persecution upon Langevin from the German Nazis who had occupied France. As early as 30/X 1940 Paul Langevin was arrested by the German Gestapo and imprisoned. This was the first arrest among French scientists. Protests by prominent representatives of French science—first and foremost by Joliot-Curie—and a student demonstration on the Place de l’Étoile, which led to the closing of the University of Paris, resulted in Langevin’s release after six weeks’ imprisonment; he was then banished from Paris to Troyes, where he spent 42 months under police surveillance. During this period the Resistance movement in France grew stronger and developed, and repression in France began rapidly to intensify. Paul Langevin’s friends in the Resistance decided that his remaining in France was dangerous for his freedom and even his life, and organized his escape to Switzerland. On September 25, 1944, a few weeks after the liberation of Paris, Paul Langevin returned to his native city and, despite his age and failing health, took an active part in the struggle for the rebirth of France. In addition to all his duties, he was elected a member of the Paris municipal council and became head of the Commission for the Reform of Public Education, which he sought to organize on a broadly democratic basis.

In May 1945, when P. Langevin turned 73, it was decided to celebrate this date, since the German occupation had made it impossible to hold a solemn celebration of his seventieth birthday. This celebration became a grand tribute to Paul Langevin, in which about a hundred delegations from various scientific and public organizations of France and many foreign countries took part.

This tribute was the last in Paul Langevin’s life. He was not fated to live to see his seventy-fifth anniversary, and on December 19, 1946, the life of the scientist and humanist came to an end—the man who said in the twilight of his days: “…throughout my already long life I have known the great joy of understanding, teaching, and acting…”.