# Broglie, Louis (Victor Pierre Raymond) De

# BROGLIE, LOUIS (VICTOR PIERRE RAYMOND) DE

(*b*. Dieppe, France, 15 August 1892; *d*. Louveciennes, France, 19 March 1987),*physics, quantum theory, wave mechanics*.

Louis de Broglie achieved a worldwide reputation for his discovery of the wave theory of matter, for which he received the Nobel Prize for physics in 1929. His work was extended into a full-fledged wave mechanics by Erwin Schrödinger and thus contributed to the creation of quantum mechanics. After an early attempt to propose a deterministic interpretation of his theory, de Broglie joined the Copenhagen school’s mainstream noncausal interpretation of the quantum theory. Stimulated by David Bohm’s revival of his views in 1952, however, de Broglie returned to his early interpretation. Throughout his career, de Broglie wrote an impressive number of specialized books, together with more general accounts aimed at popularizing modern twentieth-century physics and discussing its philosophical issues.

**Early Life.** Louis-Victor de Broglie was the son of Victor, duc de Broglie, and Pauline d’Armaillé; he was the younger of two brothers in a family of five children. His family, from a noble Italian (Piedmont) lineage, the Broglia, settled in France when Francesco Maria Broglia followed Cardinal Jules Mazarin in the seventeenth century. After that time, the family served the French kings and then the French state in military and diplomatic affairs. With the brothers Maurice and especially Louis, it added to its famous representatives (among them three state marshals) two world-rank physicists of the twentieth century.

In 1901, Louis’s family moved to Paris where he studied at the Lycée Janson de Sailly. He entered the university La Sorbonne in 1909 and first studied history, obtaining his *licence ès lettres* in 1910 before briefly switching, apparently without much conviction, to law. Soon, however, deeply impressed by Henri Poincaré’s writings, he changed his mind and enrolled in the Faculty of Sciences, studying physics and mathematics in the years 1911–1913. Louis did not have to try hard to convince his family of his choice: His elder brother, Maurice, himself a physicist, who had been in charge of supervising Louis’s education since the death of their father in 1906, encouraged this decision. To supplement the somewhat conservative education dispensed at the Sorbonne, Louis caught up with the most recent theoretical research, independently reading the works of the leading theorists of his time, Paul Drude, Paul Langevin, Hendrik Lorentz, Max Planck, and Poincaré. He obtained his *Licence ès sciences* in 1913. This same year, he joined the army to complete his military service. Maurice, who served for a long time in the navy’s wireless communications, arranged for him to fulfill his duties with the team at the Eiffel Tower’s radiotelegraphy station.

The outbreak of World War I marked a pause in Louis’s pursuit of strictly theoretical speculations. Instead of being released after the regular three years of service, he stayed in the army until 1919. Throughout the whole duration of the conflict, he remained on the team at the Eiffel Tower station. Though he was involved in rather innovative applied research (for instance, in collaboration with Leon Brillouin and his brother Maurice, he contributed to the development of wireless communication for allied submarines), he longed for more theoretical and fundamental work. When the war ended, Louis de Broglie resumed his studies and research work in physics. While attending the lectures of Langevin at the College de France, he also began to assist his brother with experimental investigations. At the time, Maurice was already an experimentalist with an international reputation. His private laboratory was renowned for pioneering research on x-ray spectra, and the young Louis there became familiar with the most advanced techniques of the field.

**Scientific Career.** During this time, de Broglie was already forming ideas which would eventually lead to his discovery of the wave nature of matter. In contrast to his more experimental research, on which he collaborated with other members of Maurice’s laboratory, the theoretical ideas that would secure Louis’s fame were developed in almost total isolation. After a series of three groundbreaking communications to the Paris Academy in 1923, where he outlined the basics of a wave theory of matter, he exposed his ideas in his PhD thesis *Recherches sur la théorie des quanta* (Researches on the quantum theory), which he defended in 1924. These works suggested that the idea of the dual nature of light (as both a localized particle and a wave extended in space) put forth by Albert Einstein in 1905 should be applied to matter as well. With the successful extension of de Broglie’s results by Schrödinger, and then especially with the discovery of electron diffraction in crystals by Clinton Joseph Davisson and Lester Halbert Germer at Bell Labs in the United States in 1927, and by George Paget Thomson at the University of Aberdeen in Scotland in 1928, which demonstrated experimentally that material particles exhibited wavelike properties, de Broglie’s ideas were spectacularly vindicated. In 1929 the Swedish Academy of Sciences conferred on him the Nobel Prize for Physics “for his discovery of the wave nature of electrons.”

After two years of lecturing at the Sorbonne, Louis was appointed *maître de conferences* in 1928 to teach theoretical physics at the Institut Henri Poincaré, which had been just created in Paris and was devoted to mathematical and theoretical physics. He obtained the chair of physical theories at the Paris Faculty of Sciences in 1933 and taught there until his retirement in 1962. De Broglie led a rather withdrawn life and never married.

De Broglie was elected a member of the Academy of Sciences in 1933 (*section des sciences mécaniques*), and was elected its permanent secretary in 1942 (*division des sciences mathématiques;* he resigned this charge in 1975). In 1929 he was the first recipient of the Henri Poincaré Prize of the Académie des Sciences and in 1932, he was granted the Albert I of Monaco Prize. In 1952 he received the first Kalinga Prize of UNESCO for his efforts to explain aspects of modern physics to the layman. He was elected a member of the Académie Française in 1944, (one of the fellow academicians to greet his election was his brother Maurice, himself elected in 1934). In 1945 he became an advisor to the French Commissariat à l’Energie Atomique (CEA). In 1956 he received the gold medal of the Centre National de la Recherche Scientifique (CNRS) and in 1961 he was decorated with the Grand Cross of the Légion d’Honneur (1961).

