Physicists worked to find mathematical models which could predict when and how such energy would be emitted. Roger Stuewer writes:
In 1859–60 Kirchhoff had defined a blackbody as an object that reemits all of the radiant energy incident upon it; i.e., it is a perfect emitter and absorber of radiation. There was, therefore, something absolute about blackbody radiation, and by the 1890s various experimental and theoretical attempts had been made to determine its spectral energy distribution — the curve displaying how much radiant energy is emitted at different frequencies for a given temperature of the blackbody. Planck was particularly attracted to the formula found in 1896 by his colleague Wilhelm Wien at the Physikalisch-Technische Reichsanstalt (PTR) in Berlin-Charlottenburg, and he subsequently made a series of attempts to derive “Wien’s law” on the basis of the second law of thermodynamics. By October 1900, however, other colleagues at the PTR, the experimentalists Otto Richard Lummer, Ernst Pringsheim, Heinrich Rubens, and Ferdinand Kurlbaum, had found definite indications that Wien’s law, while valid at high frequencies, broke down completely at low frequencies.
The work of Kurlbaum and Rubens provided the impetus and raw material for Planck’s discovery of a new law. Kurlbaum was born in 1857 and had been working in Berlin since 1891. Rubens was born in 1865, and was working in Berlin by 1888 or possibly earlier.
Kurlbaum died in 1927, and Rubens in 1922. The latter’s death may have been the result of exposure to high levels of radiation from working with radium and other unstable isotopes in a laboratory. The dangers of such radiation to human health had not yet been fully understood at that time.
The work of Kurlbaum and Rubens provided data from which Planck could construct, and then test, a mathematical model of energy emission. Werner Heisenberg describes how Planck came to make a discovery:
Als Planck im Jahre 1895 mit seiner wissenschaftlichen Arbeit in dieses Forschungsgebiet eintrat, versuchte er das Problem von der Strahlung auf das strahlende Atom zu verschieben. Durch diese Verschiebung wurden die tieferen Schwierigkeiten des Problems zwar nicht beseitigt, aber ihre Interpretation und die Deutung der empirischen Tatsachen wurden dadurch einfacher. Eben in jener Zeit, nämlich im Sommer 1900, hatten Curlbaum und Rubens in Berlin sehr genaue Messungen des Spektrums der Wärmestrahlung vorgenommen. Als Planck von diesen Ergebnissen hörte, versuchte er sie durch einfache mathematische Formeln darzustellen, die nach seinen allgemeinen Untersuchungen über den Zusammenhang zwischen Wärme und Strahlungen plausibel aussahen. Eines Tages, so wird berichtet, trafen sich Planck und Rubens in Plancks Hause zum Tee und verglichen Rubens’ neueste Resultate mit einer Formel, die Planck zur Deutung von Rubens’ Messungen vorgeschlagen hatte. Der Vergleich zeigte eine vollständige Übereinstimmung. Damit war das Plancksche Gesetz der Wärmestrahlung entdeckt.
Planck’s discovery was not the end, but rather the beginning of a series of discoveries which would together constitute a major revision of hypotheses about radiant energy. The task of systematizing or predicting the emission of energy from matter, and more specifically from an atom, would prove to be the puzzle which occasioned the emergence of quantum physics and the fabled discoveries made by Heisenberg.
