Louis De Broglie Physicist, one of the founders of quantum mechanics, Nobel Prize laureate in physics in 1929 Date of Birth: 15.08.1892 Country: France

1. Early Life and Education
2. Military Service and Early Research
3. The Birth of Quantum Theory
4. De Broglie's Wave-Particle Hypothesis
5. Doctorate and Early Career

Louis Victor Pierre Raymond de Broglie: A Pioneering Physicist in Quantum Mechanics

Early Life and Education

Louis de Broglie, one of the founders of quantum mechanics, was born in Dieppe, France, on August 15, 1892. He was the youngest of three sons born to Victor de Broglie, a diplomat, and Pauline de La Forest d'Armaillé. As the eldest male in this aristocratic family, his father held the title of Duke. For centuries, the de Broglies had served their nation in military and diplomatic roles, but Louis and his brother Maurice broke with tradition to pursue scientific careers.

Growing up in the refined and privileged milieu of French aristocracy, de Broglie developed an early fascination with various sciences. He attended the Lycée Janson-de-Sailly in Paris and later enrolled in the Faculty of Arts and Humanities at the University of Paris, where he earned a bachelor's degree in history in 1910. Under the influence of his elder brother Maurice, de Broglie increasingly gravitated towards physics and, in his own words, "the philosophies, the generalizations, and the books of [Henri] Poincaré," the renowned French mathematician. After a period of intense study, he received a bachelor's degree in physics from the Faculty of Sciences at the University of Paris in 1913.

Military Service and Early Research

That same year, de Broglie was drafted into the French Army and commissioned into the engineering corps. With the outbreak of World War I in 1914, he served in a radiotelegraph battalion and spent much of the war years stationed at the Eiffel Tower's wireless telegraph station.

After the war ended in 1918, de Broglie resumed his studies in physics at his brother's private research laboratory. He delved into the behavior of electrons, atoms, and X-rays. This was an exciting time for physicists as puzzles emerged at every turn. By the end of the 19th century, classical physics had achieved such great success that some scientists began to doubt whether any fundamental scientific problems remained to be solved. Only in the last decades of the century had striking discoveries been made, such as X-rays, radioactivity, and the electron.

The Birth of Quantum Theory

In 1900, Max Planck proposed his revolutionary quantum theory to explain the relationship between the temperature of a body and the radiation it emits. Defying the centuries-old view that light propagated as continuous waves, Planck theorized that electromagnetic radiation (shown only decades earlier to be light itself) consisted of discrete, indivisible energy packets called quanta, with the energy of each quantum being proportional to the frequency of the radiation. Planck's proposal not only solved the problem he had set out to address, but it was also too unconventional to gain immediate acceptance.

In 1905, Albert Einstein showed that Planck's theory was not a mathematical trick. Using quantum theory, he ingeniously explained the photoelectric effect (the emission of electrons from a metal surface when light strikes it). It was known that as the intensity of the light increased, the number of electrons emitted from the surface also increased, but the speed of the electrons never exceeded a particular maximum. Einstein proposed that each quantum of light transferred its energy to a single electron, ejecting it from the metal surface; the greater the light intensity, the more photons impinged on the surface, releasing more electrons; and the energy of each photon was determined by its frequency, thus setting an upper limit on the electron's velocity. Einstein not only extended the applicability of quantum theory, but he also provided confirmation of its validity. Light, which undoubtedly possessed wave-like properties, was now shown to behave like a stream of particles in certain phenomena.

Further confirmation of quantum theory came in 1913 when Niels Bohr proposed a model of the atom that combined Ernest Rutherford's concept of a dense central nucleus surrounded by orbiting electrons with certain restrictions on the electron's orbits. These restrictions allowed Bohr to explain the line spectra observed when light emitted from an excited substance (such as by burning or by electrical discharge) was passed through a narrow slit and then through a spectroscope – an optical instrument that spatially separates components of a signal according to their different frequencies or wavelengths (colors). The result is a series of lines (images of the slit), or a spectrum. The position of each spectral line corresponds to the frequency of the particular component. The overall spectrum is determined by the radiation of the atoms or molecules in the glowing substance. Bohr explained the spectral lines as "quantum jumps" of electrons in atoms from one "allowed" orbit to another, lower-energy orbit. The energy difference between the orbits, lost by the electron in the transition, is emitted as a quantum of energy, or photon – radiation with a frequency proportional to the energy difference. The spectrum was essentially a coded record of the electron's energy states. Bohr's model thus reinforced the concept of the dual nature of light as both a wave and a stream of particles.

De Broglie's Wave-Particle Hypothesis

Despite the mounting experimental evidence, the idea of the dual nature of electromagnetic radiation remained troubling to many physicists. There were also loopholes in the new theory. For instance, Bohr's model arbitrarily matched "allowed" electron orbits with observed spectral lines. The orbits did not follow from the theory but were adjusted to fit experimental data.

De Broglie had a brilliant insight: if waves could behave like particles, perhaps particles could also behave like waves. He extended Einstein and Bohr's wave-particle dualism to material objects. Waves and matter were thought to be fundamentally different. Matter had a rest mass. It could be at rest or moving with some velocity. Light, on the other hand, had no rest mass: it either moved at a certain velocity (which could change depending on the medium) or it did not exist. By analogy with the relationship between the wavelength of light and the energy of a photon, de Broglie hypothesized that there might be a relationship between the wavelength and the momentum of a particle (mass times velocity of the particle). Momentum is directly proportional to kinetic energy. Thus, a fast electron would correspond to a wave with a higher frequency (shorter wavelength) than a slower electron. Depending on the conditions of observation, a material object would manifest as either a wave or a particle.

With remarkable audacity, de Broglie applied his idea to Bohr's atom model. The negative electron was attracted to the positively charged nucleus. In order to orbit the nucleus at a particular distance, the electron had to move at a certain speed. If the electron's speed changed, so would the orbit's position. In this case, the centrifugal force would be balanced by the centripetal force. The electron's speed in a particular orbit, at a particular distance from the nucleus, corresponded to a particular momentum (velocity times electron mass) and therefore, according to de Broglie's hypothesis, to a particular wavelength of the electron. De Broglie argued that the "allowed" orbits were those in which an integer number of wavelengths of the electron could fit. Only in these orbits would the electron waves be in phase (at a particular point in the frequency cycle) with themselves and not cancel themselves out by their own interference.

Doctorate and Early Career

In 1924, de Broglie submitted his work, "Recherches sur la théorie des quanta" ("Investigations on the Theory of Quanta"), as a doctoral dissertation to the Faculty of Sciences at the University of Paris. His examiners and the members of the academic board were impressed but deeply skeptical. They regarded de Broglie's ideas as theoretical speculations without any experimental basis. However, on Einstein's urging, de Broglie was awarded his doctorate. The following year, de Broglie published his work in the form of an expanded paper, which was met with respectful attention. He became a lecturer in physics at the University of Paris in 1926, and two years later, he was appointed professor of theoretical physics at the Institute Henri Poincaré at the same university.

Einstein was deeply impressed by de Broglie's work and advised many physicists to study it carefully. Erwin Schrödinger took Einstein's advice and used de Broglie's ideas as the