He was elected Fellow of the Royal Society in 1953 and was an Officer of the Order of Leopold of Belgium. He was an honorary doctor of the universities of Warsaw, Bucharest, Athens, Lausanne, Quebec, and Brussels, and a member of eighteen foreign academies in Europe, India, and the United States.

**Development of de Broglie’s Theory.** Louis’s acquaintance with quanta went back at least to his reading of his brother’s reports and notes from the first Solvay meeting in 1911, where Maurice served as scientific secretary. According to Louis’s recollections, he was immediately caught by the puzzle of the quanta and promised himself that he would devote all of his energy to understanding it. His work in his brother’s laboratory was instrumental in providing him with firsthand knowledge of some aspects of the new phenomenology, essentially those related to the study of x-ray absorption and of the x-ray-induced photo-electric effect. The context of this research was the further study of the atomic structure, where investigation of the inner energy levels was made possible using x-rays instead of visible light. Serving as a house-theoretician, Louis de Broglie had to learn the most up-to-date theories to be able to interpret the experimental results. Alone, with Maurice, or with his collaborator Alexandre Dauvillier, Louis published numerous observations on the inner atomic levels and their occupation numbers, on the relation between absorption intensities and the number of levels and of the electrons, and on the photoelectric effect. This enabled Louis to enter the restricted field of quantum researchers; however, his important work on the absorption intensities did not bring him only praise. In spite of the agreement of de Broglie’s results with the data, Niels Bohr’s close followers from Copenhagen and Munich found his derivations rather unorthodox, if not simply inconsistent with the subtle usage of Bohr’s correspondence principle. This principle—that any quantum behavior must reproduce successful classical predictions at the limit where quantum effects can be neglected—was one they felt only “insiders” could properly handle.

Besides Bohr’s atomic theory, Louis became familiar with Albert Einstein’s light quantum hypothesis, which, at the time, was still rejected by most researchers in the quantum community. The experimentation in Maurice’s lab, especially the latter’s study of the x-ray-induced photoelectric effect in 1921, was making this hypothesis seem quite natural, and offered renewed arguments for those who, like Maurice, were defending the corpuscular hypothesis against more conservative (purely Maxwellian) views. Although he was not successful in persuading his colleagues of the validity of Einstein’s thesis at the Solvay 1921 congress, Maurice de Broglie did not renounce his ideas about the validity of the corpuscular structure of light, but went even further in advocating, in vaguer terms, a kind of general wave-corpuscle duality valid not only for light, but for electrons (matter) as well. As is known, these ideas did not really express a clear-cut ontological thesis; however, they definitely had an impact on Maurice’s younger brother.

In the years following the end of the war, many of Louis’s theoretical considerations, fueled by his brother dualistic ideas and the continuation of his own prewar meditations, were centered on the formal analogy between the geometry of light ray propagation and the classical mechanics of point particles. This analogy, which had been the starting point for William Rowan Hamilton’s famous nineteenth-century formulation of mechanics, if not simply ignored by Louis’s contemporaries, was at any rate not taken as having the slightest physical relevance. However, Louis soon felt that it could prove crucial for better understanding the Bohr-Sommerfeld quantization scheme, which selected, among a continuity of classical motions, only a discrete range of quantically allowed ones. As explained by historian Olivier Darrigol, there did exist at the time interpretations of the Bohr-Sommerfeld quantization conditions, and some might have helped Louis in reaching his own (specifically Marcel Brillouin’s “hereditary field” mechanism, or Einstein’s considerations of the multivaluedness of the action). However, Louis obtained his own interpretation within a much broader scheme of a general wave-particle duality, using relativity theory as a guide—a theory that he had ample time to ponder following Langevin’s outstanding teaching of the topic in the years 1920–1922.

**Wave Theory of Matter.** The hypothesis of light quanta was the starting point in Louis’s 1922 paper “Rayonnement noir et quanta de lumière” (received on January 26 by the *Journal de Physique*) devoted to the study of the black body radiation. This work marks the beginning of Louis’s final progression toward the discovery of waves associated with matter. Aiming at deriving Planck’s law from a purely corpuscular standpoint, de Broglie was eventually able to derive only its low-density approximation, Wien’s law, because he did not take into account the only later-derived quantum Bose statistics. However, he introduced in his paper, most significantly, the idea of a light quantum with a very small, but nonvanishing, mass.Apparently, de Broglie was guided by his desire to interpret the continuously varying energies of the light quanta as corresponding to the various (sublight) velocities these quanta could then have. As such, his light quanta did not differ from ordinary matter particles, so that the stage was set for the final step. In 1923 Louis de Broglie, in a series of three short communications to the Paris Academy, extended the wave-particles duality of light through his bold hypothesis of waves associated with matter particles. His first communication, “Ondes et quanta,” dated 10 September 1923, introduced the idea of a wave associated with a particle, making use of an important observation on the relativistic transformation properties of the frequency of a periodic process as viewed in the rest frame of the corpuscle and in the laboratory frame. To start with, an “internal” periodic process could be associated with a particle of rest mass *m*_{0} if one defined its frequency *v*_{0} by the Bohr quantum condition (where *c* is the speed of light,*h* the Planck constant)

But then, transforming to the laboratory frame, where the particle had a velocity defined as *Bc* (with B > 1), one ended up with two different frequencies, depending if one wanted to transform first the energy:

or transform the rest frame frequency using the relativistic time dilatation formula:

