Max Planck

• 1. Introduction

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  Max Karl Ernst Ludwig Planck was born on the 23rd of April 1858 in Kiel, and he would go on to crack open one of the most stubborn problems in all of physics by proposing an idea that, at the time, he himself could barely believe. That idea held that energy does not flow in a smooth and continuous stream. It comes in discrete packets, fixed parcels he called quanta. The consequences would reshape science from the atom outward.

  Planck arrived at this conclusion not through boldness but through desperation. He described the step as "an act of despair," saying he was "ready to sacrifice any of my previous convictions about physics." Here was a man who prized conservative reasoning and classical certainty, yet who handed physics its most unsettling revolution. What drove him to it? And once the idea was out in the world, why did he spend years trying to take it back?

  By the time Planck died in Göttingen on the 4th of October 1947, he had witnessed two world wars, the Nazi seizure of power, the destruction of his Berlin home by Allied bombing, and the execution of his own son. Through all of it, he returned again and again to one phrase: persevere and continue working. What that perseverance cost him, and what it ultimately produced, is the story that follows.

• 2. Roots and Formation

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  Kiel in the 1860s was not a quiet place to grow up. One of Planck's earliest memories was watching Prussian and Austrian troops march through the city during the Second Schleswig War in 1864. The Planck household itself stood on deep intellectual ground: his paternal great-grandfather and grandfather had both held theology professorships at the University of Göttingen, and his father lectured in law at Kiel University and later at the Ludwig-Maximilians-Universität München.

  The family relocated to Munich in 1867, and there a mathematician named Hermann Müller spotted something in the young Planck. Müller taught him astronomy, mechanics, and mathematics beyond the standard curriculum, and it was through Müller that Planck first encountered the law of conservation of energy. That encounter planted the seed of his lifelong preoccupation with thermodynamics.

  At university Planck received a discouraging forecast. His supervisor at Munich, Philipp von Jolly, told him around 1878 that physics was "a highly developed, nearly fully matured science" that would "soon take its final stable form." Planck pressed on anyway. He spent a year in Berlin studying under Hermann von Helmholtz and Gustav Kirchhoff, and later described Helmholtz as slow, mistake-prone, and frequently boring, while Kirchhoff lectured with precision that was nonetheless dry. Yet Planck grew genuinely close to Helmholtz, and it was in Berlin that he immersed himself in the writings of Rudolf Clausius on thermodynamics. By February 1879 he had defended his doctoral thesis on the second law of mechanical heat theory, and by 1880 he held the two highest academic degrees available in Europe.

• 3. Entropy and Equilibrium

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  Rudolf Clausius introduced the concept of entropy in 1865, defining it as a measure of the reversible supply of heat relative to absolute temperature. His reformulation of the second law stated simply that entropy can be created but never destroyed. It was this body of work that Planck absorbed and then substantially extended in his 1879 thesis, identifying contradictions in Clausius's earlier writings and resolving them.

  Where Clausius had confined the second law to reversible and thermal processes, Planck pushed its reach to all natural processes. He also insisted that entropy measures not merely a property of a physical system but the irreversibility of any process within it: if a process generates entropy, it cannot be undone. He developed this argument at length in a series of 1887 treatises titled "On the Principle of the Increase of Entropy."

  Planck pursued this work through a phenomenological approach, deliberately avoiding the molecular and probabilistic methods that dominated the field at the time. He was skeptical of atomism, and this skepticism shaped his conclusions: he focused on equilibrium states, studying the coexistence of aggregate phases and the equilibrium of gas reactions. Working independently and without access to it, he reconstructed nearly the same body of knowledge about physicochemical equilibria that Josiah Willard Gibbs had already published from 1876 onward. Gibbs's work did not appear in German until 1892. Scholars later credited Planck's approach with greater universality, even though Gibbs's equilibrium framework ultimately became the dominant one for its simplicity.

