Louis Victor Pierre Raymond de Broglie

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Louis Victor Pierre Raymond de Broglie

The French theoretical physicist Louis Victor Pierre Raymond de Broglie (1892-1987) was awarded the Nobel Prize in Physics for his discovery of the wave nature of electrons. He was the founder of wave mechanics.

Louis de Broglie the son of Victor, 5th Duc de Broglie, was born at Dieppe on August 15, 1892. After early education in Paris he entered the Sorbonne, where, as he intended to become a civil servant, he read history and graduated in that subject in 1910. He then studied the physical sciences at the Sorbonne and graduated in them in 1913. In the army during World War I he was active in wireless telegraphy. After the war he did research on theoretical physics at the Sorbonne, and in 1924 he was awarded his doctorate in science with a thesis on the quantum theory which already contained the basis of all his future work.

Origin of De Broglie's Theories

The discovery just before 1900 of the electron, x-rays, and the photoelectric effect had led to doubts regarding the accuracy of the hitherto accepted wave theory of light. Then Max Planck enunciated his quantum theory, according to which radiant energy is always absorbed in finite quantities, or quanta. In 1905 Albert Einstein postulated that light must consist of wave packets, or minute corpuscles in rapid motion, later called photons. By 1911 Lord Rutherford had explained his concept of the atom, and in 1913 Niels Bohr incorporated Planck's ideas into the Rutherford atom. The concept of the Bohr atom led at first to important results, but by about 1920 its usefulness in explaining experimental observations was rapidly declining.

De Broglie's Early Work

At the start of his researches in the early 1920s, De Broglie realized that neither the quantum theory of light nor the corpuscular theory of electrons appeared to be satisfactory. From his theoretical researches he suspected that an electron could not be regarded merely as a corpuscle, but that a wave must be associated with it. He then considered the possibility that in the case of matter, as well as light and radiation generally, it must be assumed that corpuscles are associated with waves.

De Broglie then assumed that any particle of matter, such as an electron, has "matter waves" associated with it. The velocities of propagation of these waves associated with any one particle differ slightly from each other. As a result, these waves combine at regular intervals along the direction of propagation to form a wave crest. This wave crest therefore also travels along the line of propagation, and its velocity (the "group velocity") is quite different from the velocities of the individual waves that combine to form it. The distance between two successive crests of the De Broglie matter wave is known as the De Broglie wavelength (λ).

The nature of the new matter wave postulated by De Broglie was not generally understood. But his hypothesis was not simply an imaginative attempt to envisage a vague possibility, because his theory was backed up by an elaborate mathematical analysis. The wavelength determines the character of the wave, and the moving particle is characterized by its momentum, that is, its mass multiplied by its velocity (my). He was able to deduce a very important equation for the wavelength of the De Broglie wave associated with a particle having a known momentum.

De Broglie's first two papers were published in 1922. The beginning of his theory of wave mechanics, marked by the introduction of his conception of phase waves, was made public by him in September 1923, and within a few months he published three more papers extending his views. His theoretical work was further coordinated and amplified in his doctoral thesis, published in 1924. The thesis attracted the attention of Einstein, who publicly expressed his high regard for this work. As a result, De Broglie's theory received much attention from theoretical physicists. But, as far as was then realized, there was no experimental confirmation of the theory.

Experimental Confirmation

From his theoretical work De Broglie predicted the interference phenomena that would result when a stream of electrons was directed against a solid screen having apertures approximating in size to the matter waves of the electrons. No one had then deliberately attempted such an experiment, as the technical difficulties were too great. But in 1925 Clinton J. Davisson and L. H. Germer had an accident while bombarding a sheet of nickel with electrons. To restore the nickel they heated it; they then found that it had become crystalline and that, relative to the electrons, it behaved as a diffraction grating. Their new results proved that an electron behaves not only as a particle of matter but also as a wave. The calculations made from these experimental results agreed perfectly with those obtained by using De Broglie's formula. These experimental results were not confirmed until 1927, but after that year experimental evidence favoring De Broglie's views greatly increased. This experimental confirmation was vital to the survival of his theoretical work.

De Broglie's Development of Wave Mechanics

Up to this time the De Broglie wave could be determined only in the immediate vicinity of the trajectory. De Broglie now investigated the mechanics of a swarm of particles and was thus able to define the characteristics of the matter waves in space. He was also able to predict accurately the splitting of a beam of electrons in a magnetic field and to explain this phenomenon without reference to any hypothetical electron spin.

In 1927 De Broglie put forward his "theory of the double solution" of the linear equations of wave mechanics, from which he deduced the law that a particle moves in its wave in such a manner that its internal vibration is constantly in phase with the wave that carries it. He soon modified this to his "pilot-wave theory." As a result of criticisms, he temporarily abandoned these theories. But in 1954 he developed his original theory, which now envisaged the particle as constantly jumping from one trajectory to another.

Later Life

From 1924 De Broglie taught theoretical physics in the University of Paris and from 1932 he occupied the chair in that subject for 30 years. He was awarded the Nobel Prize for Physics in 1929. In 1933 he was elected to the Académie des Sciences, and in 1942 he became its Permanent Secretary. In 1944 he was elected to the Académie Française. He was elected a Foreign Member of the Royal Society of London in 1953 and was a member of many other foreign academies, including the National Academy of Sciences of the United States and the American Academy of Arts and Sciences. He received honorary degrees from six universities.

A far-seeing man, De Broglie saw by the middle of World War II that stronger links between industry and science were becoming necessary. In an effort to forge those links, and also to give the theoretical science a practical application, he established a center for applied mechanics at the Henri Poincare Institute, where research into optics, cybernetics, and atomic energy were carried out. His efforts to bring industry and science closer together were highly appreciated by the French government, which rewarded him a post as counselor to the French High Commission of Atomic Energy in 1945.

Among De Broglie's works for theoretical physicists are Recherches sur la théorie des quanta (1924), Nonlinear Wave Mechanics (1960), The Current Interpretation of Wave Mechanics (1964), and La Thermodynamique de la particule isolée (1964). Less difficult works are Ondes et mouvements (1926), Matter and Light: The New Physics (1939), and New Perspectives in Physics (1962).

In 1960 de Broglie succeeded his brother Maurice as the 7th Duc. He died in 1987.

Further Reading

For a short biography of De Broglie see Nobel Lectures, Physics, 1922-1941 (1965), which also contains his Nobel Lecture of 1929. For a discussion of his work see N. H. deV. Heathcote, Nobel Prize Winners: Physics, 1901-1950 (1953); and A. d'Abro, The Rise of the New Physics, vol. 2 (1939). □