Louis de Broglie, initially puzzled by this discrepancy, eventually realized that one could reconcile both results provided one set the speed of the wave propagating in the laboratory equal to *c/B*. Indeed, this condition ensured that the wave of frequency *v* was constantly in phase with the internal oscillating process of the particle. De Broglie readily used this condition to show how one could then

understand Bohr’s quantization conditions in terms of the stationary character of the wave, obtained only when the electron was on one of its Bohr’s orbits. In the second communication, “Quanta de lumière, diffraction et inter-férences,” dated 24 September, de Broglie discussed the relationship between the propagation of the particle and that of its associated wave. According to Hamilton’s analogy between ray optics and mechanics, the particle had to follow the trajectories of the rays normal to the phase wave fronts. De Broglie also considered the necessity of modifying the free dynamics of the particle, as the obstacles to the propagation of the wave could curve the trajectories of the particles. He identified this as a possible experimental effect that could corroborate his phase waves. The interplay between the propagation of the particle and of the waves could be expressed in more formal terms as an identity between the fundamental variational principles of Pierre de Fermat (rays), and Pierre Louis Maupertuis (particles) as de Broglie discussed it further in his last communication “Les quanta, la théorie cinétique des gaz et le principe de Fermat” (dated 8 October 1923). Therein he also considered some thermodynamic consequences of his generalized wave-particle duality. He showed in particular how one could, using Lord Rayleigh’s 1900 formula for the number of stationary modes for *phase* waves, obtain Planck’s division of the mechanical phase space into quantum cells.

In the next months, already working on his doctoral dissertation, de Broglie generalized the relations between the particle velocity and the wave velocity for cases where there were external forces. He again used relativity as a guide. He promoted the energy-frequency relation *E = h v* to a full, four-vector (relativistic) relation between a 4-dimensional wave vector and the 4-momentum of the particle. Equating their spatial parts, de Broglie obtained his celebrated relationship between the particle momentum and the wave-length:

The more detailed derivations of his results formed the core of his dissertation, which he successfully defended on 25 November 1924 in front of a perplexed audience.

In order to reach an audience wider than the limited readership of the *Comptes rendus*, de Broglie arranged the publication of a summary of his results in *Nature*(October 1923); and a fairly complete account of his three communications appeared in the *Philosophical Magazine*(February 1924). In Germany, a summary of his communications was published in the *Physikalische Berichte*(1924). These accounts did not stir up much reaction from the community. The situation changed when, in the summer of 1924, Langevin personally informed Einstein of Louis’s ideas, which the latter embraced quite enthusiastically. Indeed, Einstein quickly recognized that they fit remarkably with his own research on quantum gases. His support made many key actors in quantum research focus on and take seriously de Broglie’s ideas in the years 1924–1925. In particular, Schrödinger extended de Broglie’s results in the winter of 1925–1926 into a genuine wave mechanics, working out the wave equation of the theory. However, Schrödinger, while extending and completing in an essential way the original framework, altered de Broglie’s original picture, granting reality only to the waves and refusing wave-particle dualism. The ensuing events, which led rapidly to the final formulation of quantum mechanics in 1926–1927, did not include the active participation of Louis de Broglie. Although he saw his ideas extended and vindicated, his conception of the meaning of his research and how it should be continued was increasingly at odds with the views of his peers.

**The Meaning of Wave Mechanics.** In his communications of 1923, and later in his 1924 PhD thesis, de Broglie did not want to commit himself to any physical interpretation of the waves. He granted physical relevance only to these wave features which could be directly related to the particle motion, namely their phase, while eluding any questions pertaining to their amplitude and proper dynamics. They were, as he dubbed them, “fictitious.” However, in the months following his PhD, de Broglie started to explore the consequences of his wave-particle model for the problem of the interaction of light with matter. He also considered the possibilities of more physically interpreting his particle-associated waves. Willing to acknowledge the reality of the particles, he tried to conceive them as embodied by the singularities of the waves. However, he had then to cope with the Schrödinger view, where only continuous matter waves were considered. He first attempted to save his dualism by conceiving Schrödinger’s equation as actually admitting pairs of solutions characterized by a common phase. He thought of each pair as consisting of a singular solution, with the singularity identified with the particle, while the corresponding continuous regular solution (the only one considered by Schrödinger) was interpreted as conveying solely statistical information. In this approach, the probabilistic (Max Born's) interpretation of the continuous wave reflected the inherent neglect of the singularity (the particle). This so-called double-solution interpretation was hence a causal one, conceiving Schrödinger waves as conveying all the potential outcomes, while concealing the realities of the underlying particle dynamics. These dynamics were non-classical, owing to the fundamental fact that the particle was coupled to the wave via the guiding mechanism which related the particle’s velocity to the gradient of the wave phase.

Louis de Broglie met substantial difficulties in justifying his ambitious proposal mathematically. At the Solvay congress of October 1927, feeling unable to defend his interpretation against the more orthodox views, he presented an intermediate position, which viewed the continuous wave as the sole undulatory entity: its role was to guide the particle, because it shared with the singular solution, now discarded, the same phase. This weakened position, called the “pilot wave” interpretation, was nonetheless heavily criticized by the Copenhagen orthodoxy, especially with respect to Bohr’s views according to which quantum mechanics was rooted in the uncontrollable disturbance that the observation necessarily brings upon the observed system—which had been presented just weeks before.