  During the same decade, Planck derived the theoretical relationship between the conductivity and dilution of solutions, and provided formal grounding for the freezing-point and boiling-point phenomena that François-Marie Raoult and Jacobus Henricus van 't Hoff had discovered empirically in 1886. He published his Treatise on Thermodynamics in 1897, by which point he held the chair at the Friedrich Wilhelm University of Berlin that Kirchhoff had once occupied.

• 4. The Birth of Quantum Theory

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  In 1894, Planck turned to a problem that had first been framed by Kirchhoff in 1859: how does the intensity of electromagnetic radiation emitted by a perfect absorber depend on the frequency of that radiation and the temperature of the body? Two existing laws had each solved half the puzzle. Wilhelm Wien's law matched experimental data at high frequencies but failed at low ones. The Rayleigh-Jeans law agreed with observations at low frequencies but predicted a catastrophic, unbounded rise in energy at high frequencies.

  Planck's first proposed solution, built on what he called the "principle of elementary disorder," arrived in 1899. It produced a modified version of Wien's law that experimental evidence quickly failed to confirm. Facing that failure, he rebuilt his derivation from a different foundation: Boltzmann's statistical interpretation of thermodynamics, an approach he had previously resisted. His new radiation law described the observed black-body spectrum cleanly across all frequencies. It was first presented to the German Physical Society on the 19th of October 1900 and published in 1901.

  The price of that success was a radical postulate. Planck assumed, on what he later called purely formal grounds, that electromagnetic energy could be emitted only in multiples of a fixed elementary unit, expressed as the product of a constant - which he had introduced in 1899 and which now bears his name - and the frequency of the radiation. He explained it this way: "Radiant heat is not a continuous flow and indefinitely divisible... It must be defined as a discontinuous mass, made up of units all of which are similar to one another." He called these units "the pennies of the atomic world."

  At first Planck did not regard this quantization as physically real. "Actually I did not think much about it," he admitted. But the constant he had defined turned out to carry enormous power: it allowed him to construct an entirely new set of universal physical units, including what are now called the Planck length and Planck mass, grounded in fundamental constants of nature. In a private conversation with his son in December 1918 he called the discovery "comparable perhaps only to the discoveries of Newton." The Nobel Prize in Physics for 1918, awarded "for the discovery of energy quanta," confirmed that the wider scientific world agreed.

• 5. Reluctant Revolutionary

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  Max Born, who knew Planck well, wrote that Planck "was, by nature, a conservative mind; he had nothing of the revolutionary and was thoroughly skeptical about speculations. Yet his belief in the compelling force of logical reasoning from facts was so strong that he did not flinch from announcing the most revolutionary idea which ever has shaken physics."

  The irony deepened almost immediately. Planck himself spent years trying to reconcile the energy quantum with classical physics, and his own attempts "extended over several years and caused me much trouble" without success. Even other leading physicists resisted: Rayleigh, Jeans, and Lorentz at various points set the Planck constant to zero in order to recover agreement with classical predictions. Planck's response to Jeans was blunt: "I am unable to understand Jeans' stubbornness - he is an example of a theoretician as should never be existing."

  When Albert Einstein published three papers in the Annalen der Physik in 1905, Planck was among the few who immediately grasped the significance of the special theory of relativity and used his influence to promote its acceptance in Germany. Yet Einstein's proposal that light itself travels in discrete quanta - building on Heinrich Hertz's 1887 discovery of the photoelectric effect - struck Planck as a step too far. He worried it would set back the theory of light "not by decades, but by centuries."

  By the end of the 1920s, when Niels Bohr, Werner Heisenberg, and Wolfgang Pauli had developed the Copenhagen interpretation of quantum mechanics, Planck rejected it, alongside Schrödinger, Laue, and Einstein. He expected that wave mechanics would make the whole quantum framework obsolete. That expectation proved wrong. The discipline Planck had founded continued to expand beyond any framework he found philosophically comfortable, and his own remark about scientific change - that new truths triumph not by converting opponents but because those opponents eventually die - came back to haunt him, though historians later noted its accuracy was more limited than often assumed.

