Conservative Revolutionary

The title of John Heilbron’s biography of the physicist Max Planck, The Dilemmas of an Upright Man, is well chosen. The chief characteristics of Planck were his integrity, his feeling for tradition, and his sense of duty. Yet throughout his adult life he faced conflicts, some of which involved reversing a stand he had taken on matters of principle; in other cases he pursued a futile struggle against inevitable disasters. Heilbron gives a very readable, and very balanced, account of the successes and disasters of this great physicist, without attempting to pass judgment. But the character of the man comes clearly through the narrative.

Planck’s work was the starting point of the quantum theory, one of the two great revolutions in twentieth-century physics. Yet he was a most unlikely revolutionary. Descended from a long line of professors of theology and law, he was deeply religious, but without belief in a personal god. He was patriotic, but never chauvinistic, and he deeply regretted one lapse in the war fever of 1914. In his science he was driven by an urge for order and unity; he regarded the edifice of physics as it then was with awe, and with the wish to complete, and not to rebuild it. Once he had formed an opinion he was slow to change it. In his many administrative functions he would prefer to act through quiet diplomacy, never through public stands or angry confrontation. He had an iron self-discipline and sense of duty, which enabled him to cope with enormous administrative burdens, and to carry on in times of the most appalling personal tragedies.

The discovery that had such revolutionary consequence was the “Planck law of radiation,” which described heat radiation. By the end of the nineteenth century it was known from observations that the color and intensity of the radiation filling a cavity in a hot body did not depend on the nature of the walls of the cavity, but only on the temperature. It was a challenge to physicists to find and explain the law determining this dependence.

Planck succeeded in finding a formula that fitted the observations accurately. Now the problem was to derive this formula from the general laws of physics. He had previously worked on thermodynamics, the science of heat, and had clarified the important concept of entropy, a measure of disorder. The concept of entropy is vital to understanding the difference between a body falling under gravity, and heat passing from a hot to a cold body. The first is reversible; if the body is elastic, like a tennis ball, it will bounce back to where it came from, whereas heat will never flow back from the cold to the hot body. This is expressed by saying that the entropy increases as the heat spreads, and that it can never decrease.

Planck now had to find how to express the entropy of the radiating atoms and the radiation. He found what seemed a suitable expression for this, no doubt working backward from the radiation law he wanted to explain. He convinced himself that this was the correct expression for the entropy according to the known laws of physics. On the strength of this he published in 1900 his paper on what is now known as “Planck’s law.”

However, he was not satisfied with the derivation, and tried hard to find better reasons for the assumptions he had had to make. But gradually it became clear that the assumptions had no place in the established laws of physics. Finally, in 1908, the great Dutch theoretical physicist H.A. Lorentz proved that the Planck law could not be derived without a revolutionary change in the principles of physics.

Einstein had already recognized this and knew that Planck had implicitly introduced the concept of the light quantum, by which light is concentrated into packets, or quanta, each of which contains a definite amount of energy, the amount being linked to the color, or frequency, of the light by Planck’s “quantum of action.”

Einstein used this concept in his explanation of the photoelectric effect in 1905. Niels Bohr used the idea in his theory of the atom in 1912; the keystone of the quantum theory, quantum mechanics, was finally put in place by Heisenberg and Schrödinger in the 1920s.

Thus Planck had started the revolution in physics, but he himself was reluctant to accept that revolution. He struggled to reconcile what he had done with the traditional principles of physics. As late as 1910 he said, “The introduction of the quantum of action…should be done as conservatively as possible, i.e., alterations should only be made that have shown themselves to be absolutely necessary.”

Characteristically, when he proposed Einstein for election to the Prussian Academy in 1913, the recommendation by him and others said, after high praise: “That [Einstein] 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.” Planck was almost like the sorcerer’s apprentice, having started the dramatic new developments in physics and being unhappy about it. His own comment in his scientific autobiography is:

My futile attempts to fit the elementary quantum of action somehow into the classical theory continued for a number of years, and they cost me a great deal of effort. Many of my colleagues saw in this something of a tragedy. But I feel differently about it. For the thorough enlightenment I thus received was all the more valuable. I now knew for a fact that the elementary quantum of action played a far more significant part in physics than I had originally been inclined to suspect.¹

He made his peace with the new physics, but he never did any work to apply or extend it.

