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One of the weirdest aspects of life on the quantum scale is the fact that all particles sometimes behave like waves. Max Planck first proposed the notion of quanta in 1900 to explain blackbody radiation, and, with Einstein’s additional insights in 1905, this led to the resolution of the longstanding debate over whether light is a particle or wave: it is both. But this strange characteristic is not limited to photons. The French physicist Louis de Broglie extended the notion of particle/wave duality to electrons in 1925.
Born in 1892 in Dieppe, Prince Louis-Victor-Pierre-Raymont was the younger son of the 5th duc de Broglie, one of the oldest noble families of France. He was a lively, charming, and precocious child, according to letters written by his elder sister, with a pronounced flair for the dramatic. He favored blue jackets with breeches and buckled shoes at dinner, and memorized entire scenes from classical theater to recite for guests of the family.
His sister envisioned a shining future as a statesman for the young Louis, given his love for history and politics. When his father died in 1906, his older brother Maurice took him in and sent him to study at the Lyćee Janson de Sailly. Louis excelled in French, history, physics and philosophy, “indifferent in mathematics,” and not good at drawing and foreign languages, but no one subject held his full attention.
De Broglie studied history and law at the Sorbonne, thinking he would join the civil service, but then he became enthralled with theoretical physics, no doubt influenced in part by Maurice, also a physicist. In fact, Maurice maintained his own home laboratory at the family residence in Paris. Louis attended Henri Poincaré’s lectures on electrodynamics, thermodynamics, and related subjects, but it was his chance reading of the report of the first Solvay Conference on quantum theory that ignited his imagination, and he chose to make physics his career.
But first Louis had to complete his mandatory military service, just as World War I broke out. Thanks to Maurice’s influence, Louis spent much of the war at the radiotelegraphy station at the foot of the Eiffel Tower, maintaining the equipment for sending wireless transmissions. When the war ended, he worked with Maurice on the latter’s experiments on x-rays and the photoelectric effect, so ably explained by Albert Einstein in 1905. He published his first papers on the underlying quantum theory of that work.
In 1923, de Broglie later wrote, “After long reflection in solitude and meditation, I suddenly had the idea… that the discovery made by Einstein in 1905 should be generalized by extending it to all material particles and notably to electrons.” Even a simple water wave is granular at the atomic level, he reasoned, since it is composed of the coordinated motion of a horde of water molecules. All “particles” and all “waves” were in fact a mix of both. Because their “wavelengths” were so small, such “matter waves” wouldn’t affect the macro-world; their effects would only appear at the atomic scale.
This work became his doctoral thesis, published in the Annales de Physique in 1925–all 100 pages. The paper made de Broglie’s career, since he had thus far mostly been known as Maurice’s younger brother. Word spread rapidly throughout the physics community, earning the admiration of Einstein himself, who wrote that de Broglie had “lifted a corner of the great veil.”
A graduate student at the University of Gottingen named Walter Elsasser suggested a possible experiment to detect the matter waves: shining a beam of electrons through a crystal. The crystal’s lattice-like structure provides a built-in array of “slits” narrow enough to scatter the electron waves.
The experiment was performed in 1927, by Bell Labs physicists Clinton Davisson and Lester Germer, and by George Paget Thomson of the University of Aberdeen in Scotland. The electrons didn’t reflect from the surface along straight lines, like tiny balls. Instead, the crystal served as a three-dimensional diffraction grating and there were sharp peaks in the intensity of the diffracted beams that occurred at predictable angles.
The Nobel Committee praised de Broglie’s courageous foresight in championing this view when it awarded him the 1929 Nobel Prize in Physics. “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.”
In 1932, de Broglie became chair of theoretical physics at the Sorbonne University, where he taught for 33 years. His lecture notes were beautifully written, but he was deemed an uninspiring lecturer, preferring to read monotonously from his notes, although his weekly seminar in theoretical physics proved more popular. He continued his research in the field of wave mechanics, which gave rise to various applications, including the development of electron microscopes.
He later tried to develop a causal model to replace the probabilistic models of quantum mechanics, which was refined by David Bohm in the 1950s and known as the de Broglie-Bohm theory. While most of his colleagues embraced the notion that the statistical nature of atomic physics was all that could be known, de Broglie believed that “the statistical theories hide a completely determined and ascertainable reality behind variables which elude our experimental techniques.”
On October 18, 1933, de Broglie was elected to the French Academy of Sciences, and became its permanent secretary when he was 50–a position he held until the age of 83. And he became the 7th duc de Broglie in 1960 when his brother Maurice died. He also published many popular books on physics, earning him UNESCO’s first Kalinga Prize for popularizing physics in 1952. He never married, and died in Louveciennes on March 19, 1987; his title passed to a distant cousin. But his mark on physics remains.
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