Some time after this confrontation, de Broglie, discouraged, joined the orthodox Copenhagen position, which he then consistently defended for the next two decades. From 1930 until 1950, de Broglie turned to study of the various extensions of wave mechanics. He worked on Paul Dirac’s electron theory and on the quantum theory of light, developed a general theory of spin particles, and considered some applications of wave mechanics to nuclear physics. This work, however, did not receive as much attention as what he achieved in the early 1920s.

The early 1950s again witnessed a major change in de Broglie’s views. Impressed by the nonlocal theory put forth by David Bohm in 1951, which reintroduced pilot-waves, de Broglie turned back to his first theoretical convictions. Surrounded by some faithful followers, de Broglie resumed his quest for a causal interpretation, this time supplementing his initial views with the idea of nonlinear dynamics for the singular wave. This was, however, increasingly perceived as a marginal research program, even in his own country, where quantum theoreticians preferred to stick to more mainstream physics, less fundamental and closer to the wealth of new experimental data emerging in the 1950s. Although revered as one of the fathers of quantum physics, de Broglie became an isolated icon.

## BIBLIOGRAPHY

*Louis de Broglie published more than 150 scientific papers, about thirty books, and many philosophical and historical studies as well as numerous popular accounts, biographical notes, and obituaries. A list of all his writings has been published in* Annales de la Fondation Louis de Broglie *17 (1992): 1–21. Louis de Broglie’s papers are deposited in the archive of the Académie des Sciences. On the Web site of the Fondation Louis de Broglie (http://www.ensmp.fr/aflb/) one can find an English translation of de Broglie’s PhD dissertation. The Fondation Louis de Broglie, dedicated to the pursuit of the ideas of Louis de Broglie, has edited texts and books of various level of scholarship commenting different aspects of the contributions of de Broglie*.

### WORKS BY DE BROGLIE

“Rayonnement noir et quanta de lumière.” *Journal de Physique* 3 (1922): 422–428.

“Ondes et quanta.” *Comptes rendus hebdomadaires des séances de l’Académie des sciences* 177 (1923): 507–510. “Quanta de lumière, diffraction et interférences.” *Comptes rendus hebdomadaires des séances de l’Académie des Sciences* 177 (1923): 548–550. “Les quanta, la théorie cinétique des gaz et le principe de Fermat.” *Comptes rendus hebdomadaires des séances de l’Académie des Sciences* 177 (1923): 630–632.

*Recherches sur la théorie des quanta*, PhD diss. Paris: Masson et Cie, 1924; also *Annales de Physique* 3 (1925): 22–128. The Fondation Louis-de-Broglie has published a facsimile of the *Annales* edition together with Langevin’s report and the three 1923 academy communications: Paris: Louis-Jean, 1992.

*Ondes et mouvements*. Paris: Gauthier-Villars, 1926.

*La mécanique ondulatoire*. Paris: Gauthier-Villars, 1928.

*Matter and Light; the New Physics*. London: Allen & Unwin, 1939.

*Perspectives nouvelles en microphysique*. Paris: Albin Michel, 1956. English translation, *New Perspectives in Physics*, translated by A. J. Pomerans. New York: Basic Books, 1962.

*Une tentative d’interprétation causale et non linéaire de la mécanique ondulatoire: la théorie de la double solution*. Paris: Gauthier-Villars, 1956. English translation: *Non-linear Wave Mechanics: A Causal Interpretation*, translated by Arthur J. Knodel and Jack C. Miller. Amsterdam and New York: Elsevier, 1960.

*Sur les sentiers de la science*. Paris: Albin Michel, 1960.

*Introduction à la nouvelle théorie des particules de M. Jean-PierreVigier et de ses collaborateurs*. Paris: Gauthier-Villars, 1961. English translation: *Introduction to the Vigier Theory of Elementary Particles*, translated by Arthur J. Knodel. Amsterdam and New York: Elsevier, 1963. *Étude critique des bases de l’interprétation actuelle de la mécanique ondulatoire*. Paris: Gauthier-Villars, 1963. English translation: *The Current Interpretation of Wave Mechanics: A Critical Study*. Amsterdam and New York: Elsevier, 1964.

*Certitudes et incertitudes de la science*. Paris: Albin Michel, 1966. *Recherches d’un demi-siècle*. Paris: Albin Michel, 1976. Apart from a selection of his technical papers, it contains a representative choice of de Broglie’s writings on science, philosophy of science, and science policy.

*Heisenberg’s Uncertainties and the Probabilistic Interpretation ofWave Mechanics*. Dordrecht, Netherlands, and Boston: Kluwer, 1990.

### OTHER SOURCES

Abragam, Anatole. “Louis Victor Pierre Raymond de Broglie, 15 August 1892–19 March 1987.”*Biographical Memoirs of Fellows of the Royal Society* 34 (1988): 23–41.

——. “Louis de Broglie: la grandeur et la solitude.” *LaRecherche* 245 (1992): 918–923. A tribute with some hints at de Broglie’s isolated position in post-war France.

Académie des Sciences. *La découverte des ondes de matière*,*Actes du Colloque Louis de Broglie, Paris, 16–17 juin 1992*. Paris: Lavoisier, 1994.

Barreau, Hervé. “Le rôle de la relativité dans la pensée de Louis de Broglie.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 juin 1992*, edited by Académie des sciences, 93–102. Paris: Lavoisier, 1994.

Ben Dov, Yoav. “De Broglie’s Causal Interpretations of Quantum Mechanics.” *Annales de la Fondation Louis de Broglie* 14 (1989): 343–360.

——. “De Broglie’s Conception of the Wave-Corpuscule Duality.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 Juin 1992*, edited by Académie des sciences, 115–122 Paris: Lavoisier, 1994.

