On ne peut pas parler de Louis sans parler de son frère Maurice

Son frère Maurice, ancien élève de l'École navale, s'était tourné vers la physique dans les premières années du siècle. Il avait créé un laboratoire privé à Paris, dans la maison familiale, dans lequel furent montées diverses expériences de physique atomique, en spectrographie des rayons X notamment. En 1911, il fut le secrétaire du premier congrès Solvay, à Bruxelles.

Louis de Broglie fut ainsi initié aux meilleures sources des derniers développements d'une science qui le disputait, dans ses études, à l'histoire.

Né à Dieppe, le 15 août 1892.Pour étrange que cela puisse paraître, l’un de nos plus brillants physicien s’orienta d’abord, après des études secondaires au lycée Jeanson-de-Sailly, vers un cursus de lettres. C’est après une licence d’histoire, qu’il se tourna vers les mathématiques et la physique. Au moment de la guerre, il fut affecté aux services de radio de l’armée. Devenu « le télégraphiste de la tour Eiffel », il y vécut pendant toute la durée des hostilités, chargé de manipuler la T.S.F. et de capter les messages allemands pour les déchiffrer.La fin de la guerre lui permit de reprendre ses études de mathématiques et de produire en 1924 sa thèse consacrée aux Recherches sur la Théorie des quantas. Ses travaux devaient le conduire à créer, à partir de ses découvertes sur les théories mathématiques de la lumière, une nouvelle branche de la physique : la mécanique ondulatoire qui embrassait dans une vaste synthèse la physique de la matière et celle de la lumière. Il obtint pour cette découverte le prix Nobel de physique en 1929, et fut en 1933 le plus jeune membre de l’Académie des Sciences, dont il allait devenir en 1942 secrétaire perpétuel. Physicien génial, Louis de Broglie se montra tout aussi soucieux de mener à bien une réflexion philosophique sur la valeur des découvertes de la science moderne. On lui doit à ce titre, en marge de ses publications strictement scientifiques, plusieurs ouvrages, et notamment : Matière et Lumière, Sur les sentiers de la science, Certitudes et incertitudes de la science.Le prince Louis de Broglie fut élu à l’Académie française le 12 octobre 1944 dans des circonstances singulières. En effet, le minimum de 20 votants exigé par le règlement ne put être atteint compte tenu des décès, emprisonnements et autres absences liées à la guerre. Il n’y eut donc, fait exceptionnel, que 17 académiciens pour élire ce jour-là, à l’unanimité, le prince de Broglie, au fauteuil d’Émile Picard. L’élection de Louis de Broglie marquait ainsi la reprise des votes à l’Académie.L’entrée de Louis de Broglie sous la coupole fut l’une des plus émouvantes, car il y fut reçu par son propre frère, le duc Maurice de Broglie, le 31 mai 1945, ce qui ne s’était jamais vu depuis trois cents ans.En 1961, le prince de Broglie était élevé à la dignité de Grand-Croix de la Légion d’honneur.Mort le 19 mars 1987.

L'expérimentation confirme la théorie
L'audace théorique de ce dernier fut récompensée en 1927, lorsque Clinton Davisson et Lester Germer, ainsi que George Thomson en Écosse, lui donnèrent une remarquable confirmation expérimentale. Observant la diffraction d'électrons par un cristal de nickel, ils montrèrent que les maxima correspondaient exactement à l'onde calculée par Louis de Broglie. Celui-ci reçut le prix Nobel de physique en 1929. Depuis l'année précédente, il enseignait au tout nouvel Institut Henri-Poincaré à Paris.

La nouvelle mécanique ondulatoire, puissamment développée par ailleurs par Erwin Schrödinger, posait un problème d'interprétation identique à celui de la dualité onde-corpuscule attribuée à la lumière. Plus précisément, il s'agissait de concevoir le rapport de la particule, en tant que lieu de concentration de l'énergie, à une onde qui, en soi, n'offre aucune singularité sous ce rapport. Parmi diverses solutions, de Broglie inclinait pour la théorie dite de la double solution, consistant à admettre une onde réelle doublée d'une solution de l'équation de Schrödinger, simple représentation statistique. Mais au congrès Solvay de 1927, il se rallia à la position majoritaire, qui ne voulait voir dans l'onde que sa valeur probabiliste.

