The dilemmas referred to in the title of this book were those faced by a leading German scientist who believed in his country right or wrong even when that country became the embodiment of evil. Max Planck is famous to this day for his introduction of the quantum theory. He was born in 1858 in Kiel, which was then part of Denmark. One of his formative memories was the triumphant entry in 1864 of Bismarck’s Prussian troops, which recovered the province of Schleswig-Holstein and united it with Prussia. His elder brother’s death in the Franco-Prussian War of 1870/71 ‘made him feel at one with the heroes who sealed their true love for the fatherland with their own blood’. They were noble sentiments in those days. At school in Munich – his father was a professor of law at the University – he nearly always earned the annual prize for religion and good behaviour. His teachers described him as conscientious, open, cheerful, gifted in all subjects, especially mathematics, yet modest and popular with his classmates. He was also intensely musical and had absolute pitch. He wondered whether to study Classics, music or physics and finally opted for the latter, even though a leading physicist advised him that there was nothing significant left to be discovered in that subject. Planck found nothing to rebel against until he was over forty when the dogged pursuit of a vital physical problem led him, almost against his will, to make a revolutionary discovery.
Until then he wrote treatises on the physics of heat, first as a Privatdozent (unpaid) in Munich, later as professor in Kiel and finally in Berlin. They aroused little interest. His appointment at Kiel was given in the confidence ‘that he would remain faithful in unbreakable loyalty to His Majesty the Emperor and to the Imperial Family’. To Planck that read, not as an empty phrase, but as a sacred duty to which he still felt bound 33 years later, after the collapse of the German Armies in October 1918. He then wrote to Einstein: ‘it would be a great stroke of fortune for us, indeed a saving grace, if the bearer of the crown would voluntarily renounce his rights. But the word “voluntary” makes it impossible for me to come forward in the matter; for first I think of my sworn oath, and second, I feel something that you will never understand at all ... namely, piety and an unbreakable attachment to the state to which I belong and which is embodied in the person of the monarch.’ Two days afterwards the Reichstag declared a parliamentary republic and Germany was thrown into chaos.
Planck’s epoch-making discovery, made in 1900, was concerned with the interaction between radiation and matter. The question that exercised German physicists concerned the colour and intensity of radiation emitted by a hot black body. The experimentalists had developed sensitive methods for measuring the radiation, and a theoretician, Wilhelm Wien, had derived an apparently well-founded mathematical theory to account for their observations. However, as techniques were refined and temperatures raised, deviations from Wien’s formula became apparent and Planck modified the formula in a way that fitted the observations exactly. A lesser man would have been content with that, but Planck had already been struggling unsuccessfully for six years with the formulation of a fundamental law for the interaction between radiation and matter: he now felt that ‘his formula had only limited value’ since even if it proved accurate, it was only ‘happily guessed’. ‘From the day of its formulation,’ he wrote, ‘I was occupied with the problem of obtaining for it a true physical meaning. Then, after several weeks of the most strenuous work of my life, the darkness lifted, and a new unexpected perspective began to dawn on me.’
Classical physicists, including Planck, were sceptical of the Austrian physicist Ludwig Boltzmann’s interpretation of heat in terms of atomic vibrations and preferred to think of matter as a continuum. Yet it soon became clear to Planck that his radiation law conflicted with the classical picture. Many years later, he confessed to the American physicist R.W. Wood: ‘what I did can be summarised simply as an act of desperation ... The two laws of thermodynamics ... must be upheld under all circumstances. For the rest I was prepared to sacrifice every one of my previous convictions about physical laws.’ Both Wien and Planck had thought of the black body radiation as being emitted by small oscillations of electric charges in that body, but neither of them had thought of the oscillators as atoms. In his desperation Planck decided to use Boltzmann’s atomistic approach to formulate the equilibrium between radiation and matter. The results of his mathematical work forced on him the strange conclusion that the energy available to the oscillators was not continuous, ‘but composed of a definite number of finite equal parts ... or energy elements’. Planck called one such element the quantum of action; it soon became known as Planck’s constant. His concept was so revolutionary that no one, not even Planck himself, immediately realised its implications. One man did, a few years later. That man was Einstein. Planck meanwhile was more concerned with another universal constant which had emerged in his new radiation law. He named it in honour of Boltzmann, to whom he owed his successful attack on the problem, and he used it to calculate accurately for the first time the true weights of single atoms and the exact charge carried by the recently discovered electron. Boltzmann’s constant was derived from nothing more than the laws of heat, yet when taken together with Newton’s gravitational constant and the speed of light, it provided a system of natural units of mass, electric charge, energy and time independent of human observers and valid throughout the universe. That discovery convinced Planck that there exists a physical universe independent of our senses.