Cormier-Delanoue, Christian. *Louis de Broglie que nous avons connu*. Paris: Fondation Louis de Broglie, 1988.

Darrigol, Olivier. “Strangeness and Soundness in Louis de Broglie’s Early Works.” *Physis* 30 (1993): 303–372.

——. “Les premiers travaux de Louis de Broglie.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 juin*

*1992*, edited by Académie des sciences, 41–52. Paris: Lavoisier, 1994.

Forman, Paul, and Raman, Varadaraja. “Why Was It Schrödinger Who Developed de Broglie’s Ideas?” *Historical Studies in the Physical Sciences* 1 (1969): 291–314.

George, André. *Louis de Broglie: Physicien et penseur*. Paris: 1953. Gerber, Johannes. “Geschichte der Wellenmechanik.” *Archive forHistory of Exact Sciences* 5 (1969): 349–416.

Hanle, Paul A. “Erwin Schrödinger’s Reaction to Louis de Broglie’s Thesis on the Quantum Theory.” *Isis* 68 (1977): 606–609.

Hund, Friedrich. *The History of Quantum Theory*. Translated by G. Reece. New York: Harper and Row, 1974.

Jammer, Max. *The Conceptual Development of QuantumMechanics*. New York: McGraw-Hill, 1960. A classic account. Klein, Martin J. “Einstein and the Wave-Particle Duality.” *Natural Philosopher* 3 (1964): 3–49.

Kragh, Helge. “The Heritage of Louis de Broglie in the Works of Schrödinger and Other Theoreticians.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 juin 1992*, edited by Académie des sciences, 65–78. Paris: Lavoisier, 1994.

Kubli, Fritz. “Louis de Broglie und die Entdeckung der Materiewellen.” *Archive for History of Exact Sciences* 7 (1970): 26–68. ———. “Conversations avec Louis de Broglie au sujet de sa thèse.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 Juin 1992*, edited by Académie des sciences, 53–64. Paris: Lavoisier, 1994.

Kuhn, Wilfried. “L’influence des images métaphysiques du monde sur le développement des idées fondamentales dans la physique, particulièrement chez Louis de Broglie.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 juin 1992*, edited by Académie des sciences, 103–114. Paris: Lavoisier, 1994.

Lochak, Georges. *Louis de Broglie: un prince de la science*. Paris: Flammarion, 1992.

Medicus, Heinrich A. “Fifty Years of Matter Waves.” *PhysicsToday* 27 (1974): 38–45.

Nye, Marie Jo. “Aristocratic Culture and the Pursuit of Science: The de Broglies in Modern France.” *Isis* 88 (1997): 397–421.

Russo, Arturo. “La découverte des ondes de matière.” In *La découverte des ondes de matière: colloque organisé à l’occasion du centenaire de la naissance de Louis de Broglie, 16–17 Juin 1992*, edited by Académie des sciences, 79–92. Paris: Lavoisier, 1994.

Taketani, Mituo. *The Formation and Logic of QuantumMechanics*. Vol. 2, *The Way to Quantum Mechanics*. Translated by Masayuki Nagasaki. Singapore and River Edge, NJ: World Scientific, 2002.

Wheaton, Bruce R. *The Tiger and the Shark: Empirical Roots ofWave-Particle Dualism*. Cambridge, U.K.: Cambridge University Press, 1983.

——. “The Laboratory of Maurice de Broglie and the Empirical Foundations of Matter-Waves.” In

*Jan Lacki*

# Louis de Broglie

# Louis de Broglie

Louis Victor de Broglie, a theoretical physicist and member of the French nobility, is best known as the father of wave mechanics, a far-reaching achievement that significantly changed modern physics. For this groundbreaking work, de Broglie was awarded the 1929 Nobel Prize for physics.

Louis Victor Pierre Raymond de Broglie was born on August 15, 1892, in Dieppe, France, to *Duc* Victor and Pauline d'Armaille Broglie. His father's family was of noble Piedmontese origin and had served French monarchs for centuries, for which it was awarded the hereditary title Ducfrom King Louis XIV in 1740, a title that could be held only by the head of the family. A later de Broglie assisted the Austrian side during the Seven Years War and was awarded the title *Prinz* for his contribution. This title was subsequently borne by all members of the family. Another of de Broglie's famous ancestors was his great-great-grandmother, the writer Madame de Stael.

The youngest of five children, de Broglie inherited a familial distinction for formidable scholarship. His early education was obtained at home, as befitted a great French family of the time. After the death of his father when de Broglie was fourteen, his eldest brother Maurice arranged for him to obtain his secondary education at the Lycée Janson de Sailly in Paris.

After graduating from the Sorbonne in 1909 with baccalaureates in philosophy and mathematics, de Broglie entered the University of Paris. He studied ancient history, paleography, and law before finding his niche in science, influenced by the writings of French theoretical physicist Jules Henri Poincaré. The work of his brother Maurice, who was then engaged in important, independent experimental research in X rays and radioactivity, also helped to spark de Broglie's interest in theoretical physics, particularly in basic atomic theory. In 1913, he obtained his Licenciéès Sciences from the University of Paris's Faculté des Sciences.