L'interprétation de la théorie
En 1934, il proposa une conception du photon qui, par généralisation aux autres particules, devint la théorie générale des particules à spin. Le photon résulterait de la fusion de deux particules de spin 1/2. Mais le principal des recherches de de Broglie continua de porter sur l'interprétation à donner à la mécanique ondulatoire. L'«interprétation de Copenhague», c'est-à-dire la position majoritaire, celle de Bohr et de Heisenberg, ne faisait pas l'unanimité. Einstein, notamment, tenait pour insuffisante l'interprétation probabiliste de la mécanique quantique. Louis de Broglie redonna la préférence à sa théorie de la double solution, cherchant une description foncièrement déterministe dont la mécanique quantique n'aurait été que la traduction statistique. Il envisagea par exemple l'existence d'un milieu subquantique responsable des variations aléatoires de l'état des particules, ainsi que l'existence de variables cachées.

Après la Seconde Guerre mondiale, il fut conseiller du CEA (Commissariat à l'Energie Atomique). Reçu à l'Académie française en 1944, il était membre de l'Académie des sciences depuis 1933. Il fut le secrétaire perpétuel de celle-ci à partir de 1942. Il écrivit de nombreux ouvrages, dont plusieurs de vulgarisation, ce qui lui valut le titre le prix Kalinga 1952 du Conseil économique et social des Nations unies.

When in 1920 I resumed my studies ... what attracted me ... to theoretical physics was ... the mystery in which the structure of matter and of radiation was becoming more and more enveloped as the strange concept of the quantum, introduced by Planck in 1900 in his researches into black-body radiation, daily penetrated further into the whole of physics. "

Taking up research in mathematical physics, de Broglie nevertheless maintained an interest in experimental physics. His brother Maurice de Broglie was at that time carrying out experimental work on X-rays and this proved a considerable interest to de Broglie during the first few years of the 1920s during which he worked for his doctorate. De Broglie's doctoral thesis Recherches sur la théorie des quanta (Researches on the quantum theory) of 1924 put forward this theory of electron waves, based on the work of Einstein and Planck. It proposed the theory for which he is best known, namely the particle-wave duality theory that matter has the properties of both particles and waves.

In a lecture de Broglie gave on the occasion when he received the Nobel Prize in 1929 he explained the background to the ideas contained in his doctoral thesis : - "Thirty years ago, physics was divided into two camps: ... the physics of matter, based on the concepts of particles and atoms which were supposed to obey the laws of classical Newtonian mechanics, and the physics of radiation, based on the idea of wave propagation in a hypothetical continuous medium, the luminous and electromagnetic ether. But these two systems of physics could not remain detached from each other: they had to be united by the formulation of a theory of exchanges of energy between matter and radiation. ... In the attempt to bring the two systems of physics together, conclusions were in fact reached which were neither correct nor even admissible when applied to the energy equilibrium between matter and radiation ... Planck ... assumed ... that a light source ... emits its radiation in equal and finite quantities - in quanta. The success of Planck's ideas has been accompanied by serious consequences. if light is emitted in quanta, must it not, once emitted, possess a corpuscular structure? ... Jeans and Poincaré [showed] that if the motion of the material particles in a source of light took place according to the laws of classical mechanics, then the correct law of black-body radiation, Planck's law, could not be obtained."

During an interview in 1963 de Broglie described how, given the above background, his discoveries came about : - "As in my conversations with my brother we always arrived at the conclusion that in the case of X-rays one had both waves and corpuscles, thus suddenly - ... it was certain in the course of summer 1923 - I got the idea that one had to extend this duality to material particles, especially to electrons. And I realised that, on the one hand, the Hamilton-Jacobi theory pointed somewhat in that direction, for it can be applied to particles and, in addition, it represents a geometrical optics; on the other hand, in quantum phenomena one obtains quantum numbers, which are rarely found in mechanics but occur very frequently in wave phenomena and in all problems dealing with wave motion. "

The wave nature of the electron was experimentally confirmed in 1927 by C J Davisson, C H Kunsman and L H Germer in the United States and by G P Thomson (the son of J J Thomson) in Aberdeen, Scotland. De Broglie's theory of electron matter waves was later used by Schrödinger, Dirac and others to develop wave mechanics.