Planck’s insistence on the reality of atoms which no one had yet seen, and on the universal validity of physical constants derived from the laws of heat, brought him into sharp conflict with the Positivist philosophers. The author of the book under review assumes that his readers know what Positivism was all about, but I did not and looked it up in the Encyclopaedia Britannica. I learnt that its founder, the 19th-century French philosopher August Comte, called his Cours de Philosophie ‘Positive’, because it was concerned only with positive facts. The sciences had to study the facts and regularities of nature and formulate them as descriptive laws, and not, as Planck had done, interpret their meaning in terms of a reality that cannot be observed directly. The 20th-century Viennese school of Positivists distinguished between a metaphysical utterance and a genuine proposition by requiring the latter to be conclusively verifiable, which neither atoms nor quanta then were. Plank’s chief opponent, the philosopher physicist Ernst Mach, went so far as to argue that all factual knowledge consists of conceptual organisation and elaboration of immediate experience, and therefore denied the existence of Kant’s ‘things in themselves’, the ultimate entities underlying phenomena.
For the first time the peaceful and good-natured Planck turned into a fighter. He vigorously asserted the reality of the unseen world which had emerged from his mathematical work and declared that ‘the philosopher who limits himself to asking to what extent the meaning of a new idea is evident a priori hampers the development of science. What matters is that the idea gives rise to useful work. A positivism that rejects every transcendental idea is as one-sided as a metaphysics which scorns individual experience.’ Mach replied disdainfully: ‘If belief in the reality of atoms is so essential to you, I will have no more truck with physical thinking.’ Many years later, Planck complained: ‘I have never succeeded in getting everyone to agree to a new result, the truth of which I could demonstrate by a conclusive, but purely theoretical argument. That has been one of my most painful experiences.’
When Planck formulated his radiation formula, he parcelled into quanta only the energy of the oscillators in the black body and left the physical meaning of his quantum of action unclear. Five years later, Einstein published his famous trio of papers, among them one ‘On a Heuristic Viewpoint concerning the Production and Transformation of Light’ which extended and deepened Planck’s work on the interaction between radiation and matter. Einstein opened his paper with the revolutionary proposal that radiation, i.e. light itself, consisted of quanta. An ingenious mathematical argument then led him to conclude that these quanta were the same as Planck’s. Einstein showed that his theory leads to an understanding of an enigmatic effect discovered, ironically, by Heinrich Hertz, the very man who was thought to have proved that light consisted of electromagnetic waves. This is the photo-electric effect, so called because light falling on certain metals, such as silicon, causes an electric current. It is now used in solar panels to provide electric power for satellites and other devices. Einstein showed that this effect was due to a quantum of light absorbed by a metal atom transferring its energy to, and liberating, an electron. Einstein’s paper implied that all energy is parcelled into quanta, no matter whether it is contained in radiation or atoms. This was the true meaning of Planck’s radiation law.
Planck was deeply impressed by Einstein’s papers. A meaner character might have feared to be overshadowed, but Planck engineered the establishment of a research professorship free of any duties for Einstein in Berlin. Their admiration was mutual: Einstein, who was no respecter of persons, later wrote that the best thing about Berlin was the sheer joy of being near Planck, even though his views differed from Planck’s on almost every subject other than science. Planck was revered by all who knew him. He was honest, modest and gentle, entirely free of the pomp of the proverbial German professor. Lise Meitner, who had worked with him for forty years, wrote that he never decided anything on the grounds that it might be either useful or harmful to himself, but always on the true merits of the case. Contrast this with Maynard Keynes’s dictum about Lloyd George: ‘He made every decision on grounds other than the real merits of the case.’ In fact, Planck was something of a saint. He was, he said, a deeply religious man, even though he did not believe in a personal, let alone a Christian God. His beliefs are well expressed in his lectures on the philosophy of physics, where he wrote:
There is a real world independent of our senses; the laws of nature were not invented by man, but forced upon him by that natural world. They are the expression of a rational world order. Both religion and natural science need a belief in God, but to the former God is the starting-point, to the latter the goal of every train of thought ... Physics must contain no contradictions; in terms of ethics this comes down to honesty and truthfulness. Since justice is inseparable from truthfulness, it must be the same for all, just as the laws of Nature.
