Milestones
MILESTONE 3 described by his formula, partially guided by the work of Ludwig Boltzmann on entropy. However, The quantum leap there was one revolutionary assump- tion that he had to make: that light aimed at constructing a standard for was emitted and absorbed in discrete the measurement of illumination packets of energy — quanta. These intensities, directed him towards were not a feature of heat radiation heat radiation. He revisited Gustav alone, but, as Albert Einstein showed Kirchhoff’s theoretical studies of in 1905, also of light. Einstein used black-body radiation, which implied the term Lichtquant, or quantum that when a substance capable of of light. Only in 1926 was the word absorbing and emitting radiation is ‘photon’ introduced, by the chemist enclosed in a cavity with perfectly Gilbert Lewis. His theory of a “hypo- reflecting walls, the spectral distribu- thetical new atom that is not light tion of the observed radiation at but plays an essential part in every equilibrium is a function only of process of radiation” did not hold up, temperature and is independent of but the name ‘photon’ stuck. the substance involved. Intrigued by Without setting out to do so, such an ‘absolute’ law, Plank devoted Planck had rocked the edifice of himself, from 1896, to finding an physics to its very foundations. “His explanation for it. was, by nature, a conservative mind,” Parallel works on black-body wrote Max Born in an obituary radiation produced confusing results. of Planck, “he had nothing of the Lord Rayleigh had found a law revolutionary and was thoroughly (which, with James Jeans, he later sceptical about speculations. Yet refined) that well described the emis- his belief in the compelling force of sion spectrum at long wavelengths, logical reasoning from facts was so but failed at short ones. By contrast, strong that he did not flinch from an earlier law by Wilhelm Wien announcing the most revolutionary describing the frequency position idea which ever has shaken physics.” of the radiation maximum — which Andreas Trabesinger, Image courtesy of Senior Editor, Nature Physics Rudolf Dührkoop had been observed experimentally, “With the discovery of the principle but was not reproduced by the ORIGINAL RESEARCH PAPERS Wien, W. Ueber of energy conservation, the edifice of Rayleigh–Jeans theory — held for die Energievertheilung im Emissionsspectrum theoretical physics is fairly complete. short, but not for long, wavelengths. eines schwarzen Körpers. Ann. Phys. 294, 662–669 There will be a mote to wipe out in a By October 1900, Planck had found (1896) | Rayleigh. Remarks upon the law of complete radiation. Phil. Mag. 49, 539–540 (1900) | corner here or there, but something a formula that interpolated between Planck, M. Entropie und Temperatur strahlender fundamentally new you won’t find.” the curve of Rayleigh and Jeans and Wärme. Ann. Phys. 306, 719–737 (1900) | Planck, M. Ueber das Gesetz der Energieverteilung im So spoke Philipp von Jolly when, in that of Wien. He sent his result, by Normalspectrum. Ann. Phys. 309, 553–563 (1901) | 1877, his student Max Planck left postcard, to Heinrich Rubens, who Planck, M. Ueber die Elementarquanta der Materie Munich for Berlin, to spend his last immediately compared it to experi- und der Elektricität. Ann. Phys. 309, 564–566 (1901) | Einstein, A. Über einen die Erzeugung year of studies there. mental data. It fitted all observations und Verwandlung des Lichtes betreffenden Planck, undeterred, went into perfectly. Spurred by the agreement, heuristischen Gesichtspunkt. Ann. Phys. 322, theoretical physics — not hoping to Planck set about finding the physical 132–148 (1905) | Jeans, J. H. On the partition of energy between matter and aether. Phil. Mag. 10, make new discoveries, but driven by character of his empirical formula. 91–98 (1905) | Lewis, G. N. The conservation of his admiration of its elegance. His On 14 December 1900, he photons. Nature 118, 874–875 (1926) FURTHER READING Planck, M. Filmed self- main interest was thermodynamics, presented the outcome in a lecture portrayal [online]
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