Quantum of action
The smallest portion of energy that a material oscillator of a given frequency emits or absorbs: energy turned out to be not continuous but divided into equal steps proportional to frequency, with a new physical constant h.
The assumption of these portions was made by the German Max Planck in order to derive a formula for the black-body spectrum that agreed with the long-wavelength measurements of Heinrich Rubens and Ferdinand Kurlbaum. The formula was found in October 1900, the derivation with energy elements was presented on 14 December 1900, the paper appearing in Annalen der Physik in 1901.
Planck himself at first regarded the assumption as purely formal and accepted the quantum of action as physically irreducible only around 1908; historians still debate what exactly he meant in 1900. But it is here that classical physics ceased to be complete.
Why this date
exact for the year: on 7 October 1900 Rubens told Planck of the long-wavelength measurements and Planck found the formula the same day (DSB); the derivation with energy elements was presented on 14 December 1900 (Kragh); the Annalen der Physik paper appeared in 1901. The dates of the presentation of the formula (October 1900) and of the paper's receipt are not confirmed by the sources cited and are not given here.
What it grew from
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Spectral analysis
influenced · Confidence: medium
Kirchhoff's spectral work of 1859 produced the radiation law and the notion of the black body; the energy distribution of the black body over wavelength, measured at the Physikalisch-Technische Reichsanstalt (Lummer and Pringsheim, Rubens and Kurlbaum, 1899-1900), is the very spectrum for which Planck derived his formula with the quantum of action.
Dispute: The line of descent holds, but it runs not through chemical spectral analysis (the lines of the elements) but through Kirchhoff's radiation law and measurements of the spectral energy distribution of the black body. The decisive measurements by Rubens and Kurlbaum (October 1900) used the residual-ray method (multiple reflection from crystals), not a prism spectroscope; prism spectrometers were used by Lummer and Pringsheim (1899). Historians (Kragh, DSB) trace the quantum back to Kirchhoff's law of 1859, not to the spectroscope as an instrument.
Link sources
- Kragh H. «Max Planck: the reluctant revolutionary», Physics World, December 2000 (2000) Verified 20 Sep 2026
The study of black-body radiation had begun in 1859, when Robert Kirchhoff, Planck's predecessor as professor of physics in Berlin, argued that such radiation was of a fundamental nature.
- Encyclopedia of World Biography, «Gustav Robert Kirchhoff» Verified 20 Sep 2026, quotation assembled from fragments
Spectroscopy turned out to be the magic key...half a century later it ushered in the era of modern atomic physics.
- Deutsches Patent- und Markenamt, «Gustav Robert Kirchhoff» (2024) Verified 20 Sep 2026
This direction of development ultimately led to Planck's quantum hypothesis. Kirchhoff is therefore one of its pioneers.
- Kangro H. «Rubens, Heinrich», Dictionary of Scientific Biography (Encyclopedia.com) (1975) Verified 20 Sep 2026
sets out the decisive measurements that led to Planck's new radiation formula.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
- Kragh H. «Max Planck: the reluctant revolutionary», Physics World, December 2000 (2000) Verified 20 Sep 2026
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Kinetic theory of gases and statistical mechanics
prerequisite for · Confidence: high
Planck derived the radiation formula by applying the combinatorial counting of states from Boltzmann's 1877 paper, and it was that counting which required energy to be divided into finite elements, whence the quantum of action.
Dispute: Historians disagree about exactly what Planck took from Boltzmann. Klein and those following him hold that in 1900 Planck introduced something genuinely new into physics, finite elements of energy, and knew it. Kuhn in 1978 objects: for Planck energy remained continuous, the combinatorics was a technical device, and discontinuity appeared only in 1905-1906 with Einstein and Ehrenfest. Gearhart adds a third point: Planck adopted Boltzmann's formalism but not his probabilistic view of the second law, which Planck held to be absolute ('Planck thought it entirely impossible that there could be any exceptions…to the law of entropy increase').
Link sources
- Martin J. Klein, Max Planck and the Beginnings of the Quantum Theory, Archive for History of Exact Sciences 1 (1962) Verified 20 Sep 2026
During those two months PLANCK had successfully changed the direction of all of his previous thought in recognizing and adopting BOLTZMANN'S insight into the relationship between entropy and probability.
- Clayton A. Gearhart, Planck, the Quantum, and the Historians, Physics in Perspective 4 (2002) Verified 20 Sep 2026
In December 1900, Planck cited Boltzmann, 'especially' his 1877 paper, and said that by introducing probability considerations, he could derive an expression for the entropy of a resonator.
- M. Planck, quoted in Martin J. Klein, Max Planck and the Beginnings of the Quantum Theory, Archive for History of Exact Sciences 1 (1962) Verified 20 Sep 2026
I was not only indifferent but to a certain extent even hostile to the atomic theory which was the foundation of his entire research.
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong.
- Martin J. Klein, Max Planck and the Beginnings of the Quantum Theory, Archive for History of Exact Sciences 1 (1962) Verified 20 Sep 2026
Node sources
- Kragh H. «Max Planck: the reluctant revolutionary», Physics World, December 2000 (2000) Verified 20 Sep 2026, quotation assembled from fragments
In his seminal paper published in late 1900 and presented to the German Physical Society on 14 December - 100 years ago this month - Planck regarded the energy...
- Gearhart C. A. «Planck, the quantum, and the historians», Physics in Perspective 4, 170-215 (2002) Verified 20 Sep 2026
In late 1900, the German theoretical physicist Max Planck derived an expression for the spectrum of black-body radiation. That derivation was the first step in the introduction of quantum concepts into physics.
- Kangro H. «Rubens, Heinrich», Dictionary of Scientific Biography (Encyclopedia.com) (1975) Verified 20 Sep 2026
On 7 October 1900, however, Rubens mentioned to Planck that his results concerning residual rays of great wavelength were actually consistent with a law that Lord Rayleigh had suggested in June of that year
"Verified" means that on that day the quotation was found at the address given, word for word. No date means we have not been able to open the source yet, not that the quotation is wrong. "Assembled from fragments" means the words do not stand together on the page: we found each fragment separately and joined them with an ellipsis.
Further reading
Not evidence: these carry no quotation and support nothing we wrote. They are books and scans to read next.