1 (page 1)p. 1C1A richly furnished mind

C1.P1The very original mind of Niels Bohr came into existence in Copenhagen in 1885. It arose from the union of a professor of physiology at the city’s university, Christian Bohr, and Ellen Adler, the daughter of a liberal philanthropic Jewish banker (Figures 1 and 2). Although (and perhaps because) Christian came from a family of pastors, he was an atheist; and although Ellen’s family were prominent among ‘Viking Jews’—Jews who settled and prospered in Denmark before the immigration of Yiddish speakers from the shtetl—she was not observant. Nonetheless, some accommodation for the religious training of their three children was required. Christian agreed that they be raised as Jews. Ellen did not insist, however, and Niels and his brother Harald were sent to the Lutheran church so that they would have the same religious experience as other Danish boys. Of the couple’s eldest child, Jenny, few accessible records exist.

C1.P2The forces of parental indifference and state religion had to do battle with Ellen’s sister Hanna, the first woman to earn an advanced degree in physics in Denmark and the proprietor of the first coeducational secondary school in the kingdom. Aunt Hanna (Figure 3) was close to her nephews, especially Niels. An expert (page 2)p. 2on the history of Danish Jewry cites Hanna Adler’s work as exemplary of the ‘special, very concrete idealism, expressed in an intensive and persistent striving to realize an idea or thought’, that constituted the Jewish ‘impulse’ in Danish science. In contrast (page 3)p. 3with her sister Ellen, who had little or no connection with the synagogue, Hanna had close ties to the Jewish community.

C1.P3Naturally religion loomed large in Niels’s adolescent mind as it struggled to balance the views of his parents, his pastors, and his aunt. One day he told his father that although he had struggled very, very hard, he could not believe. As he later described the incident to his fiancée, Margrethe Nørlund, his father responded with a smile. Bohr interpreted the smile as a sign that ‘I too could think.’ For a time he thought to write a book to help others think (page 4)p. 4themselves free from the errors of organized religion, and echoes of this unrealized project may be discerned in other forms in his later writings.

C1.P4Unbelievers are of course not free from the cultures in which they grow up. In Bohr’s case both Christian and Jewish elements continued to occupy his mind until the simultaneous arrival of his wedding and his quantized atom filled it with other things. The Jewish element of his boyhood came not only from Aunt Hanna, but also from his Adler cousins, notably Edgar Rubin, who became a leading experimental psychologist. Rubin had an opportunity to observe some extraordinary minds in a club of university students dominated by himself and the Bohr brothers. Many of the topics debated in the Ekliptika, as the twelve overachievers who constituted the club called it, (page 5)p. 5started from Høffding’s teaching. At least half of this academic zodiac were Jewish.

C1.P5The remarkable representation of students of Jewish descent in the Ekliptika and their subsequent high academic achievement were consonant with the traditional Jewish emphasis on study, the tendency of liberal reform Jews to assimilate, and the relative tolerance of Danish society. Five of the members were cousins of one sort or another, in keeping with the tendency, even among assimilated Jews, to stick together. Two of the gentile members of the club, the brothers Niels Erik Nørlund (a mathematician) and Poul Nørlund (a historian), joined this large family when their younger sister Margrethe became engaged to Niels (Figure 4).

C1.P6When Bohr returned to Copenhagen from Manchester during the Easter vacation of 1912 to help plan his wedding, the cultural hit of the season was an improbably popular play, Indenfor murene (‘Inside the walls’), by the Danish Zionist Henri Nathansen. It tells the story of a Jewess, Esther, who falls in love with a gentile professor whose lectures she attended, just as Ellen Adler had done with Christian Bohr. Esther’s engagement distresses both families, but the action takes place primarily ‘within the walls’, in the warm, cosy, middle-class Jewish home that Esther’s desertion threatens to destroy (Figure 5). Similarly, the story of Niels and Margrethe unfolded within the Adler side of the family; Margrethe lived with Ellen Bohr for some time before her marriage and saw very little of the Bohr side and its Lutheran theologians. But whereas Margrethe joined the assimilated Adlers easily, Esther could not be comfortable with her bigoted in-laws.

