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Louis de Broglie
Discoverer of the Wave Nature of Particles
 
 
 
Dr Subodh Mahanti

 

“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, I was drawn towards the problems of atomic physics…It was the conceptual difficulties which these problems raised; it was the mystery which surrounded that famous Planck’s constant, h, which measures the quantum of action; it was the disturbing and badly-defined character of the dualism of waves and corpuscles, which appeared to assert itself more and more in the realms of physics…”

Louis de Broglie



Prince Louis-Victor-Pierre-Raymont, 7th duc de Broglie, generally known as Louis de Broglie, is best known for his research on quantum theory and for his discovery of wave nature of electrons. He was awarded the 1929 Nobel Prize for Physics “for his discovery of the wave nature of electrons.” The wave-like behaviour of particles discovered by de Broglie, was used by Erwin Schrodinger in his formulation of wave mechanics.
Louis de Broglie was born in Dieppe (Seine-Maritime), France on August 15, 1892. He was the younger son of Victor, 5th duc de Broglie. His mother was Pauline d’Armaille.The de Broglie family is one of the most illustrious families of France. De Broglies came to serve the French kings in the seventeenth century. The family came from Piedmont, then in France and now in Italy. In 1740 Louis XIV had conferred on the head of the family the hereditary title of duc (duke). The German title Prinz (Prince) dated in the family from service to the Austrians during Seven Years War (1756-63). The de Broglies achieved high distinctions as politicians, diplomats and soldiers. Four members of de Broglies became Marshals of France. The French Revolution was a difficult and trying time for de Broglie family and at least one de Broglie died at the guillotine. The family remained prominent in French public life throughout the nineteenth century.

De Broglie was exceptionally charming in his childhood. His elder sister wrote: “This little brother had become a charming child, slender, svelte, with small laughing face, eyes full of mischief, curled like a poodle. Admitted to the great table, he wore in the evenings a costume of blue velvet, with breeches, black stockings and shoes with buckles, which made him look like a little prince in a fairy tale. His gaiety filled the house. He talked all the time even at the dinner table where the most severe injunctions of silence could not make him hold his tongue, so irresistible were his remarks. Raised in relative loneliness he had read much and lived in the unreal….he had a prodigious memory and knew by heart entire scenes from the classical theatre that he recited with inexhaustible verve….he seemed to have a particular taste for history, in particular political history….hearing our parents discuss politics he improvised speeches inspired by the accounts in the newspapers and could recite unerringly complete lists of ministers of the Third Republic, which changed so often…a great future as a statesman was predicted for Louis.”

De Broglie was educated at home by private tutors. In 1906, his father died. His elder brother Maurice, then at 31, took charge of his upbringing. It was at Maurice’s advice he was sent to Lycee Janson de Sailly, where he spent three years before completing his secondary school education in 1909. Maurice wrote: “Having experienced myself the inconvenience of a pressure exercised on the studies of a young man I refrained from imparting a rigid direction to the studies of my brother, although at times his vacillation gave me some concern. He was good at French, history, physics, philosophy, indifferent in mathematics, chemistry and geography, poor in drawing and foreign language.”

