ABSTRACT
By the nineteenth century classical physics was well established. Newtonian dynamics provided the way to calculate the behaviour of bodies in motion, and Maxwell’s electromagnetic theory accounted for electric and magnetic phenomena. Yet in a few years around the turn of the century several experimental discoveries were made that opened up a whole new world. Becquerel discovered radioactivity, and Planck’s explanation of the spectra of black-body radiation required the introduction of the quantum. These and other discoveries could not be explained by existing classical concepts, and it was only after two decades of intensive work that the foundations of a new understanding were laid. The advent of the quantum was totally unexpected. Unlike the Special
Theory of Relativity, it was in no sense a logical extension of what had been known before. It was forced on reluctant physicists by the results of measurements of the spectrum of black-body radiation, and subsequently confirmed by the photoelectric effect and by Compton scattering. Later on, it proved to be the key to understanding the structure of the atom and the nucleus. Max Planck came from a distinguished family of scholars, lawyers and
doctors. He was a rather severe and upright Prussian, in contrast to the more Bohemian Einstein. He was said to personify the German Protestant ideal of an ‘excellent, incorruptible, idealistic man, devoted to the service of Church and State’ (Heilbron, 1986, p. 168). His early work was on thermodynamics, and he realized that the spectrum of black-body radiation is a universal and fundamental characteristic of matter, and therefore worthy of serious study. Black-body radiation is emitted from a small hole in the side of a furnace. The interior walls of the furnace are rough, and the hole is small, to ensure that the radiation reaches statistical equilibrium before emission. The spectrum of the black-body radiation was measured accurately, and it was a challenging problem to try to understand it. Rayleigh and Jeans used classical physics to derive the frequency distribution
of the radiation at a temperature T and found it to be
where n is the frequency and k is Boltzmann’s constant. This agrees quite well with measurements at low frequencies, but tends to infinity for high frequencies, which is obviously wrong. This is the ‘ultraviolet catastrophe’.
