Max Planck

Max Karl Ernst Ludwig Planck was born on April 23, 1858, in Kiel, then part of the Duchy of Holstein, into a family of scholars, jurists, and clergymen. His father, Johann Julius Wilhelm Planck, held a professorship in constitutional law at the University of Kiel, and his grandfather and great-grandfather had both been theology professors at Göttingen ^[1]. The household prized duty, learning, and Lutheran piety, values that shaped Planck's temperament for the rest of his life.

In 1867 the family moved to Munich, where the elder Planck took a chair at the university. Young Max enrolled at the Maximiliansgymnasium, a demanding classical school, and came under the influence of the mathematics teacher Hermann Müller, who introduced him to the principle of conservation of energy ^[1]. Planck later recalled that this idea struck him as a revelation, an absolute law standing above human affairs. He was a capable rather than dazzling pupil, graduating near the top of his class at sixteen.

Music competed seriously with science for his attention. Planck was a gifted pianist, sang in choirs, composed songs, and even considered a musical career before deciding on physics ^[2]. When he consulted the Munich physicist Philipp von Jolly about his choice around 1874, Jolly reportedly discouraged him, describing physics as a nearly complete science with little left to discover. Planck answered that he did not want to make discoveries, only to understand the existing foundations, a modest ambition that history would overturn ^[2].

Planck studied at the University of Munich from 1874 and spent the academic year 1877 to 1878 in Berlin, where he attended lectures by Hermann von Helmholtz and Gustav Kirchhoff, two of the most eminent physicists of the age ^[1]. He found their lecturing dry but their published work on thermodynamics compelling. Working largely on his own, he absorbed the writings of Rudolf Clausius on entropy, and the second law of thermodynamics became the anchor of his early research.

He earned his doctorate at Munich in 1879, at the age of twenty-one, with a dissertation on the second law, and qualified as a university lecturer the following year ^[3]. Positions were scarce, and for five years he taught in Munich without salary beyond student fees, living with his parents. In 1885 the University of Kiel appointed him associate professor of theoretical physics, giving him financial independence in his native city ^[1].

The decisive move came in 1889, when Planck was called to Berlin to succeed the recently deceased Kirchhoff. He became a full professor at the Friedrich Wilhelm University in 1892 and was elected to the Prussian Academy of Sciences in 1894 ^[3]. Berlin placed him at the center of German physics, close to the experimentalists at the Physikalisch-Technische Reichsanstalt whose precise measurements of thermal radiation would soon set the stage for his most famous work.

Among Max Planck achievements, none compares with his resolution of the blackbody radiation problem. Physicists in the 1890s could not derive a formula that correctly described the spectrum of radiation emitted by an idealized heated body across all wavelengths. Wilhelm Wien's law worked at short wavelengths but failed at long ones, where new measurements by Otto Lummer, Ernst Pringsheim, Heinrich Rubens, and Ferdinand Kurlbaum in Berlin exposed the discrepancy ^[4]. In October 1900 Planck produced an interpolation formula that matched the data across the entire spectrum with remarkable accuracy.

To justify the formula theoretically, Planck took a step he later called an act of desperation. In a paper presented to the German Physical Society on December 14, 1900, he assumed that the energy of the oscillators in the walls of the cavity could only be emitted or absorbed in discrete packets, each proportional to the frequency of the radiation ^[4]. The constant of proportionality, which he labeled h, is now known as Planck's constant, and the date is conventionally treated as the birthday of quantum theory. Planck himself regarded the quantum as a mathematical device for years and hoped it could eventually be reconciled with classical physics ^[2].

Others carried the idea further than its author intended. Albert Einstein used quantization of light itself to explain the photoelectric effect in 1905, and Niels Bohr built quantum conditions into his 1913 model of the atom. Planck received the Nobel Prize in Physics for 1918, awarded in 1919, in recognition of his discovery of energy quanta ^[3]. Beyond the quantum, he made lasting contributions to thermodynamics, wrote an influential treatise on heat theory, and was one of the first prominent physicists to champion Einstein's special theory of relativity, publicly defending it as early as 1906 ^[1].

Planck married Marie Merck, the sister of a school friend and daughter of a Munich banker, in 1887. The couple had four children: Karl, twin daughters Emma and Grete, and Erwin ^[1]. Their Berlin home in the Grunewald district became a gathering place for scientists and musicians, with Planck at the piano accompanying colleagues, among them Einstein on violin in later years ^[2].