This was the greatest, but not the first dilemma in his scientific work. He had started his career with work on the laws of thermodynamics at a time when the kinetic theory (now called statistical mechanics) was developing. This explains heat as the random motion of atoms. Planck regarded it as an unnecessary hypothesis to introduce atoms, about which so little was known; his sense of order and simplicity required all physics to be expressed in terms of thermodynamics. This put him in opposition to Ludwig Boltzmann, the great pioneer of the atomistic theory of heat.

But he got shaken in this conviction when it became clear that no progress could be made about such phenomena as electrolysis and electric conduction without introducing atoms and ions. Finally his work on the radiation problem demonstrated to him that he had to approach entropy in Boltzman’s way, and he was converted to atomism. He now vigorously took Boltzmann’s side against Ernst Mach, who denied the existence of atoms.

Another conflict, though perhaps not felt by Planck as a dilemma, was of a more philosophical nature: it was related to the problem of positivism in physics. The interpretation of quantum mechanics by Bohr and Heisenberg says that, when the velocity of an electron is known and, by the “uncertainty principle” its position cannot be determined, the question “Does the electron really have a position?” is meaningless. This attitude is described by some as positivism. Planck was always strongly opposed to positivism and inveighed against it in many lectures and essays, which again put him in opposition to Mach, a strong exponent of positivist views. When he had to accept the uncertainty principle of modern quantum mechanics, he argued his way out of this dilemma² by saying essentially that it is the Schrödinger wave function that has reality, a view with which few physicists will agree.

Planck’s career had a slow start, but he became an extraordinary (associate) professor in Berlin in 1889, at the age of thirty-one, and a full professor in 1892. He was a very conscientious teacher, and during most of his time in Berlin delivered a course on theoretical physics, which consisted of four lectures and one problem class a week, and covered the subject in a six-semester cycle. He maintained this lecture course at least until 1930, when he was seventy-two. I attended his lectures as a first-year undergraduate, and found them unhelpful—he was reading verbatim from one of his books, and there was nothing inspiring in his delivery. He had something in common with his predecessor, Kirchhoff, whose lectures he describes. “It would sound like a memorised text, dry and monotonous. We would admire him but not what he was saying.”³

After his great success with the radiation law he became an established figure in science, and acquired many honors and duties. He was much in demand for his clarity of thought, his coolness of temper, and for what at a celebration at the Berlin Academy was called “the spotless purity of his conscience.” He had responsibilities at the University of Berlin, the German Physical Society, and the Berlin Academy, and faithfully attended the scientific meetings of all these organizations.

His efforts did not slacken when his wife, to whom he had been very close in twenty-three years of marriage, died in 1909. He married again soon afterward. In Heilbron’s words, “He needed another wife for his house and children, for companionship, and because a professor customarily had one.”

At the outbreak of the First World War he was caught up in the patriotic fervor. He felt the war was an occasion for sacrifice in the service of the nation. He signed the famous Appeal of Ninety-three Intellectuals in October 1914, which supported the German Army and denied that Germany had committed atrocities in Belgium. But he opposed other, more aggressive, manifestoes. In the Berlin Academy he defeated a proposal that after the (of course, victorious) was German scientists would not cooperate with any foreign ones.

Later correspondence with Lorentz in Holland and an exchange of visits convinced him that many things had happened in Belgium “that do not conduce to the honor of Germans.” In 1916 he sent an open letter to Lorentz, in which he tried to explain the Appeal of the Ninety-three Intellectuals as expressing only support for the army, but he admitted that individual Germans might have done wrong. At Planck’s request, Lorentz circulated this letter to Allied scientists. It helped to raise Planck’s reputation in the “enemy” countries, and above all to relieve his conscience.

His elder son, Karl, was fatally wounded at the front. In spite of his grief, the father felt that the sacrifice was necessary. “Everyone should be happy and proud to be able to sacrifice something for the whole.” He had had a low opinion of Karl, who could not settle down to any worthwhile occupation; his war service had shown his real worth. “Without the war I would never have known his value, and now that I know it, I must lose him.”

