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The Great Quantum Leap

When light comes into contact with matter it has a particle and not a wave nature, Einstein showed in the first article of "The Year of Wonder", which won him the Nobel Prize

Amit Hagar

Thomas Young's two-slit experiment, one of the most important experiments that pointed to the wave nature of light, which was considered a fait accompli until Einstein. Light falling on a photographic plate (left) through two slits creates an image, in which areas are dark and areas are light

Quantum mechanics is one of the most successful theories of modern physics, and despite its oddities and the paradoxes it poses to common sense, it is difficult to describe the second half of the 20th century and the beginning of the 21st century, with its abundance of technological innovations, without it. But what seems obvious to scientists today was considered heresy mainly in the early years of the 20th century. The continuous nature of space, as expressed in Newton's theory of gravitation or the electric and magnetic field theories of James Clerk Maxwell, and especially the continuous nature of light, which was revealed to scientists in dozens of experiments on the diffraction, refraction and refraction of light rays, the most famous of which is the "two slit experiment ” of Thomas Young, were considered at that time a fait accompli. Every physicist then knew what light was - continuous electromagnetic radiation, or in simple words, a wave.

Then came Einstein, and in a short article he sent to the scientific monthly "Annalen der physik" in March 1905, he turned the bowl upside down. The title of the article seemed completely innocent: "On a heuristic (speculative and fruitful, a.h.) point of view regarding the creation of light and its conversion". But it hid one of the greatest revolutions of modern physics, if not the greatest of them. "According to the hypothesis discussed here," Einstein wrote in the article, "when a light beam spreads through space and exits from some point in it, its energy is not distributed in a continuous distribution... but it consists of a finite number of energy packets (quanta)... which are absorbed and formed as complete units only."

Einstein was of course aware of the interference, refraction and diffraction experiments in which the wave nature of light was discovered, but those "wave" optical phenomena did not represent cases in which light was emitted ("created") from some source or absorbed ("converted") by some source, i.e. cases in which light comes into contact with the world of atomic matter. Indeed, five years earlier, the German physicist Max Planck had tried to analyze the behavior of light when it came into contact with atoms, and in the process discovered Planck's constant - the discrete and minimal amount of energy, measuring 6.626X10-27 arg-second, which must be assumed to solve a problem known as Black body radiation.

A black body is a body that absorbs all the radiation that hits it. Since such a body heats up as a result of this radiation, it begins to emit electromagnetic radiation. What scientists could not explain is the fact that the intensity of this radiation depends solely on the temperature of the black body, and not on any other parameter. Planck provided a physical model for the phenomenon - his hypothesis that energy is emitted from matter only in discrete portions allowed him to explain the distribution of radiation and its dependence on temperature.

Einstein, then, was not the first to talk about discrete energy packets. But Planck, in his solution to the radiation problem of a black body, considered the isolated energy packets that he had to assume "a product of desperation", and the physical model he built was somewhat forced. Einstein, unlike him, showed in his groundbreaking article how fertile the quantum hypothesis is, according to which energy is "created" or "converted" in discrete units and not as a continuous wave, especially when it is applied to electromagnetic radiation.

Einstein's paper on the quantum hypothesis of light is considered one of the most beautiful theoretical papers written in the history of modern physics, and in 1921 it earned Einstein the Nobel Prize in Physics. Einstein describes in the article how, from thermodynamic and probabilistic considerations, which were first put forward by the Austrian physicist Ludwig Boltzmann, they arrive at the hypothesis that light is of a particle nature and not a wave. Einstein used in the article a model familiar to every physicist, in which radiation (light) and matter (molecules of gas and electrons located inside a closed container) "mix" with each other, when during the collisions of the gas molecules with the walls of the container and with the electrons inside, light is emitted and absorbed by the electrons. But this innocent description, Einstein showed, contains a catastrophe, since without the assumption that the light radiation emitted by the electrons or absorbed by them is discrete and discontinuous, the model predicts that the energy of the light radiation inside the container will become infinite - a prediction that is obviously absurd.

Einstein demonstrated the fruitfulness of the quantum hypothesis later in the article, when he showed how the "particle" point of view of light leads to the solution of additional physical problems, which until that time were a mystery. The most famous of them is the problem of the photoelectric effect. This effect was first discovered in 1887 by the German physicist Heinrich Hertz, and is based on the phenomenon that light striking a metallic body causes the emission of electrons. This phenomenon was a major stumbling block for the wave theory of light, since it was impossible to reconcile the interaction that resulted in the emission of individual electrons with the continuous nature of light. The assumption that light is not continuous but "made up" of discrete packets, which will later be called photons, solved the problem.

It should be noted that Einstein did not prove the existence of photons, but only showed how fertile is the hypothesis that they exist, and even provided accurate predictions for confirmation in the context of the photoelectric effect. But even though these predictions were verified with astonishing accuracy in 1916 in a series of experiments carried out by the American physicist Robert Milliken, until 1923, the year in which the American physicist Arthur Compton showed that light behaved exactly as Einstein predicted - as if it were composed of tiny billiard balls colliding with each other - among scientists Many recognize the correctness of the quantum hypothesis. Also interesting was the reaction of Planck himself, who "recruited" Einstein to the Prussian Academy of Sciences. In a speech he gave at the ceremony of Einstein's acceptance into the academy in 1913, he said that "the fact that Einstein sometimes exaggerates his speculations, as in the case of the quantum hypothesis of light, should not be considered a point against him."

The particles of light, the photons, quickly became an iron sheep property of modern physics, and it is impossible to imagine the development of quantum mechanics without them. Indeed, according to quantum field theory and quantum electrodynamics, the photon is the particle that "mediates" the electromagnetic forces. But quantum mechanics, in addition to the particle nature of light, also adopted the wave nature of matter, and this duality caused paradoxes and oddities that Einstein himself, even though he is considered one of the fathers of the theory, did not have the pleasure of accepting. 30 years after the article on the quantum hypothesis, Einstein launched an unprecedented attack on quantum mechanics. Before his death, he wrote about photons in a letter to a friend: "After fifty years of wrestling with the question 'What is a quantum of light,' I am still far from the answer."

Dr. Hagar is a philosopher of physics

Tomorrow: The big innovation and the small errors in Einstein's doctoral thesis

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