June 9, 1905 - A giant step in quantum physics is taken


Between March and September 1905, the German scientific journal "Annales der Physik" mailbox received four studies that would forever change the laws of physics and, ultimately, the conception of the reality of light, matter, time, and space.

The author was a 26-year-old Albert Einstein who, at the time, was working at the patent office in Berne (Switzerland). His career as a physicist had stalled after the rejection of his doctoral thesis, and his scientific passion had been relegated to his spare time and long idle hours in the office. However, this year was a miraculous one for Einstein with these four significant studies.

On 9 June 1905, the journal Annalen der Physik (1905, 17, 132) published the article with its first study, entitled "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt" ("On a heuristic point of view concerning the production and transformation of light"), authored by Albert Einstein (1879-1955).

In this article, Einstein gave a theoretical explanation of three phenomena involving the interaction of matter with energy (in the form of electromagnetic radiation): the photoelectric effect, the Stokes effect, and the absorption of light by molecules. The results published in that paper confirmed the existence of a quantum world.

In 1900, Max Planck (1858-1947), in trying to explain black body radiation, found an equation that explained the phenomenon. To do so, he had to assume that energy was not continuous, but that only certain energy levels were allowed; he introduced the constant h (now known as Planck's constant) to adjust the equations. Planck, who was a classically trained theoretical physicist, thought that what he had done was simply a mathematical contrivance and believed that, by refining the equations, he could eliminate this constant.

Although Planck's paper marks the beginning of quantum physics, it went largely unnoticed at the time. It received a decisive boost with the publication of the paper we commemorate today. In this article, Einstein explains the phenomenon of light absorption by matter, which is complementary to that of black body radiation, which is a phenomenon of light emission. Einstein starts from different premises than Planck, but reaches the same conclusions: energy is not continuous, but is absorbed by matter in the form of discrete 'packets', which Einstein called quanta. Of course, Einstein derived equations in which Planck's constant appeared, demonstrating that it is a universal constant.

This paper demonstrates that light is corpuscular in nature (back to Newton's theory!). Since light had also been shown to have a wave nature, these results paved the way for the formulation of a wave-corpuscle theory to explain the nature of light (and, in turn, to extend it to matter on a quantum scale).

It should be borne in mind that the three phenomena explained by Einstein are very frequent in nature with many practical applications, from detectors and readers based on the photoelectric effect to the absorption of light by matter, which is responsible for its colour. In addition, another important consequence of Einstein's research was the theoretical proposal for very intense monochromatic light beams (vibrating at a single frequency), i.e. the laser, which was theoretically proposed by Einstein himself in 1917.

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