Lecture 4 of the "Quantum Revolution" course (Spring 2025)
In this lecture, we examine the prehistory of quantum mechanics, examining the pioneers who put forward new ideas that later formed the foundation of the new science. Pioneers should not be confused with "discoverers," that is, with the physicists who developed the new theory. In the case of quantum mechanics, the discoverers are Heisenberg, Born, Jordan, Schrödinger, Dirac, Pauli, Bohr, and others. The pioneers are Planck, Einstein, Rutherford, Bohr, Pauli, Sommerfeld, de Broglie, Goudsmit, Uhlenbeck, and others.
Two areas of research played a key role in the development of the theory of the microworld: the study of thermal radiation and spectral analysis. The fact that at the same temperature, all heated objects appear the same color was known to potters and blacksmiths long before 1859, when Gustav Kirchhoff, a thirty-four-year-old German physicist at the University of Heidelberg, began a theoretical study of the nature of this correlation. To simplify the problem, Kirchhoff introduced the concept of a "black body." Kirchhoff imagined a black body as a hollow box with a small hole in one wall. Since any radiation, visible or invisible, enters the box through the hole, it models a perfect absorber and behaves like a black body. Once inside, the radiation is reflected alternately from one wall, then the other, until it is completely absorbed.
If Planck, Einstein, and Bohr were the fathers of quantum theory, its grandfather was Gustav Robert Kirchhoff. Since he was the founder of spectroscopy (jointly with Robert Bunsen, 1860), one of the arrows leads from him and Bunsen to Johann Jakob Balmer, the author of the formula named after him. From Balmer, we move to Bohr, the founder of atomic quantum dynamics. Returning to Kirchhoff as the discoverer of the universal nature of blackbody radiation, we note that his work influenced Wien, and through him, Planck. The arrow that points from Wien to Planck corresponds to the latter's formulation of the law of blackbody radiation; the Wien-Planck-Einstein triangle corresponds to their mutual influence, which led to the proposal of the light quanta hypothesis. The arrow leading from Bose to Einstein corresponds to Bose's work on electromagnetic radiation and its influence on Einstein's contribution to the quantum statistics of a real gas. The Einstein-de Broglie-Schrödinger triangle reflects the role Einstein played in the transitional period of the development of wave mechanics.
The problem posed by Kirchhoff is known as the blackbody problem: it requires measuring the spectral distribution of radiation energy from a blackbody—that is, the amount of energy emitted for each wavelength, from infrared to ultraviolet—and deriving a formula for this distribution at an arbitrary temperature.
In 1860, Kirchhoff demonstrated that the spectral distribution of thermal radiation is a universal function of frequency and temperature. The task arose of finding this function. This problem was ultimately solved by Max Planck. His discovery is now considered the birth of quantum physics. This is not entirely true!
The development of the theory was facilitated by practical problems. Only in the early 1880s did German companies set themselves the goal of producing incandescent lamps that could compete with British and American ones. Measuring the spectrum of blackbody radiation and deriving the equation Kirchhoff dreamed of became a priority. Edison received a patent for the incandescent lamp and its base. To this day, lamps are marked with the letter E, for Edison. Max Planck proved to be the right man in the right place at the right time: in addition to improving the quality of light bulbs, he discovered the quantum field.