This comes close to Jacques Monod’s later attempt to build an ethic upon scientific truth. While Planck was the grandson of a Protestant pastor and Einstein the son of a Jewish businessman, both believed in a rational God-made order of the world, independent of man. Both men also believed in determinism and absolute causality, and were therefore equally reluctant to accept Erwin Schrödinger and Max Born’s probabilistic wave mechanics as the ultimate picture of the atomic world.
I was sad to read that Planck’s faith and goodness did not prevent him being infected by Germany’s chauvinistic fever at the start of the First World War. Certain of the righteousness of the German cause and of the civilised nature of all German people, Planck indignantly rejected as lying propaganda Allied reports of German atrocities in Belgium until, two years later, his friend the Dutch physicist Hendrick Lorentz convinced him otherwise. His naive faith in German goodness was to be shaken further soon after the war by vicious attacks on Einstein. I had been under the illusion that the excesses of anti-semitism started only after Hitler’s accession to power in 1933, but I learnt from Heilbron’s book that attacks on Einstein began in 1919, the very year that Sir Arthur Eddington’s confirmation of the Theory of Relativity brought him world fame. They came from anti-semitic physicists who rejected his theories, from anti-semitic newspapers and from Nazi thugs who actually threatened to kill him, a pacifist who had done no one any harm, and whom Planck regarded as one of the greatest physicists of all time. Planck had stood up against official anti-semitism on two earlier occasions: in 1894, when the Prussian Minister of Education tried to evade the science faculty’s recommendation to appoint a Jew, Emil Warburg, to the chair of experimental physics in Berlin, and in 1895, when the Ministry demanded the dismissal of a Jewish Privatdozent whose only offence was his support of the Social Democratic Party. Planck responded to the attacks on Einstein, not by a public denunciation of the anti-semites, but by arranging a public discussion between him and the chief anti-relativist, Phillip Lenard, where ‘opposing conceptions of the epistemological foundations of science confronted each other in a dignified manner,’ but that did nothing to silence the slanderous campaign.
In 1933, when Hitler took power, Planck, as the doyen of German science, was Secretary of the Prussian Academy of Sciences and President of the Kaiser Wilhelm Gesellschaft, the forerunner of the present Max-Planck Gesellschaft, which ran Germany’s independent research institutes, including Einstein’s in Berlin. Einstein happened to be in the United States. Having learnt of the dismissal of Jewish scientists, Einstein publicly announced that he would not return to Germany because there no longer existed ‘civil liberty, tolerance and equality of citizens before the law’. The Nazi press responded to his statement with a flood of abuse and the Reichskommissar appointed by the Prussian Minister of Education to take charge of the Academy demanded the instigation of disciplinary procedures against him. Planck realised the hopelessness of a compromise and advised Einstein to resign ‘so as to conserve his honourable relations with the Academy and save his friends an immeasurable amount of grief and pain’. Planck had no understanding of Einstein’s public protests, believing that, as a German, Einstein should have stood up for Germany abroad whatever the faults of its new regime. He told Einstein that his public statements had given all his friends great pain. ‘There has taken place a collision between two conflicting views of the world. I have no understanding for either. I feel remote from yours, as you will remember from our talks about your propaganda for refusing military service.’ At an official meeting of the Academy, Planck declared that its members had a special duty of loyalty to the Government and regretted that Einstein’s political attitude had made his continued membership impossible. For the record, he stressed that Einstein was not just an eminent physicist, but the physicist through whose papers, published by the Academy, physical knowledge in our century had gained in depth to an extent comparable only with what had been achieved by Kepler and Newton. I find it hard to see how Planck could have expected Einstein to be loyal to a government composed of men who had long treated him as a criminal. Einstein never forgave Planck for what he regarded as his cowardly failure to stand up for him and other Jewish colleagues.