C1.P7The real-life story of another member of the Ekliptica, Lis Rubin Jacobsen, who received her doctorate in 1910 when already married and became an authority on Nordic runes, is similar and similarly cogent. Her non-observant father Marcus Rubin was close to the leading Danish-Jewish man of letters, Georg Brandes, who fervently favoured assimilation; yet Marcus would not allow (page 6)p. 6his daughter to attend Christian religious instruction in school. When she asked permission to participate in these prohibited exercises, Marcus gave her some Christian books to read. That was enough. She grew up an unbeliever and aimed to be a schoolteacher until she fell in love with a gentile intellectual. (page 7)p. 7Though without faith, married into a Christian family, and urged to assimilation by Brandes, she could not shake off her Jewish identity. In later life she promoted Jewish causes and is now cited along with Hanna Adler among the most distinguished Jewish women in Danish history.

C1.P8Bohr and Jacobsen shared the traits to which Nathansen ascribed Jewish success in surviving persecution. The leading items in his inventory, as itemized in Nathansen’s sensitive biography of his friend Brandes, are strength and joy in work. To the world Jews are fierce competitors, who can come across as over-critical, domineering, and arrogant. Yet their struggle for equality made them champions of truth, justice, freedom, and human rights. With family and friends, ‘inside the walls’, Nathansen’s Jews have an ‘intimate special life … whose passwords [are] “respect” and “discipline”—respect for tradition, discipline in the family’. There competitiveness turns to humour, irony, satire, word play, banter, (page 8)p. 8‘wily, equivocal, ambiguous, double-edge wit combined with irony and self-irony’. ‘The world of the mind was the home of the homeless Jewish people, the life of the mind their only free state. … From the special exclusivity of this life of the mind Jewish “chutzpa”, boldness, something between courage and insolence, has developed, and also the Jewish “chain”, the artistic, sensitive union of grace and taste, something between enchantment and enticement.’ Niels Bohr refined and combined these characteristics into his special ways of thought and expression.

C1.P9While planning his wedding Bohr encountered a religious problem of the same intensity as the difficulty that had precipitated his loss of faith. He refused to be married in church, from which he and Margrethe resigned; the decision upset her pious mother, whose hurt Bohr tried to assuage with the reassurance that he did not believe that science governed or could govern everything. Moreover, he thought that he could prove it. And that, he said, was a source of joy to him: ‘[life] would be so infinitely trivial if I thought I could understand it’. Thus he was particularly aware of the necessity and difficulty of reconciling conflicting religious beliefs and cultures, and of the probable existence of aspects of experience beyond rational explanation, just before he encountered the contradictions of the nuclear atom.

C1.P10The Viking in Bohr’s make-up was not only Jewish. He had a great love for the North, the land of volcanoes and glaciers, fire and ice, mirages, glories, and auroras—and of sagas, heroes, giants, kobolds, trolls, snow queens, and ice palaces. There was romance in his make-up. ‘When I see the briefest reference to the old Nordic countries, then my heart flares up so wildly, so wildly, my little one, I dream I am among Norway’s cliffs and skerries.’ The occasion of this outburst was reading the chapter on Odin in Thomas Carlyle’s On heroes and hero worship, which the little one, Margrethe, had sent him as an example of her favourite reading. He went on to pose a pair of crazy questions: ‘[T]ell me whether you would come with me to Iceland in a Viking ship; tell me (page 9)p. 9whether you would, and tell me, whether you, in addition, would stay behind on your own in Iceland, when I had to leave in the summer.’ She replied in the literary idiom in which they clothed their emotions. ‘I will come to you, Niels, as Solveig came to Peer Gynt.’ This was to grant more than was required, however, since the innocent Solveig, who offered herself freely to the vagabond Peer, wasted her life waiting for his return. The catechism continued. ‘Will you care for my work?’ ‘Dear Niels, I cannot at all describe to you how much I love you and how much I love your work.’ But will you be a mother to my students? ‘I set no limit at all to how much I wish that I could be allowed to be a mother to your students.’ The limit would be the size of Bohr’s Institute.

C1.P11Another indication of the content of Bohr’s mind around the time of his great invention is a story that he wrote for Margrethe, then still his fiancée, for Christmas of 1911. The story begins with their watching a little boy accompanying his father to church on Christmas day, the only day in the year the father entered a church; the little boy soon discovered, ‘after his own lonely battles’, the reason that his father otherwise kept away. ‘[The boy] could not imagine anything as dreadful and terrible’ as the realization of the things taught in church. After doing honour to this perfect parent, then recently deceased, the couple fly to the far north, where they again spy the boy, now an adult, discussing philosophy with an old gentleman who no doubt answered to the name of Høffding. Eventually they return to Bohr’s digs in Cambridge, whence Margrethe modestly leaves for Denmark while Niels retires to bed, ‘his courage roar[ing] so wildly, so wildly, for he thinks that he too could think’. With Margrethe’s help, he will try to combine courage and thinking, Viking virtue with something else: ‘My own little darling, if you will care for him, he will try to find meaning in his wild courage.’