More tragedy not connected with the war came to the family. His twin daughters died, two years apart, each shortly after childbirth. Planck’s sense of duty was too strong to let these events interrupt his activities, which now included much work for the Notgemeinschaft der Deutschen Wissenschaft (Emergency Association of German Science); this tried to secure the desperately needed funds for supporting research.

In the postwar period he worked to restore, or maintain, international contacts in science. Many international organizations had rules excluding German scientists, and he felt this as an injustice. When he was invited to the 1927 Solvay conference, he was at first inclined to refuse, because the physicist Arnold Sommerfeld was not invited. Only when the reasons for the choice were explained, and he was assured they did not involve any prejudice, did he agree to go.

His role was always that of a peacemaker. When Philipp Lenard started his ill-tempered anti-Semitic outbursts against Einstein and relativity, Planck called a meeting of scientists in Bad Nauheim in 1920, for a confrontation between Lenard and Einstein. As chairman he managed to conduct the debate in a civilized manner.

The greatest disaster that befell him—and Germany—was the coming to power of Hitler. He was shocked by the behavior of the Nazi regime, in particular by the dismissal of Jewish and liberal academics. But he made no public protest—that was not his way. He did consider resignation, but decided to remain and use his influence to work for a more reasonable course and to protect those under him as best he could. This required cooperating with the regime, at least in form; he had to give the Hitler salute at meetings.

His position is perhaps best characterized by a remark made by Robert Oppenheimer, when he was reminiscing about having remained too long in a post under a hostile authority. Oppenheimer referred to the attitude “As long as I ride on this train, it will not go to the wrong destination.”

Planck had some successes. He prevented the appointment of some pro-Nazi nonentities to leading positions. As president of the Kaiser-Wilhelm-Gesellschaft, a government-sponsored and privately supported research organization, he could protect “non-Aryan” employees like Lise Meitner—for a time. When Lise Meitner finally had to go, posts had become much scarcer and it was much harder for her to find support abroad than it had been for earlier refugees. He succeeded in keeping the Kaiser Wilhelm Institute for Physics working on pure physics under Peter Debye, an excellent and independent-minded Dutch physicist—for a time. A few years later Debye stayed abroad and the institute was taken over for work on atomic energy.

Planck even went to Hitler to tell him that the expulsion of Jews would weaken German science. Hitler replied that he had nothing against Jews, only against communists, and then flew into a rage. Planck was depressed, since this meant he had no basis for any further negotiation. Heisenberg, to whom Planck reported the conversation, took Hitler’s words at face value and told Max Born and James Franck that they had nothing to fear, and could stay in their jobs. They left anyway.

Eventually Planck had to resign from the presidency of the Kaiser-Wilhelm-Gesellschaft in 1937 and from the secretaryship of the academy in 1938. He was now eighty, but he continued giving lectures and writing essays, mainly on philosophical and religious subjects, but always inserting remarks critical of the regime. In 1944, while he was on a lecture tour, his house was destroyed in an air raid, and with it his library and all his papers. Then his only surviving child, his son Erwin, was accused of taking part in the abortive assassination attempt against Hitler in 1944, and was executed in a particularly brutal way.

After this shattering blow his health broke down. The war zone came to his temporary home in the country, and he and his wife had to hide in the woods and sleep in haystacks. He was rescued by an American officer and taken to Göttingen, where he had relatives, and where he could get hospital treatment. After his release from the hospital he worked, in spite of his age and frailty, for the recovery of German science, and particularly for the reestablishment of the Kaiser-Wilhelm-Gesellschaft. The Allied occupying authorities were reluctant to allow this, because of the name, but to everybody’s satisfaction the society now became the Max-Planck-Gesellschaft.

Heilbron’s book is based on scholarly research, and the sources for every statement are given. To the relief of the reader, the footnotes appear at the bottom of each page, and not in a list at the end. There is a good collection of photographs. I suppose only a native German will be upset by the excessive use of the umlaut: Planck’s house was in Grunewald, not Grünewald; his wartime home was Rogatz, not Rogätz. But this is the only complaint I have to make about this solid and readable book.