De Broglie's studies were interrupted by the outbreak of World War I, during which he served in the French army. Yet even the war did not take the young scientist away from the country where he would spend his entire life; for its duration, de Broglie served with the French Engineers at the wireless station under the Eiffel Tower. In 1919, after what he considered to be six wasted years in uniform, de Broglie returned to his scientific studies at his brother's laboratory. Here he began his investigations into the nature of matter, inspired by a conundrum that had long been troubling the scientific community: the apparent physical irreconcilability of the experimentally proven dual nature of light. Radiant energy or light had been demonstrated to exhibit properties associated with particles as well as their well-documented wave-like characteristics. De Broglie was inspired to consider whether matter might not also exhibit dual properties. In his brother's laboratory, where the study of very high frequency radiation using spectroscopes was underway, de Broglie was able to bring the problem into sharper focus. In 1924, de Broglie, with over two dozen research papers on electrons, atomic structure, and X rays already to his credit, presented his conclusions in his doctoral thesis at the Sorbonne. Entitled "Investigations into the Quantum Theory," it consolidated three shorter papers he had published the previous year.

In his thesis, de Broglie postulated that all matter—including electrons, the negatively charged particles that orbit an atom's nucleus—behaves as both a particle and a wave. Wave characteristics, however, are detectable only at the atomic level, whereas the classical, ballistic properties of matter are apparent at larger scales. Therefore, rather than the wave and particle characteristics of light and matter being at odds with one another, de Broglie postulated that they were essentially the same behavior observed from different perspectives. Wave mechanics could then explain the behavior of all matter, even at the atomic scale, whereas classical Newtonian mechanics, which continued to accurately account for the behavior of observable matter, merely described a special, general case. Although, according to de Broglie, all objects have "matter waves," these waves are so small in relation to large objects that their effects are not observable and no departure from classical physics is detected. At the atomic level, however, matter waves are relatively larger and their effects become more obvious. De Broglie devised a mathematical formula, the matter wave relation, to summarize his findings.

American physicist Albert Einstein appreciated the significant of de Broglie's theory; de Broglie sent Einstein a copy of his thesis on the advice of his professors at the Sorbonne, who believed themselves not fully qualified to judge it. Einstein immediately pronounced that de Broglie had illuminated one of the secrets of the universe. Austrian physicist Erwin Schrödinger also grasped the implications of de Broglie's work and used it to develop his own theory of wave mechanics, which has since become the foundation of modern physics. Still, many physicists could not make the intellectual leap required to understand what de Broglie was describing.

De Broglie's wave matter theory remained unproven until two separate experiments conclusively demonstrated the wave properties of electrons—their ability to diffract or bend, for example. American physicists Clinton Davisson and Lester Germer and English physicist George Paget Thomson all proved that de Broglie had been correct. Later experiments would demonstrate that de Broglie's theory also explained the behavior of protons, atoms, and even molecules. These properties later found practical applications in the development of magnetic lenses, the basis for the electron microscope.

De Broglie devoted the rest of his career to teaching and to developing his theory of wave mechanics. In 1927, he attended the seventh Solvay Conference, a gathering of the most eminent minds in physics, where wave mechanics was further debated. Theorists such as German physicist Werner Karl Heisenberg, Danish physicist Niels Bohr, and English physicist Max Born favored the uncertainty or probabilistic interpretation, which proposed that the wave associated with a particle of matter provides merely statistical information on the position of that particle and does not describe its exact position. This interpretation was too radical for Schrödinger, Einstein, and de Broglie; the latter postulated the "double solution," claiming that particles of matter are transported and guided by continuous "pilot waves" and that their movement is essentially deterministic. De Broglie could not reconcile his pilot wave theory with some basic objections raised at the conference, however, and he abandoned it.

The disagreement about the manner in which matter behaves described two profoundly different ways of looking at the world. Part of the reason that de Broglie, Einstein, and others did not concur with the probabilistic view was that they could not philosophically accept that matter, and thus the world, behaves in a random way. De Broglie wished to believe in a deterministic atomic physics, where matter behaves according to certain identifiable patterns. Nonetheless, he reluctantly accepted that his pilot wave theory was flawed and throughout his teaching career instructed his students in probabilistic theory, though he never quite abandoned his belief that "God does not play dice," as Einstein had suggested.

In 1928, de Broglie was appointed professor of theoretical physics at the University of Paris's Faculty of Science. De Broglie was a thorough lecturer who addressed all aspects of wave mechanics. Perhaps because he was not inclined to encourage an interactive atmosphere in his lectures, he had no noted record of guiding young research students.

In 1929, at the age of thirty-seven, de Broglie was awarded the Nobel Prize for physics in recognition of his contribution to wave mechanics. In 1933, he accepted the specially created chair of theoretical physics at the Henri Poincaré Institute—a position he would hold for the next twenty-nine years—where he established a center for the study of modern physical theories. That same year, he was elected to the Académie des Sciences, becoming its Life Secretary in 1942; he used his influence to urge the Académie to consider the harmful effects of nuclear explosions as well as to explore the philosophical implications of his and other modern theories.

In 1943, anxious to forge stronger links between industry and science and to put modern physics, especially quantum mechanics, to practical use, de Broglie established a center within the Henri Poincaré Institute dedicated to applied mechanics. He was elected to the prestigious Academie Francaise in 1944 and, in the following year, was appointed a counsellor to the French High Commission ofAtomic Energy with his brother Maurice in recognition of their work promoting the peaceful development of nuclear energy and their efforts to bridge the gap between science and industry. Three years later, de Broglie was elected to the National Academy of the United States as a foreign member.

During his long career, de Broglie published over twenty books and numerous research papers. His preoccupation with the practical side of physics is demonstrated in his works dealing with cybernetics, atomic energy, particle accelerators, and wave-guides. His writings also include works on X rays, gamma rays, atomic particles, optics, and a history of the development of contemporary physics. He served as honorary president of the French Association of Science Writers and, in 1952, was awarded first prize for excellence in science writing by the Kalinga Foundation. In 1953, de Broglie was elected to London's Royal Society as a foreign member and, in 1958, to the French Academy of Arts and Sciences in recognition of his formidable output. With the death of his older brother Maurice two years later, de Broglie inherited the joint titles of French duke and German prince. De Broglie died of natural causes on March 19, 1987, at the age of ninety-five, having never fully resolved the controversy surrounding his theories of wave mechanics.