On completion of school education, de Broglie joined the University of Sorbonne. At the time of joining the university, he had no definite plan for a career. He was not attracted to the idea of a military or diplomatic career. At the beginning he studied history but he did not like the uncritical way the history was taught those days. From history he shifted to law with a view to make a career in the civil service. At the age of 18 he graduated with an arts degree. He was then assigned a research topic in history of his choice. But he did not complete his research in history. Instead he decided to study theoretical physics, a subject he chose to devote his life to. In deciding to study physics he was greatly influenced by Poincare’s masterworks, La valeur de la science and La science et l’hypothese. In choosing theoretical physics as a career, he was also influenced by his elder brother Maurice, who was also a physicist and made notable contributions to the experimental study of the atomic nucleus. Maurice kept a well-equipped laboratory at the family mansion in Paris. However, his journey in theoretical physics was not very smooth. In those days at the Sorbonne University the teaching of physics did not include the recent developments in the subject like Maxwell’s electromagnetic theory or statistical thermodynamics. The course was based on standard subject like mechanics and wave optics. Books on these topics were also not available in French. French translations of foreign textbooks were often of poor quality. De Broglie made it a point to attend Poincare’s lectures on electrodynamics, thermodynamics, celestial mechanics and other subjects. In his initial years he passed through emotional and psychological problems. Apparently this was triggered by the marriage of his elder sister Princess Pauline, to him he was deeply attached. Princess Pauline was 20 years older than de Broglie. After her marriage de Broglie lost her youthful personality full of gaiety and spirits. He lost an examination in general physics. Following these developments when his self-confidence at its lowest, he chanced upon reading the report of the first Solvay Conference on quantum theory. At the end of reading this report in depth he was confident that theoretical physics would be his career. In 1913 de Broglie obtained a science degree.

After attaining the required age, he had to join the military service as it was mandatory for everyone in France. He had to stay in army for six years as the First World War broke out. Initially he was sent to the fort at Mont Valerien, where he had nothing much to do and it was a very difficult situation for him. However, his brother exerting his influences got him transferred to the radiotelegraphy section situated at the bottom of the Eiffel Tower, on which a radio transmitter had been installed. De Broglie served as telegraph operator. Commenting on his war-time work de Broglie later commented that he “was able to serve his country while working as an electrician, taking care of machines and wireless transmissions and perfecting heterodyne amplifiers then in their infancy.” De Broglie later admitted that the practical experience gained during the war time helped him in his scientific research. After the war was over, de Broglie resumed his studies of physics with his elder brother, Maurice, who worked on experimental physics in his well-equipped laboratory in the family mansion in Paris. Unlike his brother, de Broglie was interested in the theoretical aspect of physics. Immediately after he was decommissioned from the army, he attended a seminar given by Langevin on quantum theory and then a course on relativity. De Broglie wrote: “...demobilized in 1919 I returned to the studies I had given up, while following closely the work pursued by my brother in his private laboratory with his young collaborators on X-ray spectra and on the photoelectric effect. Thus I made my first steps towards research by publishing a few results in the fields studied by my brother.

In a first series of publications I considered the adsorption of X-rays, its interpretation by the theory of Bohr, and its relation with thermodynamic equilibrium…some of the reasonings I used were questionable but they led me to formulae which gave an acceptable account of the facts. At the same time I had long discussions with my brother on reinterpretation of the beautiful experiments that he pursued on the photoelectric effect and corpuscular spectra. I published, with him or separately, a series of notes on the quantum theory of these phenomena which, although classical now, was not well established then.”

In 1923 de Broglie brilliantly brought together the concepts of the particle and the wave. He was influenced by Einstein’s work on particle nature of light. De Broglie wrote: “After long reflection in solitude and meditation, I suddenly had the idea, during the year 1923, that the discovery made by Einstein in 1905 should be generalized by extending it to all material particles and notably to electrons.” At the beginning of the twentieth century physicists explained physical phenomena in terms of particles like electrons or protons and electromagnetic radiation like light, ultraviolet radiation etc. While particles were visualized as discrete entities forming atoms and molecules but electromagnetic radiation were conceived as a wave motion involving changing electric and magnetic fields. This conventional visualization of the physical world was changed by Einstein’s work. The special theory of relativity founded by Einstein showed that matter itself was a form of energy. While explaining the photoelectric effect, Einstein proposed that electromagnetic radiation, a wave, can also behave as particle (photon).

