Max Planck: The Reluctant Revolutionary Who Discovered the Quantum

Of all the figures who shaped the physics of the twentieth century, none is more poignantly self-divided than the grave, courteous, deeply conservative Berlin professor who, on the afternoon of 14 December 1900, presented to the German Physical Society a short paper that contained, almost as an accidental algebraic device, the most revolutionary idea in the history of natural philosophy since Isaac Newton. Max Planck had been studying the spectrum of the radiation emitted by a heated cavity — the so-called blackbody — for nearly six years. He had constructed an empirical formula that fitted the experimental data with extraordinary precision. To derive that formula from first principles, however, he had been forced to suppose that the energy of the oscillators inside the cavity could not vary continuously, as classical physics required, but could take only discrete values that were integer multiples of a quantity proportional to the frequency. The proportionality constant — a number so small that it had no parallel in the macroscopic world, scarcely six and a half divided by ten to the thirty-fourth in the units of his calculation — became, in the years that followed, one of the three fundamental constants of nature. He called it h, and it has been called h ever since. The introduction of h into the equations of physics was, in retrospect, the moment at which the classical worldview began its long unravelling, but Planck himself did not understand at the time what he had done, and for the better part of two decades he would labor, often desperately, to reconcile his discovery with the deterministic and continuous physics in which he had been educated and which he revered as a deep expression of the rationality of nature.

He was, by temperament, the least likely revolutionary the history of physics has produced. The grandson of a theologian, the son of a jurist, the descendant on both sides of generations of Prussian and Schleswig-Holstein scholars, pastors, and civil servants, he was reared in the high humanistic culture of the German Bildungsbürgertum — a culture in which Bach and Beethoven were household intimates, in which Goethe and Schiller were the standards of literary judgment, in which classical philology and the disciplined cultivation of the inner life of the mind were considered the highest achievements of which a human being was capable. He was a musician of considerable accomplishment, a pianist whose ability had been at one point sufficient to suggest a possible career as a concert performer; he was an accomplished singer in the choral tradition of the north German Protestant church; he was a mountain climber who through his long life would spend his summer holidays scaling the peaks of the Bavarian and Tyrolean Alps. He carried himself, by every account, with the gentle gravity of an old-fashioned German Herr Professor of the imperial type: tall, thin, gray-eyed, sparely dressed in the dark suit and white linen of the Prussian academic class, always courteous, never raising his voice, treating students and colleagues alike with a formal and slightly distant kindness that masked an interior life of considerable depth and considerable suffering. He was a Christian of the most reserved and rational German Protestant sort, who at the end of his life would write that "religion and natural science are not in conflict but only stand in a complementary relation," and who at the same time refused to attribute personal qualities to the God of his belief, preferring to describe the divine as the "rational order behind the phenomena."

The course of his personal life was as harrowed as his public career was distinguished. He was twice married — first to Marie Merck, his childhood sweetheart from Munich, who died young of tuberculosis after twenty-two years of happy marriage; then in late middle age to her niece Marga von Hoesslin, who survived him by sixteen years. He had five children, four of whom died before him. The eldest son, Karl, was killed at Verdun in 1916 in the First World War. The twin daughters Grete and Emma, born inseparable and devoted to each other, both died in childbirth within two years of each other in 1917 and 1919, each leaving behind a single surviving infant; in a moment of awful symmetry, the second daughter Emma married the widower of the first daughter Grete, only to die giving birth to her own first child eighteen months later. The youngest son, Erwin, was hanged by the Nazis in early 1945 for his participation in the July plot against Hitler. Only one child, the second son Hermann, who survived his father in greatly straitened circumstances in postwar Göttingen, lived past Planck's own death in October 1947. The Berlin house in which Planck had lived for more than fifty years, with its grand piano, its library of philosophical and scientific works, its correspondence with Einstein and Bohr and Schrödinger, its files of unpublished papers and lecture notes, was destroyed in a single allied bombing raid in February 1944. He lived through the four great convulsions of modern German history — the Wilhelmine Empire of his youth, the catastrophe of 1914–1918, the Weimar Republic and the hyperinflation, the rise and ruin of National Socialism — and he saw the country whose science he had labored to build into the most distinguished in the world brought to physical and moral ruin by leaders whom he loathed but whom he was too old, by 1933, to do anything to oppose except by acts of personal courage that, in the conditions of the Third Reich, were nearly futile.