## Further Reading

Cline, Barbara Lovett, *Men Who Made a New Physics,* University of Chicago Press, 1987.

Guillemin, Victor, *The Story of Quantum Mechanics,* Scribner, 1968.

Heathcote, Niels H., *Nobel Prize Winners in Physics, 1901-1950,* Books for Libraries Press, 1953.

*Modern Men of Science,* Volume II, McGraw-Hill, 1968.

Weber, Robert L., *Pioneers of Science: Nobel Prize Winners in Physics,* Institute of Physics, 1980.

*Proceedings of the Royal Society,* Volume 34, 1988. □

# Broglie, Louis Victor De (1892-1987)

# Broglie, Louis Victor de (1892-1987)

*French physicist*

Louis Victor de Broglie, a theoretical physicist and member of the French nobility, is best known as the father of wave mechanics, a far-reaching achievement that significantly changed modern physics. Wave mechanics describes the behavior of matter, including subatomic particles such as electrons, with respect to their wave characteristics. For this groundbreaking work, de Broglie was awarded the 1929 Nobel Prize for physics. De Broglie's work contributed to the fledgling science of microbiology in the mid-1920s, when he suggested that electrons, as well as other particles, should exhibit wave-like properties similar to light. Experiments on electron beams a few years later confirmed de Broglie's hypothesis. Of importance to **microscope** design was the fact that the wavelength of electrons is typically much smaller than the wavelength of light. Therefore, the limitation imposed on the light microscope of 0.4 micrometers could be significantly reduced by using a beam of electrons to illuminate the specimen. This fact was exploited in the 1930s in the development of the **electron microscope** .

Louis Victor Pierre Raymond de Broglie was born on August 15, 1892, in Dieppe, France, to Duc Victor and Pauline d'Armaille Broglie. His father's family was of noble Piedmontese origin and had served French monarchs for centuries, for which it was awarded the hereditary title Duc from King Louis XIV in 1740, a title that could be held only by the head of the family.

The youngest of five children, de Broglie inherited a familial distinction for formidable scholarship. His early education was obtained at home, as befitted a great French family of the time. After the death of his father when de Broglie was fourteen, his eldest brother Maurice arranged for him to obtain his secondary education at the Lycée Janson de Sailly in Paris.

After graduating from the Sorbonne in 1909 with baccalaureates in philosophy and mathematics, de Broglie entered the University of Paris. He studied ancient history, paleography, and law before finding his niche in science, influenced by the writings of French theoretical physicist Jules Henri Poincaré. The work of his brother Maurice, who was then engaged in important, independent experimental research in x rays and radioactivity, also helped to spark de Broglie's interest in theoretical physics, particularly in basic atomic theory. In 1913, he obtained his Licencié ès Sciences from the University of Paris's Faculté des Sciences.

De Broglie's studies were interrupted by the outbreak of World War I, during which he served in the French army. Yet, even the war did not take the young scientist away from the country where he would spend his entire life; for its duration, de Broglie served with the French Engineers at the wireless station under the Eiffel Tower. In 1919, de Broglie returned to his scientific studies at his brother's laboratory. Here he began his investigations into the nature of matter, inspired by a conundrum that had long been troubling the scientific community: the apparent physical irreconcilability of the experimentally proven dual nature of light. Radiant energy or light had been demonstrated to exhibit properties associated with particles as well as their well-documented wave-like characteristics. De Broglie was inspired to consider whether matter might not also exhibit dual properties. In his brother's laboratory, where the study of very high frequency radiation using spectroscopes was underway, de Broglie was able to bring the problem into sharper focus. In 1924, de Broglie, with over two dozen research papers on electrons, atomic structure, and x rays already to his credit, presented his conclusions in his doctoral thesis at the Sorbonne. Entitled "Investigations into the Quantum Theory," it consolidated three shorter papers he had published the previous year.

In his thesis, de Broglie postulated that all matter—including electrons, the negatively charged particles that orbit an atom's ** nucleus** —behaves as both a particle and a wave. Wave characteristics, however, are detectable only at the atomic level, whereas the classical, ballistic properties of matter are apparent at larger scales. Therefore, rather than the wave and particle characteristics of light and matter being at odds with one another, de Broglie postulated that they were essentially the same behavior observed from different perspectives. Wave mechanics could then explain the behavior of all matter, even at the atomic scale, whereas classical Newtonian mechanics, which continued to accurately account for the behavior of observable matter, merely described a special, general case. Although, according to de Broglie, all objects have "matter waves," these waves are so small in relation to large objects that their effects are not observable and no departure from classical physics is detected. At the atomic level, however, matter waves are relatively larger and their effects become more obvious. De Broglie devised a mathematical formula, the matter wave relation, to summarize his findings.

American physicist Albert Einstein appreciated the significant of de Broglie's theory; de Broglie sent Einstein a copy of his thesis on the advice of his professors at the Sorbonne, who believed themselves not fully qualified to judge it. Einstein immediately pronounced that de Broglie had illuminated one of the secrets of the Universe. Austrian physicist Erwin Schrödinger also grasped the implications of de Broglie's work and used it to develop his own theory of wave mechanics, which has since become the foundation of modern physics.

