From Positivism to Objectivity
Max Planck (1858–1947) was born in the city of Kiel in the Duchy of Holstein—now part of Germany.1 As a youth, he attended the classical Königliche Maximilian Gymnasium, where he was introduced to the principle of the conservation of energy by one of his teachers, Hermann Müller. Although the young Planck was gifted in music—he excelled at performing on both the piano and the organ—he opted to study physics and mathematics when he enrolled at the University of Munich in 1874. Three years later, he traveled to Berlin for a year to study under Hermann von Helmholtz, Gustav Kirchoff and the mathematician Karl Weierstrass. Inspired by a careful reading of the theoretical papers of Rudolph Clausius during his time in Berlin, Planck wrote his 1879 doctoral dissertation on Clausius’ second law of thermodynamics. After completing his habilitation thesis on The equilibrium states of isotropic bodies at different temperatures, Planck returned to Munich to work as a lecturer (Privatdozent) for a few years before being appointed Associate Professor of Theoretical Physics at the University of Kiel in 1885. His subsequent work began to combine Clausius’ theory of thermodynamics with Maxwell’s electromagnetic theory of light. An outstanding problem, identified by Kirchoff, was obtaining a proper theoretical account of the electromagnetic radiation emitted by a heated body which was itself coated with a completely absorbing substance—the so-called “black-body radiation.” Existing theories, which treated the body as a collection of tiny oscillating electrical charges, failed to account for the observed black-body emission spectrum. After completing his Treatise on Thermodynamics in 1897, he moved to the University of Berlin where he succeeded Kirchoff as assistant professor and director of the new Institute of Theoretical Physics. It was during his time in Berlin that Planck solved the problem of black-body radiation. He did so by making the radical assumption that electromagnetic energy can only exist in discrete units, or quanta. The publication of Planck’s quantum hypothesis in 1900 is now regarded as marking the birth of modern quantum theory. In the reading selection below, Planck presents some of his mature views on the relationship between heat, energy and probability.