De Broglie, influenced by Einstein’s work, proposed that just as waves can behave as particles, for instance electrons, can also behave as it were a wave motion (a de Broglie wave) with wavelength h/p, where p is the momentum of the electron and h is Planck’s constant. He summed up his discovery in the following words: “Because the photon, which, as everyone knows, is a wave, is also a particle, why should not the electron (or any material particle) also be a wave?” His revolutionary idea was put forward in his doctoral thesis of 1924, entitled Recherches sur la theorie des quanta (Research on Quantum Theory). It contained the idea of matter waves. The thesis was published as a paper of over 100 pages in Annales der Physique in 1925. Today it may seem to be very logical to think that way but for de Broglie it was a daring act. This was rightly taken note of by the Nobel Committee. In its citation noted: “When quite young, you threw yourself into the controversy raging over the most profound problem in physics. You had the boldness to assert, without the support of any evidence whatsoever, that matter had not only a corpuscular nature but also a wave nature. Experiments came later and established the correctness of your view.”

The idea which later proved to be of far-reaching implications was taken very seriously by many scientists. Ralph Fowler reported de Broglie’s discovery to the British scientific journals and Langevin apprised Einstein of the development, who in turn reported it to Berlin Academy of Sciences. These developments ensured rapid spread of the “bizarre” ideas of de Broglie, till then an obscure theoretical physicist and mostly known to the scientific community as “Maurice’s younger brother.” Einstein was very sympathetic to de Broglie’s idea. He wrote to Langevin: “Louis de Broglie’s work has greatly impressed me. He has lifted a corner of the great veil. In my work I obtain results which seem to confirm his. If you see him please tell him how much esteem and sympathy I have for him.”

The experimental verification of de Broglie’s discovery of wave nature of particles became feasible after Walter Elsasser, a graduate student at the University of Gottingen, suggested that like X-rays, electrons could be diffracted by a crystals. Compared to the spacings between atomic layers in crystal, the wavelengths of de Broglie’s waves corresponding to high speed electrons were shorter. Thus for de Broglie’s waves, a crystal lattice would serve as a three-dimensional diffraction gratings and sharp peaks in the intensity of the diffracted beams should occur at specific angles. This was actually experimentally verified in 1927 by Clinton Davisson and Lester Germer at the Bell Labs in New York City and by George Paget Thomson at the University of Aberdeen, Scotland.

The fact that particles can behave as waves and radiations (waves) can behave as particles is called wave-particle duality. This has caused intense debate as to the “real” nature of particles and electromagnetic radiations—whether there is determinacy in quantum mechanics. De Broglie himself believed that there is true determinable physical process underlying quantum mechanics and that the current indeterminate approach in terms of probability can be replaced by a more fundamental theory.

After receiving his PhD degree of the Sorbonne University in 1924 de Broglie completed two years’ free lectures at the Sorbonne University before he was appointed Professor of Theoretical Physics at the Henri Poincare Institute, which had just been built in Paris with the purpose of teaching and developing theoretical physics. In 1932, he was appointed to the chair of theoretical physics at the Faculty of Sciences of the Sorbonne University, where he taught till 1962. Joan James, in his Remarkable Physicists: From Galileo to Yukawa, wrote: “For thirty-three years duc Louis lectured at the Sorbonne. He took a very exalted view of his duties as a teacher, the books that originated from his lectures, beautifully written and carefully produced, brought instruction and enlightenment. In his teaching he took great care when presenting his own ideas to explain that they were not generally accepted. However, as a lecturer in the classroom he was uninspiring. Starting scrupulously on time, he read in his high-pitched voice and in a somewhat monotonous tone from abruptly at the end of the hour and departed immediately. He also ran a well-attended weekly seminar at which young and not so young theorists could expound their ideas.” Many students from France and other countries came to work with him and a large umber of PhD theses were prepared under his guidance.

