Obituary notice for M Marcel Brillouin, by M Henri Villat

Henri Villat, President of the Académie des Sciences, delivered the obituary address for Marcel Brillouin on Monday 21 June 1948.

The President expresses himself in these terms:

Our Academy is once again cruelly struck when our great colleague Marcel Brillouin was taken from us on the morning of 16 June, in his 94th year; he was one of our oldest elected members, belonging to the section of General Physics since 21 November 1921.

The career of Marcel Brillouin was a beautiful unity for, when he entered the École Normale, he already was a celebrity in the university world, having obtained, at the Concours Général, the first prize in elementary mathematics in 1873, and the prize of honour for special mathematics in 1874. He therefore seemed destined to be oriented towards mathematics; it was, however, Physics which had his preferences, either as a result of the influence of Pierre Auguste Bertin (then Deputy Director of the École Normale), or as a consequence of a predestination which escapes our weak reasoning.

But what was essential was that Brillouin remained, throughout his career, as much a mathematician as a physicist, and that for the greater good of science. This double competence, so rare, especially at the time when his career began, explains why he succeeded so brilliantly in work which would have been impossible for a scientist who was only a mathematician, or only a physicist. In all his research, the two sciences are intimately linked, and although many of his works are in pure physics, most of them are theoretical physics research, where mathematics plays the essential role of an indispensable auxiliary. As he himself wrote, he "did not seek experimental investigation for the fortuitous revelation of new phenomena." To be less attractive at first sight, the current facts, known for a long time, were for Brillouin of an interest as great as the exceptional phenomena. Thus he devoted great efforts to the study of plasticity and brittleness of solids, as well as to the theory of friction and viscosity.

In hydrodynamics, Brillouin showed how to initiate the in-depth study of discontinuity surfaces introduced by Helmholtz; the instability of these sliding surfaces, so clearly demonstrated by our colleague Jacques Hadamard, made him guess a mode of formation of vortices. Generally speaking, Brillouin's penetrating remarks on this subject were the basis of research carried out up to now by a team of young mathematicians, who have transformed modern hydrodynamics. Brillouin was also concerned with the theory of gases, he gave a remarkable theory of the singular phenomenon of acoustic dispersion observed for the first time by Violle.

In thermodynamics, he undertook the difficult study of permanent deformations, and of general cases where there is no relationship between pressure, temperature and specific heat. The thermodynamic theory of radiation led him to this result, that the medium transmitting the radiation has a specific heat proportional to the cube of the absolute temperature, this is what is called, paradoxically, the specific heat of the vacuum.

We know what was, just a few decades ago, the magnificent development of New Physics, with quantum theory, relativity, and the extraordinary success of spectral theory. In the prodigious effervescence caused by this renaissance, or to put it better, by this birth, Brillouin played a leading role, and his personal contributions were considerable. For example, Brillouin was very much concerned, at that time, with kinetic theory, seeking to find there the solution of the paradoxes which oppose the reversibility of the equations of Mechanics (those which the kinetic theory applies to atoms), and irreversibility of real phenomena of diffusion, viscosity and friction. It was the passage of molecules or atoms through unstable equilibrium positions that seemed to allow him to overcome most of these difficulties.

In addition, as a bold precursor of Wave Mechanics, Marcel Brillouin has shown how it is possible to find the discontinuity of quanta, by a theory, of a continuous nature. Likewise, the theory of Relativity owes to him the solution of essential questions.

Brillouin was still very concerned with the Physics of the Globe and Meteorology. His work on the circulation of the atmosphere, and on the formation of rain, is well known. He made high precision measurements on the curvature of the geoid. His observations in the Simplon tunnel are a real scientific monument, where he examined with a rare critical mind the most diverse points of geodesy, involving the rotation and deformations of the lithosphere and the oceans.

In his later years, the theory of the tides caught his attention. He showed how inadmissible are the usual hypotheses on the thickness of the oceans, as well as on the horizontal speeds independent of the depth, and he underlined how mysterious the question of critical latitudes remained. Long discussions, in which in-depth knowledge of the mathematical methods of partial differential equations plays a primary role, lead him to numerical applications, and he undertook a gigantic effort for this purpose, of which he has given detailed results.

What strikes us most in all the work of M Brillouin, whose main stages have been presented in his courses at the College de France, is, with an ardent faith in effort, the taste for completed work, and above all this concern to understand, and to explain from simple notions, apparently complex facts, without hesitating in front of the multitude, often disconcerting, of the difficulties to be overcome.

To these general considerations, which are too brief, on the beauty of Marcel Brillouin's work, I would now like to add a few personal words, which I hope will not be out of place in this chamber.

It is not without great emotion that I have just undertaken to speak here of my former master how could I forget the day of 1911, when I crossed the well-known courtyard of the College de France, in order to submit to Marcel Brillouin my first manuscript, relating to a question of Hydrodynamics which he had posed, I had just learned from M Charles Maurain, the subject of his most recent lessons. How can I forget the welcome I received from him, and the amount with which his encouragement (today and later) has influenced my entire career? No one can put into words clear enough what the affability of his welcome, the comfort of his friendship, and the stimulus of his example could create around Marcel Brillouin. Thus are formed, between a venerated master and a beginning pupil, these bonds over which time has no hold; thus I had the inestimable privilege of never losing contact with a scholar who was much more than a master, a man of heart whose example remains incomparable.

I was thus able to witness the beginning of old age, accompanied by an incredible spirit of youth, of the great deceased; he had known how to give everything to science and conscience, and nothing to ambition; a precious recompense has been granted to him, in the form of a long and last period of calm goodness, and of moral serenity, which seems given, at the end of their age, to very few of the children of men. He passed on the torch to an admirable family, which was his pride and joy, and he was fortunate to see one of his sons, who had become one of the best theorists of modern physics, succeed him in his chair at the College of France, realising one of its dearest hopes.

Today I salute not only the eminent physicist whom we have lost, but also the man, deeply understanding of all greatness and all human miseries, who knew how to live, and who will remain in our hearts, a light and an example. I respectfully convey to all his family the testimony of the feelings of deep mourning that our Academy is experiencing.

Last Updated January 2021