C1.P12Then came a question unusual for a Viking. Will you pay my debts, ‘all the debts that my poor soul might incur?’ Although Niels repeated this question several ways, he never specified the (page 10)p. 10obligations he needed Margrethe’s help to discharge. Perhaps he meant that with her psychological support he could justify, and so repay, the belief in his abilities entertained by his family, teachers, and friends. To his mother he was a ‘rare treasure’, to his father ‘gold’, to his brother, ‘the greatest and wisest human being we have known’. The family had helped him to develop the gifts of nature. He dictated most of his thesis to his mother. His father put his laboratory and assistant at Niels’s disposal when he needed them. The entire family helped in computing tables, doing calculations, and writing out fair copies of his papers. Margrethe as amanuensis and also her brother Niels Erik as calculator became part of this machinery even before her marriage. There was much to repay.

C1.P13The Viking in Bohr may also be seen in his strength and endurance as a skier and football player. He could ski for days and his size made him a good goalie as long as he kept his mind on the game. Harald was more agile in sport as well as more practical in life than Niels and made it to the Danish Olympic football team.

C1.P14Even before entering the university, Bohr spent time with Høffding (Figure 6). The philosopher was a great friend of Christian Bohr and also of Christian Christiansen, who would be Bohr’s professor of physics. The three met regularly with the philologist Vilhelm Thomsen, and Niels and Harald would listen to them airing deep questions of science and philosophy when Christian hosted their symposium. This eavesdropping gave the brothers at least two precious insights: that experienced scholars can advance their discipline before securing its foundations and that responsible scholars strive to deepen as well as enlarge their discipline’s structure. The most basic of the foundational issues the quartet of professors discussed concerned ‘the nature, condition and limits of knowledge, the nature and worth of evidence, and the principles which underlie our valuation of human actions and institutions’, that is, the problem of Truth.

(page 11)p. 11C1.P15Høffding had a lot to say about truth. He began with Søren Kierkegaard, whom Bohr was to rank as the greatest of Danish thinkers and stylists, and the author of one of the best books ever written. This was Stages on life’s way, which Høffding took to be representative of Kierkegaard’s philosophy and a great help to (page 12)p. 12people undergoing religious crises. It had helped Høffding himself in early life, and his writing about truth, like Kierkegaard’s, owed much to struggling with a Christian worldview. He arrived at the capacious principle that no single truth can capture a domain, for no matter how promising a line of analysis is at the beginning, if pushed ever further it must eventually expose an irremediable, inaccessible, irrational residuum. This was the proposition that Bohr told Margrethe’s mother he could prove logically. Enthusiasm over the necessity of renouncing the search for a theory of everything marked Høffding’s modest epistemology; as Rubin recalled, ‘this state of affairs caused him great and profound satisfaction’, for, like Bohr, he regarded its contrary, in which everything would stand revealed, as the destruction of ‘an essential condition for the value of human life’.

C1.P16Høffding had begun his university studies in theology. After a long internal fight guided by Kierkegaard’s similar struggle he decided that he could not live his life ‘by the ideals and commandments of religious ethics’ and looked to philosophy to find ‘equivalents for the loss of belief in those goods which the vanishing of religion entails’. Bohr gave expression to the same programme when he assured his future mother-in-law that he believed in many things: ‘in the goodness and love of human beings, for that I have experienced’; ‘in the duties of a human being, although I cannot say exactly what they are’; and ‘in so many many other things that I do not understand’. How can these things be justified, grounded, in absence of religion? Bohr could only hope, ‘with all my soul’ and without supernatural help or threats, that he could stay true to his ideals of ‘the good and great and true’. This was a moral, if not a philosophical solution, to the great problem of Truth, which, as he knew from Høffding, could not be solved.

C1.P17For Høffding, free enquiry in the religious sphere was the pre-eminent means for awakening and encouraging thought. ‘He to whom the problem [of religion] does not present itself has of course no ground for thought, but neither has he any ground (page 13)p. 13for preventing other people from thinking.’ Høffding’s even-handed consideration of religion persuaded his students, ‘for whom his lectures were the experience of their university years’, and worried their parents, who feared, rightly, that he might dissolve their traditional beliefs. He put an extravagant value on intellectual life. So did young Bohr (‘it is the most valuable and only thing I possess’), who hoped to enter the only class of scientist that, according to Høffding, required true scientific culture. These were the creators of new theories.