After his doctorate, de Broglie remained at the Sorbonne where he taught for two years, becoming professor of theoretical physics at the Henri Poincaré Institute in 1928. From 1932 he was also professor of theoretical physics at the Faculté des Sciences at the Sorbonne. De Broglie taught there until he retired in 1962. From 1944 he was a member of the Bureau des Longitudes. In 1945 he became an adviser to the French Atomic Energy Commissariat (CEA).

His greatest honour was being awarded the Nobel Prize in 1929. We have quoted above from his lecture given at the award ceremony. Let us quote further from the lecture : - "Thus I arrived at the following general idea which has guided my researches: for matter, just as much as for radiation, in particular light, we must introduce at one and the same time the corpuscle concept and the wave concept. In other words, in both cases we must assume the existence of corpuscles accompanied by waves. But corpuscles and waves cannot be independent, since, according to Bohr, they are complementary to each other; consequently it must be possible to establish a certain parallelism between the motion of a corpuscle and the propagation of the wave which is associated with it. "

After receiving the Nobel Prize in 1929 De Broglie worked on extensions of wave mechanics. Among publications on many topics he published work on Dirac's theory of the electron, on the new theory of light, on Uhlenbeck's theory of spin, and on applications of wave mechanics to nuclear physics. He wrote at least twenty-five books including Ondes et mouvements (Waves and motions) (1926), La mécanique ondulatoire (Wave mechanics) (1928), the last three mentioned books were published in English translations as Non-linear Wave Mechanics: A Causal Interpretation (1960), Introduction to the Vigier Theory of elementary particles (1963), and The Current Interpretation of Wave Mechanics: A Critical Study (1964).

He wrote many popular works which demonstrate his interest in the philosophical implications of modern physics, including Matter and Light: The New Physics (1939); The Revolution in Physics (1953); Physics and Microphysics (1960); and New Perspectives in Physics (1962).

In 1933 de Broglie was elected to the Académie des Sciences becoming Permanent Secretary for the mathematical sciences in 1942. The Académie awarded him its Henri Poincaré Medal in 1929 and the Albert I of Monaco Prize in 1932. Other honours which he received included the Kalinga Prize which was awarded to him by UNESCO in 1952 for his efforts towards the understanding of modern physics by the general public. The French National Scientific Research Centre awarded him its gold medal in 1956. Further honours included the awarding of the Grand Cross of the Légion d'Honneur and Belgium made him an Officer of the Order of Leopold. He received honorary doctorates from the Universities of Warsaw, Bucharest, Athens, Lausanne, Quebec, and Brussels. He was elected to honorary membership of eighteen academies and learned societies in Europe, India, and the United States.

De Broglie described himself as : - " ... having much more the state of mind of a pure theoretician than that of an experimenter or engineer, loving especially the general and philosophical view ... . "

The central question in de Broglie's life was whether the statistical nature of atomic physics reflects an ignorance of the underlying theory or whether statistics is all that can be known. For most of his life he believed the former although as a young researcher he had at first believed that the statistics hide our ignorance. Perhaps surprisingly, he returned to this view late in his life stating that : - " ... the statistical theories hide a completely determined and ascertainable reality behind variables which elude our experimental techniques."

Let us end our biography with the tribute paid to de Broglie by C W Oseen, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences : - " When quite young you threw yourself into the controversy raging round the most profound problem in physics. You had the boldness to assert, without the support of any known fact, that matter had not only a corpuscular nature, but also a wave nature. Experiment came later and established the correctness of your view. You have covered in fresh glory a name already crowned for centuries with honour. "