A brief history of quantum mechanics
At the end of the 19th century, physics was considered to be the most perfect of all sciences. There were only a few unexplored problems that were soon to be solved, although these results were not expected to have a significant impact on the physical picture of the world. Very few understood the importance of unsolved problems, which included in particular the problem of explaining the phenomenon of heat radiation from the perfect black body. Closer examination of perfectly black body radiation, the photoelectric effect and the Compton effect completely changed the way physicists thought about the world. The pioneer of quantum physics was Max Planck. In 1900, after years of trying to explain the phenomenon of heat radiation from bodies based on classical physics, he assumed - contrary to Maxwell's theory - that the energies of electromagnetic waves emitted by bodies are sudden (quantified). Similarly, when the body absorbs electromagnetic waves, it absorbs energy impulses. He also assumed that the quantity of energy is proportional to the frequency of the wave (later called the Planck constant) and insisted that the theory was consistent with the results of measurements of thermal radiation.
In 1905, Albert Einstein explained the photoelectric effect by assuming that a beam of monochromatic radiation carries discrete particles of Planck's values and is absorbed into them when it interacts with matter. It was a bold extension of Planck's quantum concept. The need to quantify electromagnetic radiation, regardless of whether it interacts with matter or moves freely in space, was fully justified only with the development of the so-called quantum field theory.
In 1913, Niels Bohr explained the quantification of the energy levels of the hydrogen atom. To do so, he required the existence of a previously unknown law that allowed the electrons of the hydrogen atom to occupy only certain levels of energy. This concept solved the paradoxes of Rutherford's earlier experimental work, which showed that the entire mass of the atom was concentrated in its nucleus, the so-called planetary model of the atom. However, the question of the stability of the atom remained unresolved. Since the electrons are supposed to orbit around the nucleus, they should continuously radiate energy and eventually fall into the nucleus. Bohr was the first scientist to argue that a stable model of the atom in classical physics was impossible, and proposed a series of heuristic rules explaining the stability of matter and created a new branch of physics: quantum physics.