• 6. Science Under Nazi Rule

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  When the Nazi government took power in 1933, Planck was 74 years old and had been president of the Kaiser Wilhelm Society since 1930. He watched colleagues lose their positions and saw hundreds of scientists leave Germany. His response was to counsel staying put and persevering, a posture that placed him in impossible positions.

  Otto Hahn asked him to organize a public protest by prominent professors against the treatment of Jewish scientists. Planck declined, reasoning that any statement from thirty senior figures would draw a counter-statement from a hundred and fifty others eager to take the vacated positions. In May 1933 he sought a personal meeting with Adolf Hitler to argue that expelling Jewish scientists would destroy German science and that Jewish Germans could be loyal citizens. Hitler responded that he had nothing against Jews, only against communists, an equation Planck recognized as eliminating any ground for further argument.

  The limits of Planck's strategy became visible in a series of specific confrontations. Johannes Stark, a champion of the Deutsche Physik movement, attacked Planck, Arnold Sommerfeld, and Heisenberg for continuing to teach Einstein's theories and labeled them "white Jews." A Nazi government office then opened an investigation into Planck's ancestry, claiming he was "1/16 Jewish." Planck denied it. In 1938, when the Prussian Academy of Sciences was absorbed into the Nazi system as part of Gleichschaltung, Planck resigned his presidency in protest. That same year the Max-Planck Medal, which the German Physical Society had established as its highest honor in 1928, was awarded at his 80th birthday celebration to French physicist Louis de Broglie.

  The war brought destruction on a personal scale. Allied bombing raids forced Planck and his wife out of Berlin. In February 1944 an air raid obliterated their home, along with all his scientific records and correspondence. Then, in 1944, his son Erwin was arrested by the Gestapo for alleged involvement in the July assassination attempt against Hitler. Erwin was sentenced to death by the People's Court in October 1944 and hanged at Berlin's Plötzensee Prison on the 23rd of January 1945. Planck wrote in 1942 that he harbored an ardent desire to live long enough to witness "the beginning of a new rise." The execution of Erwin, to whom he had been particularly close, took much of that will from him.

• 7. Faith, Philosophy, and Legacy

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  Planck began his career as a follower of Ernst Mach's positivism, which held that science should concern itself only with observable phenomena. His discovery of the quantum of action moved him in the opposite direction. He became a committed scientific realist, arguing that physics must be grounded in objective realities that exist independently of any observer. This shift produced a public dispute with Mach in 1908 and later made Planck an early and firm defender of Einstein's relativity, which he read as confirming the existence of absolute structures beneath measurable appearances.

  The same philosophical conviction made him a persistent critic of the Copenhagen interpretation, which he viewed as abandoning the causal and objective universe he believed in. He wrote that the quest for laws applying to an absolute, independent external world "appeared as the most sublime scientific pursuit in life."

  Planck was a member of the Lutheran Church in Germany throughout his life. In a 1937 lecture titled "Religion und Naturwissenschaft," he argued that religious symbols provide an imperfect but genuine pathway to worship, and he criticized both atheists for mocking those symbols and believers for over-estimating them. Historian of science John L. Heilbron characterized his views as deistic: Planck acknowledged being deeply religious but said he did not believe in a personal God, let alone a specifically Christian one.

  His musical life ran in parallel with his science for his entire adult life. He composed the opera Die Liebe im Walde during his university years and held weekly musical soirées at his Berlin home, where Albert Einstein would play violin alongside the celebrated violinist Joseph Joachim. Einstein, writing the introduction to Planck's book Where Is Science Going?, placed him among "those few worshipers in the Temple of Science who would still remain should an angel of God descend and drive out of the temple all those lesser scientists, who under different circumstances might become politicians or captains of industry."

  After the war ended, Planck was brought to Göttingen, where he died on the 4th of October 1947. He is buried at the Stadtfriedhof there. The Kaiser Wilhelm Society was renamed the Max Planck Society in 1948, and by the time of this writing it encompasses 83 institutions across a wide range of scientific disciplines.