Max Planck

Max Karl Ernst Ludwig Planck was born on April 23, 1858 in Kiel, then a port town in the duchy of Holstein under Danish administration. His father was a professor of constitutional law; his grandfather and great-grandfather had been theologians at Göttingen. The Plancks were the sort of north-German family that produced civil servants, pastors, and academics by the generation: sober, observant, conservative, and steeped in the conviction that intellectual life was a duty owed to the state. When the family moved to Munich in 1867 the nine-year-old Max enrolled in the Maximiliansgymnasium and presented himself in 1874 to the University of Munich with no particular ambition beyond a quiet life in physics.

He was warned off the field at the door. Philipp von Jolly, the senior physicist at Munich, told the young man that essentially everything important had already been worked out; what remained was polishing constants in the sixth decimal place. Planck thanked him and enrolled anyway. He took his doctorate at Munich in 1879 on the foundations of the second law of thermodynamics, and after a long unpaid stretch as Privatdozent succeeded Gustav Kirchhoff in 1889 as professor of theoretical physics in Berlin. There he would remain for the rest of his working life.

Berlin in the 1890s housed the imperial standards lab, whose experimentalists at the Physikalisch-Technische Reichsanstalt were measuring the spectrum of black-body radiation with unprecedented precision. Planck, by then forty-two and known as a careful classical thermodynamicist, was asked to derive that spectrum from first principles. He spent most of 1900 producing one interpolation formula after another. On the afternoon of October 7, after Rubens visited his apartment to share fresh infrared data, Planck found a formula by midnight that fit at every wavelength they had measured. For eight weeks he tried to derive it from anything resembling classical physics, and could not. On December 14, 1900 he presented to the German Physical Society the only derivation that worked: oscillators in the cavity walls could only hold energy in integer multiples of a small quantity proportional to their frequency. The constant of proportionality, which he extracted from Kurlbaum’s data, was h ≈ 6.626 × 10⁻³⁴ J·s.

Max Karl Ernst Ludwig Planck (; ; 23 April 1858 – 4 October 1947) was a German theoretical physicist. He won the 1918 Nobel Prize in Physics "for the services he rendered to the advancement of physics by his discovery of energy quanta."

He never made peace with what he had done. In a 1931 letter to Robert Wood he called the leap an act of desperation: he had been ready, he wrote, to sacrifice every one of his previous convictions about physics. For more than a decade he hoped quietly that the discontinuity lived only in the cavity walls and not in light itself, and he was slow to endorse Einstein’s photon hypothesis even after nominating Einstein to the Prussian Academy in 1913.

The rest of his life was overtaken by Germany’s catastrophes. His first wife Marie died of tuberculosis in 1909. Their elder son Karl was killed at Verdun in 1916. Twin daughters Grete and Emma both died in childbirth, two years apart. By 1919 Planck had lost four of his five children to disease, war, or labor. He continued teaching, and in 1918 was awarded the Nobel Prize in Physics for the quantum hypothesis he still half-disbelieved.

When the Nazis came to power in 1933 Planck was seventy-five and the senior figure in German physics, president of the Kaiser Wilhelm Society. He stayed at his post and tried to use what authority remained to him to protect Jewish colleagues. He requested and received an audience with Hitler in May 1933, attempting to explain that the forced removal of Fritz Haber and other Jewish scientists would gut German science for a generation. Hitler refused to discuss specifics and dismissed him. The dismissals went ahead. Planck attended Haber’s memorial in 1935 in defiance of a government ban, a stubborn old man trying to slow the destruction of an institution he had spent fifty years building.

The institution did not survive intact and neither did his family. His house in Berlin-Grunewald was destroyed by Allied bombing in February 1944, taking his library and correspondence with it. His younger son Erwin was implicated in the July 20, 1944 plot against Hitler, arrested by the Gestapo, sentenced to death by the People’s Court, and executed by hanging at Plötzensee on January 23, 1945. Planck was eighty-six. American forces evacuated him from the ruins of Rogätz to Göttingen in May 1945. He lived two more years, helped re-found the German scientific society that would soon bear his name, and died on October 4, 1947.

The Planck constant, or '''Planck's constant''', denoted by h, is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and a particle's momentum is equal to the wavenumber of the associated matter wave (the reciprocal of its wavelength) multiplied by the Planck constant.

His name is now everywhere in physics. Planck’s constant sets the scale of every quantum process; the Planck length, mass, and time are the units that fall out when one combines h with G and c; the Max Planck Society funds the descendants of the Kaiser Wilhelm institutes he tried to save. The act of desperation he committed on a December afternoon in Berlin opened every chapter that follows in this book. He spent the rest of his life trying to understand what he had done.