Fritz Haber, the Jewish chemist whose synthesis of ammonia from nitrogen of the air had saved the German Army from running out of explosives soon after the start of the First World War, had committed no indiscretions, but was dismissed all the same. Planck sought an audience with Hitler to get Haber reinstated. When he extolled Haber’s and other Jewish Germans’ contributions to science, Hitler replied that he had nothing against Jews as such, but that they were all Communists. When Planck tried to argue, Hitler shouted, ‘People say that I get attacks of nervous weakness, but I have nerves of steel,’ slapped his knee and whipped himself into a rage which continued until Planck took his leave. Planck told Max Born, another eminent Jewish physicist dismissed by the Nazis, that this interview extinguished any hopes he might have had of openly exerting his influence in favour of his Jewish colleagues. He did so on only one further occasion, two years later, after Haber had died in exile, shunned as a Jew by the Germans and as the initiator of Germany’s gas war by his British and French colleagues. Planck decided that the Kaiser Wilhelm Gesellschaft should hold a meeting in his memory. After the invitations had been sent out, the Minister of Culture forbade all public employees to attend. Planck said that he would hold the meeting unless the police dragged him out. Another state-employed physicist, the Nobel laureate Max von Laue, attended despite the Minister’s veto, but suffered a heart attack afterwards. It was perhaps symptomatic of the tensions under which Germans were then living.
When Laue went for a lecture tour to the United States, Planck asked him to make people understand the difficulties he had to face and to assure them that ‘quieter and more normal times would return.’ This had been Planck’s experience during the revolution, civil war and inflation after the First World War, when, despite the turbulent times, his unique standing had made it possible for him to rebuild and even strengthen the fabric of German physics. Planck felt confident of being able to do the same again, and meanwhile tried to minimise the damage by keeping administrative power in his own hands rather than letting it be seized by the Nazis. He refrained from public protests, paid lip-service when required to do so on public occasions, exercised quiet diplomacy to try to prevent the dismissal of Jewish heads of Kaiser Wilhelm Institutes, and dissuaded German scientists from emigrating. One of these was Werner Heisenberg, the founder of quantum mechanics, who had been the subject of vicious attacks by the Nazi press for his support of Einstein’s Theory of Relativity. Heisenberg stayed, and later worked for the German atomic energy project. Fortunately that project remained unsuccessful. Planck’s failure to stand up in public against the atrocities of the Nazi regime and his discreet manoeuvres to save German science were a disaster, because they corroborated Nazi propaganda which branded reports about the real goings-on in Germany as Jewish inventions.
Planck’s 19th-century ideal of ‘a clear conscience that expresses itself in conscientious fulfilment of one’s duty’ proved an insufficient guide in those evil times, and his hope for better days was shattered by the final, terrible tragedy of his life. His eldest son Karl had been killed in the First World War, and both his daughters had died giving birth to their first children. The only child left was his son Erwin, who was also his closest friend. After the attempt on Hitler’s life in July 1944, Erwin, who had been a high official in the Ministry of Defence in the Weimar Republic, was arrested. According to the historians who unearthed the police evidence against him, he had discussed ways of overthrowing the Nazi regime with several groups from 1934 onwards and had known some of the conspirators in the July plot, but had not been aware of the actual plot. Since father and son were very close, Heilbron believes that the father must have known of these activities. Perhaps this partly explains Planck’s public reticence and his confidence in better days to come. For several months after Erwin’s arrest, Planck was torn between hope and despair, until he received the shattering news that the People’s Court had sentenced Erwin to death. Planck then wrote to both Hitler and Himmler assuring them that his son had had no knowledge of the plot, and in mid-January 1945 was told of an imminent reprieve. Five days later Erwin was hanged. In a letter to his old friend, the physicist Arnold Sommerfeld, Planck wrote: ‘My sorrow cannot be put into words. I struggle for the strength to give meaning to my future life by conscientious work.’ Even now, this characteristically German virtue remained his guiding star.
Heilbron’s book developed from an introduction he was asked to write to a forthcoming reprint of Planck’s essays on science. It is a well-documented, lively, readable and balanced account which stimulated me to read Planck’s essays as well as other people’s writings about him and his work. It has some weaknesses: it gives too brief an account of the formative experiences which provide the key to Planck’s staunch nationalism; it does not explain his conflict with the Positivist philosophers clearly enough, and it fails to quote some key documents such as Planck’s letter to R.W. Wood or his tragic letter to Sommerfeld. On the whole, however, it gives a true and vivid picture of one of the great and tragic figures of science.