Grief pursued him with unusual cruelty. Marie died in 1909, probably of tuberculosis. His elder son Karl was killed in action at Verdun in 1916 during the First World War. Grete died in 1917 after childbirth, and Emma died in 1919 under the same circumstances, having married her late sister's widower ^[2]. Planck bore these losses with a stoicism rooted in his sense of duty and his religious conviction, though letters from the period show how deeply they wounded him.

In 1911 he married Marga von Hoesslin, a niece of his first wife, and they had a son, Hermann ^[1]. Colleagues consistently described Planck as reserved, scrupulously fair, and almost incapable of self-promotion. He hiked and climbed mountains into old age, ascending Alpine peaks in his eighties, and kept a strict daily routine of work, music, and walks.

Planck's standing made him the effective spokesman of German science through decades of upheaval. He served as permanent secretary of the mathematics and physics section of the Prussian Academy of Sciences from 1912, and from 1930 to 1937 he was president of the Kaiser Wilhelm Society, the umbrella organization of Germany's elite research institutes ^[3]. In 1914 he signed the notorious Manifesto of the Ninety-Three defending Germany's conduct in the war, a decision he came to regret and partially retracted in later statements to foreign colleagues ^[2].

The Nazi seizure of power in 1933 confronted him with choices he never fully mastered. Planck deplored the dismissal of Jewish scientists and, according to his own later account, met Adolf Hitler in May 1933 to argue against the purge, without effect ^[2]. He chose a strategy of quiet preservation, protecting what he could of German science from within rather than resigning or protesting openly, a stance that has drawn both sympathy and criticism from historians ^[5]. He continued to speak Einstein's name with respect and delivered a memorial lecture for the chemist Fritz Haber in 1935 in defiance of official displeasure.

The Second World War destroyed almost everything he had left. Allied bombing leveled his Berlin home in February 1944, consuming his library and papers. In January 1945 his son Erwin, who had been connected to the circles behind the July 1944 plot against Hitler, was executed by the Gestapo ^[2]. Friends said the death of Erwin extinguished much of Planck's remaining will to live.

In the final months of the war, Planck and his wife fled the bombing to an estate at Rogätz on the Elbe, and in 1945 American forces brought the eighty-seven-year-old physicist to the relative safety of Göttingen ^[2]. Despite frailty and severe spinal problems, he resumed lecturing and gave talks on science and religion, themes he had explored in essays arguing that faith and physics addressed different questions and need not conflict ^[5].

His name was attached in 1946 to the reconstituted successor of the Kaiser Wilhelm Society: the occupying authorities objected to the imperial name, and the organization was rebuilt as the Max Planck Society in 1948, with Otto Hahn as its first president ^[6]. Planck served briefly as honorary president of the interim body, lending his untainted reputation to the revival of German research.

Max Planck died in Göttingen on October 4, 1947, at the age of eighty-nine, and was buried in the city cemetery there ^[3]. He had lived long enough to see the quantum idea he introduced in 1900 mature into a complete mechanics in the 1920s at the hands of Heisenberg, Schrödinger, Born, and Dirac, a development he watched with a mixture of pride and philosophical unease.

Any Max Planck biography ends with the constant. The quantity h, fixed by international agreement at exactly 6.62607015 times 10 to the minus 34 joule seconds, became the basis for the redefinition of the kilogram in 2019, tying a base unit of measurement to Planck's discovery ^[7]. Physicists also use the Planck length, Planck time, and Planck mass, natural units he proposed in 1899, to mark the scales at which quantum gravity is expected to matter.

Institutional memory of Planck is unusually broad. The Max Planck Society today operates more than eighty research institutes and ranks among the world's leading scientific organizations ^[6]. The German Physical Society's highest honor, the Max Planck Medal, was created in 1929, and its first awards went jointly to Planck and Einstein. Craters on the Moon, an asteroid, and a European Space Agency space observatory launched in 2009 to study the cosmic microwave background all carry his name ^[3].

Basic Max Planck facts, the birth in Kiel in 1858, the December 1900 quantum paper, the 1918 Nobel Prize, the death in Göttingen in 1947, only sketch the outline. Historians continue to debate his conduct under National Socialism and the meaning of his conservatism in science, since the man who insisted he only wanted to understand existing foundations ended up demolishing them ^[5]. That tension, between a cautious temperament and a revolutionary result, is what keeps his story compelling more than a century after the quantum was born.