Louis de Broglie

Louis Victor Pierre Raymond de Broglie was born on August 15, 1892 in Dieppe, on the Normandy coast, into one of the great aristocratic houses of France. The Broglies had served kings and republics for three centuries as marshals, ambassadors, and ministers; the family’s Italian branch traced itself to Piedmontese nobility from the 1600s. The young Louis was the fifth child, raised in a house full of tutors and politics, and he was expected, as a younger son, to enter the diplomatic service. He took his first university degree in medieval history at the Sorbonne in 1910. It was his elder brother Maurice, who had broken with custom to become a serious experimental physicist, who quietly turned him toward science.

Maurice ran a private X-ray laboratory in the family’s Paris mansion on the rue Châteaubriand, with proper apparatus and a paid assistant. Louis began drifting in to read and to argue, and after taking a second degree in physics in 1913 he enlisted for the First World War. He spent the war at the Eiffel Tower’s radio station, operating one of the most powerful transmitters in Europe, learning the practical electromagnetism that would later shape his thinking about waves. He emerged in 1919 with the rank of adjudant, a six-year hole in his scientific education, and the conviction that he wanted to do theoretical physics for the rest of his life.

For the next four years he worked in Maurice’s lab on X-ray spectra and the photoelectric effect, gradually building up his own picture of what light actually was. By 1923 he had convinced himself that Einstein’s photons were not the whole story: if waves can behave as particles, then by simple symmetry particles must behave as waves. In three short notes to the Paris Academy in September and October 1923, and then in his doctoral thesis of November 1924, Recherches sur la théorie des quanta, he wrote down the relation λ = h/p and showed that the Bohr quantization condition was nothing more than the requirement that an electron’s wave fit a whole number of wavelengths around its orbit.

Louis Victor Pierre Raymond, 7th Duc de Broglie (; 15 August 1892 – 19 March 1987) was a French theoretical physicist and aristocrat known for his contributions to quantum theory. In his 1924 Ph.D. thesis, de Broglie postulated the wave nature of electrons and suggested that all matter has wave properties. This concept is known as the de Broglie hypothesis, an example of wave–particle duality, and forms a central part of the theory of quantum mechanics. In 1929, de Broglie won the Nobel Prize in Physics, after the…

The thesis was so far outside the consensus that his examining committee at the Sorbonne hesitated. They sent a copy to Einstein in Berlin for an opinion. Einstein read it carefully, replied that de Broglie had “lifted a corner of the great veil,” and the degree was duly granted. Three years later Davisson and Germer at Bell Labs accidentally diffracted electrons off a nickel crystal, G.P. Thomson did the same in Aberdeen, and the wavelength came out exactly as de Broglie had predicted. The Nobel Prize in Physics for 1929 was awarded to de Broglie alone, the first time the prize had ever gone to a thesis. He was thirty-seven.

He spent the rest of his very long career at the Sorbonne and the Henri Poincaré Institute, training generations of French physicists. He never accepted the probabilistic Copenhagen interpretation of his own equation, and from the 1950s onward he worked with David Bohm on a “pilot wave” picture in which a real wave guided a real particle along a definite trajectory. He inherited the title of 7th duc de Broglie in 1960, on the death of his elder brother Maurice, and he died unmarried in Louveciennes on March 19, 1987, at the age of ninety-four. The brief paper he had written in the summer of 1923, scarcely longer than a magazine article, had outlived him by a margin nothing in the rest of physics could match.

He was also, almost incidentally, an unusually graceful writer. He won the inaugural UNESCO Kalinga Prize in 1952 for popularising science, sat as Perpetual Secretary of the Académie des sciences from 1942 onward, and was the first senior physicist to publicly call for the multinational laboratory that became CERN.

Studying the nature of X-ray radiation and discussing its properties with his brother Maurice, who considered these rays to be some kind of combination of waves and particles, contributed to Louis de Broglie's awareness of the need to build a theory linking particle and wave representations. In addition, he was familiar with the works (1919–1922) of Marcel Brillouin, which proposed a hydrodynamic model of an atom and attempted to relate it to the results…

In the arc of this book, de Broglie is the hinge: the man who took Einstein’s photons and turned them around, asking what it would mean for an electron to also be a wave. Every wavefunction, every orbital, every interference pattern in the rest of these pages is a footnote to the short thesis he wrote in 1924.