Recalling the origins of his discovery, de Broglie in a lecture delivered in 1945 said: “Thirty years ago, physics was divided into two camps:…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 formation of a theory of exchanges of energy between matter and radiation …the intervention of quanta and of Planck’s constant h, as much in the theory of photons as in that of the quantization of the electronic movements, seemed to me to show clearly that the link between the two terms of the wave-corpuscle dualism took place through the intermediary of the quantum of action, and must in consequence be expressed mathematically for formulae in which the constant h would appear. This was already the case for the relations which, in the theory of the photon, expressed the energy and momentum of the corpuscle of light as a function of the frequency and of wavelength of the luminous wave, and the form of these relations gave an indication of the interaction that had to be established in the general case of any corpuscle whatever…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 corpuscle accompanied by waves. But corpuscles and waves cannot be independent, since, according to Bohr, they wre 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 his path-breaking discovery de Broglie’s work chiefly devoted to various extensions of wave mechanics—Dirac’s theory of the electron, the new theory of light, Uhlenbeck’s theory of spin, applications of wave mechanics to nuclear physics, etc. Towards the later part of his scientific career de Broglie worked towards developing a causal explanation of wave mechanics, in opposition to the wholly probabilistic models which dominate quantum theory but he had to abandon it in the face of severe criticism of fellow scientists.

It was Louis de Broglie, who in 1949 at the Lausanne European Cultural Conference, issued the first high level call for establishing multinational laboratory as an instrument to revive European research. He was joined by Raoul Dautry, administrator general of the French Atomic Energy Commission; Pierre Augur, Director of UNESCO’s Department of Exact and Natural Sciences (1948-1959); and Edoardo Amald, one of the founders of Italy’s National Institute for Nuclear Physics. This led to establishment of the European Organisation for Nuclear Research (CERN).
In 1933, de Broglie was elected a member of the French Academy of Sciences. He became a permanent secretary of the Academy at the age of fifty, a post he held till he resigned at the age of 83. But he continued to be associated with the Academy in an honorary capacity till his death. The Academy awarded him its Henri Poincare Medal in 1929 and the Albert I of Monaco Prize in 1932. The French National Scientific Research Centre awarded him its gold medal in 1956. Among other awards received by him included Grand Cross of the Legion d’Honneur of France and Officer of the Order of Leopold of Belgium. He was among the few scientists to be elected to the literary Academy of France. He was elected a foreign associate of the US National Academy of Sciences in Washington and a foreign member of the Royal Society of London. In 1960, upon the death without heir of his elder brother, Maurice, 6th duc de Broglie, Louis de Broglie became the 7th duc de Broglie.

De Broglie published more than 25 books on various subjects of physics. Some of his important publications are: Waves and Motions (1926), Wave Mechanics (1928), 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). In addition to his strictly scientific work De Broglie wrote on popular aspects of physics, and philosophy of science including the value of modern scientific discoveries. Among his popular books on physics included Matter and Light: The New Physics (1939); The Revolution in Physics (1953), Physics and Microphysics (1960) and New Perspectives in Physics (1962). In 1952, de Broglie was awarded the first Kalinga Prize by UNESCO for his efforts to explain modern physics to laymen.
Louis de Broglie died on March 19, 1987 in Louveciennes (Yvelines).

References

  1. 100 Years with Nobel Laureates. New Delhi: Encyclopaedia Britannica (India) Pvt. Ltd, 2001.

  2. Dardo, Mauro. Nobel Laureates and Twentieth Century Physics. Cambridge: Cambridge University Press, 2004.

  3. Heilbron. J. L. The Oxford Companion to the History of Modern Sciences. Oxford: Oxford University Press, 2003.

  4. James, Joan. Remarkable Physicists: From Galileo to Yukawa. Cambridge: Cambridge University Press, 2004.

  5. Dictionary of Scientists. Oxford: Oxford University Press,1999.

  6. The Cambridge Dictionary of Scientists (Second Edition). Cambridge: Cambridge University Press, 2002.

  7. Louis de Broglie—Biography. Nobelprize.org

  8. Louis Victor Pierre Raymond duc de Broglie by J. J. O’Connor and E. F. Robertson. www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Broglie.html.