Home > Positioning > Persons > Planck

Max Planck (1858–1947)

Planck showed that energy is not continuous but comes in discrete packets — quanta — and in doing so opened the door to quantum mechanics. His resolution of the ultraviolet catastrophe (1900) was not a revolutionary manifesto; it was a reluctant mathematical adjustment that happened to shatter the foundations of classical physics. Planck himself was a conservative physicist who spent years trying to reconcile his result with classical thermodynamics. The revolution was in the result, not in the intent. The quantum of action — Planck’s constant h — became the fundamental unit of the new physics, appearing in the uncertainty principle, the Schrödinger equation, the photoelectric effect, and every subsequent formulation of quantum mechanics.

Life

Born 23 April 1858 in Kiel, Schleswig-Holstein (then under Danish sovereignty, transferred to Prussia in 1864). His father Julius Wilhelm Planck was a professor of constitutional law. The family was academic and Lutheran; Planck inherited both traits. Educated at the Maximilians-Gymnasium in Munich, then the University of Munich (1874–77) and the University of Berlin (1877–78), where he attended lectures by Hermann von Helmholtz and Gustav Kirchhoff. PhD at Munich (1879), with a thesis on the second law of thermodynamics.

Habilitation at Munich (1880). Associate professor at the University of Kiel (1885–89). Appointed professor of theoretical physics at the University of Berlin (1889), succeeding Kirchhoff — a position he held until his retirement in 1926. Schrödinger succeeded him. Berlin under Planck was the centre of German theoretical physics; Planck’s seminar and the Berlin Physical Society were the institutional contexts in which quantum theory developed.

Nobel Prize in Physics (1918, awarded 1919) for “the discovery of energy quanta.” Planck’s personal life was marked by extraordinary losses: his first wife Marie died in 1909; his elder son Karl was killed in the First World War (1916); his twin daughters Margarete and Emma both died in childbirth (1917 and 1919). His younger son Erwin was executed by the Nazis in 1945 for involvement in the July 20 plot against Hitler.

Planck remained in Germany through the Nazi period. He met Hitler in 1933 to appeal on behalf of Jewish colleagues, including Fritz Haber; the appeal was rebuffed. He did not emigrate, choosing — controversially — to stay and protect what he could of German science. As president of the Kaiser Wilhelm Society (1930–37), he navigated between institutional survival and complicity. The moral judgment on his accommodation is contested; he was not a collaborator, but neither was he a resister. Died 4 October 1947 in Göttingen.

The quantum of action

The ultraviolet catastrophe. Classical physics predicts that a heated body radiates energy across all frequencies, with the energy per frequency increasing without limit toward the ultraviolet — an absurd result known as the ultraviolet catastrophe (or the Rayleigh-Jeans catastrophe, after the physicists whose classical calculation produced it). The prediction follows from the equipartition theorem: in classical statistical mechanics, every mode of oscillation receives an equal share of the available energy, and there are infinitely many high-frequency modes. The result contradicts observation: real heated bodies radiate a spectrum that peaks at a wavelength determined by their temperature and falls off at both high and low frequencies.

Planck’s law (1900). Planck derived a radiation formula that matched the observed spectrum exactly. The mathematical trick: he assumed that the energy of each oscillator in the heated body is not continuous but restricted to discrete values — integer multiples of a fundamental unit hν, where ν is the frequency of oscillation and h is a new constant, Planck’s constant. The assumption was initially a formal device — Planck called it “an act of desperation” — intended to fix the mathematics, not to make a physical claim. But the discrete energy levels could not be removed without destroying the agreement with experiment.

Planck’s constant (h). The fundamental constant of quantum physics: h ≈ 6.626 × 10⁻³⁴ J·s. Its dimensions are energy × time (action), and it sets the scale at which quantum effects become significant. At macroscopic scales, h is so small that quantum discreteness is invisible — energy appears continuous. At atomic scales, it dominates. The constant appears in Heisenberg’s uncertainty principle (Δx · Δp ≥ ℏ/2, where ℏ = h/2π), in the Schrödinger equation, in the photoelectric effect, and in the Boltzmann-Planck relation for entropy (S = k_B ln Ω, where Boltzmann’s constant k_B was first determined precisely by Planck from his radiation formula).

The reluctant revolutionary

Planck did not intend a revolution. His intellectual commitments were conservative: he believed in the absolute validity of the second law of thermodynamics, in the continuity of physical processes, and in the existence of objective physical reality independent of observation. The quantum hypothesis violated the second of these commitments, and Planck spent years attempting to derive his radiation law without it.

He initially treated the energy quantisation as a property of the oscillators in the heated body (a peculiarity of the emission mechanism) rather than of radiation itself. It was Einstein who drew the radical consequence: in his 1905 paper on the photoelectric effect, Einstein proposed that light itself comes in quanta (later called photons) — that quantisation is a property of the electromagnetic field, not just of matter. Planck resisted this step for over a decade. When nominating Einstein for the Prussian Academy in 1913, Planck praised his work but noted apologetically: “That he may sometimes have missed the target in his speculations, as, for example, in his hypothesis of light-quanta, cannot really be held too much against him.”

The conservatism is not a failing but a structural feature of the discovery. Planck’s constant was introduced not by a visionary who saw where physics was heading but by a careful, traditional physicist who was forced by the mathematics into a conclusion he did not want. The revolution happened despite its originator’s intentions.

Where Planck stops

Planck’s quantum hypothesis — energy comes in discrete packets — was the starting point of quantum mechanics, but Planck did not develop the theory that followed. The uncertainty principle (Heisenberg, 1927), the wave equation (Schrödinger, 1926), the probability interpretation (Born, 1926), and the mathematical unification (von Neumann, 1932) were the work of a younger generation. Planck’s own research after 1900 remained within classical thermodynamics and radiation theory; he refined and extended his radiation law but did not build the broader quantum framework.

His resistance to Einstein’s photon hypothesis — maintained for over a decade after the 1905 paper — illustrates a pattern: Planck saw quantisation as a property of matter, not of radiation. The step from quantised oscillators to quantised fields was conceptually larger than it appears in retrospect, and Planck’s reluctance was shared by many physicists of his generation. The photoelectric effect, Compton scattering (1923), and the eventual development of quantum electrodynamics all confirmed that light is quantised; Planck’s conservatism on this point delayed his own acceptance of the implications of his discovery.

The philosophical question Planck raised — whether the quantum is a real feature of nature or a useful mathematical device — was not settled by his own work. Planck himself believed in an objective physical reality underlying the mathematical formalism, a position closer to Einstein’s realism than to the Copenhagen interpretation that Bohr and Heisenberg developed. Whether the quantum of action reflects a genuine discreteness in nature or a feature of our interaction with nature is a question that the interpretive debates of the century after Planck — Copenhagen, many-worlds, relational QM — are still contesting.

Key works

See also: Maxwell · Boltzmann · Schrödinger