De Broglie's wave matter theory remained unproven until two separate experiments conclusively demonstrated the wave properties of electrons—their ability to diffract or bend, for example. American physicists Clinton Davisson and Lester Germer and English physicist George Paget Thomson all proved that de Broglie had been correct. Later experiments would demonstrate that de Broglie's theory also explained the behavior of protons, atoms, and even molecules. These properties later found practical applications in the development of magnetic lenses, the basis for the electron microscope.

In 1928, de Broglie was appointed professor of theoretical physics at the University of Paris's Faculty of Science. De Broglie was a thorough lecturer who addressed all aspects of wave mechanics. Perhaps because he was not inclined to encourage an interactive atmosphere in his lectures, he had no noted record of guiding young research students.

During his long career, de Broglie published over twenty books and numerous research papers. His preoccupation with the practical side of physics is demonstrated in his works dealing with cybernetics, atomic energy, particle accelerators, and wave-guides. His writings also include works on x rays, gamma rays, atomic particles, optics, and a history of the development of contemporary physics. He served as honorary president of the French Association of Science Writers and, in 1952, was awarded first prize for excellence in science writing by the Kalinga Foundation. In 1953, Broglie was elected to London's Royal Society as a foreign member and, in 1958, to the French Academy of Arts and Sciences in recognition of his formidable output. With the death of his older brother Maurice two years later, de Broglie inherited the joint titles of French duke and German prince. De Broglie died of natural causes on March 19, 1987, at the age of ninety-four.

* See also * Electron microscope, transmission and scanning; Electron microscopic examination of microorganisms; Microscope and microscopy

# de Broglie, Louis

# de Broglie, Louis

**FRENCH PHYSICIST1892–1987**

Louis-Victor-Pierre-Raymond de Broglie was born into a noble French family. He initially studied history at the Sorbonne in Paris, intending to enter the diplomatic service. His elder brother Maurice had chosen to forgo a diplomatic career for one in physics, despite opposition from his family. Louis also became interested in science and decided to pursue a degree in **theoretical physics** . His plans, however, were interrupted by World War I, during which time he served in a wireless telegraphy unit stationed at the Eiffel Tower.

In 1920 de Broglie returned to his studies; later he stated that his attraction "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 Max Planck in 1900 in his researches into black-body radiation, daily penetrated further into the whole of physics" (quoted by Heathcote, pp. 289–290).

During this same period de Broglie's brother Maurice was studying experimental physics, and he was particularly interested in x rays. The brothers frequently discussed x rays, and their dual nature (both wavelike and particle-like behavior) suggested to Louis that this same particle-wave duality might also apply to particles such as electrons.

In his doctoral dissertation in 1924, Louis de Broglie developed the equation *λ = h/mυ*, which predicts that the wavelength *λ* of a particle is inversely proportional to its mass *m* and velocity *υ* where *h* is Planck's constant.✷ The wavelength associated with a submicroscopic object—an electron, for example—is large relative to the size of the object and is therefore significant in describing its behavior, whereas the wavelength associated with a macroscopic object—a basketball, for example—is negligibly small relative to its size, and therefore the wavelike behavior of such an object is unnoticeable.

✷ See Max Planck article for more about Planck's constant.

The dual nature of electrons proposed by de Broglie, together with the dual nature of electromagnetic radiation proposed by Max Planck, led to the development of **quantum mechanics** by the Austrian physicist Erwin Schrödinger in 1926. The following year American physicists Charles J. Davisson and Lester H. Germer and others demonstrated experimentally that electrons can be diffracted just like light. That is, as electrons pass through a narrow slit, they spread out in a wavelike pattern similar to that of diffracted light.

De Broglie accomplished his most important work in physics while still a young man, receiving the Nobel Prize in physics in 1929. After obtaining his doctorate in 1924, he taught at the Sorbonne, and in 1928 he was named professor of theoretical physics at the Henri Poincaré Institute in Paris. In 1932 he also became professor of theoretical physics at the Sorbonne, retiring from that post in 1962.

Throughout his long life, de Broglie remained active in the development and interpretation of quantum mechanics and wrote more than twenty-five books on various topics related to this field of study. As a member of the French Commission on Atomic Energy, he was a long-time advocate for the peaceful use of atomic power. De Broglie also wrote a number of popular books to help promote public understanding of modern physics, and in recognition of these efforts, the United Nations Educational, Scientific, and Cultural Organization (UNESCO) awarded him the Kalinga Prize in 1952. He was the recipient of many awards and honors for his work in quantum mechanics.

As a young scientist de Broglie had believed that the statistical nature of modern physics masks our ignorance of the underlying reality of the physical world, but for much of his life he also believed that this statistical nature is all that we can know. Toward the end of his life, however, de Broglie turned back toward the views of his youth, favoring causal relationships in place of the accepted probabilistic picture associated with quantum mechanics.

see also Planck, Max; SchrÖdinger, Erwin

*Richard E. Rice*

## Bibliography

Heathcote, Niels H. de V. (1971). *Nobel Prize Winners in Physics, 1901–1950.* Freeport, NY: Books for Libraries Press.

Weber, Robert L. (1988). *Pioneers of Science: Nobel Prize Winners in Physics,* 2nd edition. Bristol, U.K.: Adam Hilger.

### Internet Resources

O'Connor, J. J., and Robertson, E. F. "Louis Victor Pierre Raymond duc de Broglie." Available from <http://www.history.mcs.st-andrews.ac.uk/history/Mathematicians/Broglie.html>.

# de Broglie, Louis Victor

**de Broglie, Louis Victor** See Broglie, Prince Louis Victor de

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