Yet through all of these calamities he continued to think, to teach, to compute, to write. His professional output between 1900 and his retirement from the Berlin chair in 1928 was prodigious. After the quantum hypothesis came a long series of papers on radiation theory and on the foundations of thermodynamics. He produced, in 1906, the textbook on the Theory of Heat Radiation that became the canonical reference for the new physics and that went through five editions in his lifetime. He helped to organize the great Solvay Conferences that, from 1911 onward, drew the most important physicists of the world together in Brussels at the expense of the Belgian industrialist Ernest Solvay. He served as one of the four permanent secretaries of the Prussian Academy of Sciences from 1912 until 1938 — a post in which he largely determined the appointments to senior physical sciences positions in Berlin and which made him, for the better part of a generation, the most powerful figure in German physics. He was president of the Kaiser Wilhelm Society from 1930 to 1937, a position from which he resigned in protest at the regime's increasing persecution of Jewish colleagues, though he never resigned from the academy itself, on the principle that to leave was to abandon the institution to its enemies. He was awarded the Nobel Prize in Physics for 1918 — held over from that year of war and announced in 1919 — in recognition "of the services he rendered to the advancement of Physics by his discovery of energy quanta," and he gave a Nobel lecture in Stockholm in June 1920 that remains one of the most lucid expositions of the foundations of the new physics ever set down.

But for all his honors he remained, to those who knew him best, a man of considerable inward sorrow, formed by personal tragedy and by the historical disasters of the German Catastrophe, sustained in adversity by an unshowy religious faith, by his music, by his books, and by an iron discipline of professional duty that did not falter even in the last years of his life when, after the death of Erwin and the bombing of his house, he was reduced to lodging in a relative's farmhouse near Magdeburg and reading proofs by candlelight in the bitter winter of 1945. The biography that follows traces in detail the long path from the Schleswig-Holstein parsonage to the Göttingen grave: the boyhood in Kiel and Munich, the gymnasium training and the choice between music and physics, the doctoral thesis at Munich on the second law of thermodynamics, the long apprenticeship in junior teaching positions at Kiel and finally the call to Berlin in 1889, the patient labors on radiation theory that culminated in the great discovery of December 1900, the slow and often reluctant accommodation to the implications of his own work, the family tragedies and the political disasters of the Wilhelmine and Weimar and Nazi years, and the postwar effort to rebuild the institutions of German science from the rubble of defeat. It is a story in which scientific genius and personal grief, deep religious feeling and rigorous rationalism, the highest classical culture and the lowest historical brutality, are all interwoven in a single human life of unusual moral seriousness.

Birth and Family in Kiel

Max Karl Ernst Ludwig Planck was born on 23 April 1858 in the harbor city of Kiel, then the principal naval town of the duchies of Schleswig and Holstein and a place of some twenty-five thousand inhabitants at the mouth of the long inlet that the Baltic forces inland from the Bay of Lübeck. The Kiel of 1858 was a contested city. The duchies of Schleswig and Holstein had been linked to the Danish crown since the late Middle Ages, but their population was overwhelmingly German-speaking, and the question of their political and constitutional future had been one of the great unresolved European disputes of the previous decade. The First Schleswig War of 1848–1851 had ended in Danish victory and the restoration of the union; the Second Schleswig War of 1864 would end in Danish defeat and the absorption of the duchies into the Prussian-led German state. The Plancks were Holsteiners of long standing, by every cultural and political instinct German, and the tense political atmosphere of the duchies in the years before unification formed the earliest political consciousness of the future physicist. He would remember, into old age, the sight of Austrian and Prussian troops marching through the streets of Kiel in the spring of 1864, and the family's nervous calculations of which side would prevail and what would become of his father's professorship in the event of one outcome or the other.