C1.P18Høffding stayed in contact with the Bohr family after Christian Bohr’s death in 1911. In his old age his ‘good friend Niels Bohr’ would visit him to talk about physics and philosophy, and read from their favourite poets, for ‘Niels Bohr is not only a great physicist, but also is interested in philosophy and literature.’ Høffding was able to make use of Bohr’s ideas in a widely published essay on the concept of analogy and Bohr, returning the compliment, credited Høffding with ‘ideas that helped physicists to “understand” their work’. On Høffding’s death, Bohr succeeded him in the Aeresbolig, the villa left by the founder of the Carlsberg Brewery as the home of the greatest intellectual among the Danes as determined by the Danish Academy of Sciences. The succession might serve as a symbol of Bohr’s place in Danish philosophy and culture.

C1.P19Only the boldest of thinkers could expect to refashion his science with the very same tools he used to prove ‘logically’ that human beings can never know everything. And how is that proved? Let us begin with physics. Our mode of understanding is continuity of thought; hence physics posits continuity in motion, in action, in cause and effect; as the old philosophers said, nature does not make jumps. Is this assumption a requirement of thought? ‘The great question is, whether the idea of the continuity of motion or activity can be carried out in all spheres.’ If not, room opens for ‘an (page 14)p. 14irrational relationship’ between nature and knowledge. And that in fact is how things are. ‘For us, existence can never be absorbed into thought without remainder.’

C1.P20Our usual analyses suppose a clean division between the subject (the observer) and the object (the observed). Alas, it is but an indulgent delusion. Object and subject mutually determine one another: a pure subject is as illusory as a thing-in-itself. Not only is there no pure case, but no place to stop: the subject influenced by the object becomes a new observer of a new object, and so on and on. ‘Here again we run up against the irrational and here perhaps we see most clearly how inexhaustible being is in comparison to our knowledge.’ There is no reason for despair in the realization that human beings cannot create ‘an exhaustive concept of reality’; for it is just in ‘the irrationality in the relation between thought and reality [that] … the possibility of progress lies’. Thus Høffding.

C1.P21Kierkegaard says the same things even better. There is no way for us to create a complete account of Being because our knowledge and experience grow and change; and as we are part of the Being we are trying to capture in thought, we are attempting to grasp something unformed or continually forming. (Kierkegaard snickered that academic philosophers had missed this point because they are such non-entities they excluded themselves from existence in general.) This problem of the Subject altered by the Object can be traced back further, to a story by Kierkegaard’s major patron, Poul Møller, a professor of philosophy considered by many to have been the archetypical Danish writer of his time. In Møller’s story a student addicted to thought drives himself into intellectual impotence by thinking about himself thinking about a second self thinking … and into physical impotence by finding no sufficient reason to perform an action at any particular time and, hence, at no time at all. Bohr thought this story so expressive of the problems of quantum physics and the Danish way of handling (page 15)p. 15them that he later urged it on all his foreign students as soon as they knew enough of the language to read it. For it presented not only the problem of the division between subject and object, but also the need to break off a logical line of thought arbitrarily in order to progress at all.

C1.P22Introducing considerations of time provides another way of demonstrating limitations in the reach of rational analysis: whatever understanding we achieve can only be retrospective. As Høffding put the point in 1904, ‘we live forward but understand backward’. Not everything lends itself to backwards comprehension: we will never be able to explain how we can understand retrospectively the necessity of what was open-ended prospectively. This was to phrase the problem of free will in precisely the terms in which Bohr approached it, by the doctrine of multiple partial truths: we are free in prospect, bound in retrospect. ‘[A] situation that calls for a description of our feeling of volition and a situation demanding that we ponder on the motives for our actions have quite different conscious contents.’

C1.P23The irrationality of development in time appears arrestingly in Kierkegaard’s Stages on life’s way. Bohr had a copy of it with him at the rural parsonage to which he withdrew from the bustle of quiet Copenhagen to prepare for his master’s thesis and examination. It was just the place for romantic intellectuals like Bohr and Kierkegaard. ‘I walk here in solitude and think about so many things.’ Thus Bohr. He thought about physics, of course, and mathematics and logic, but also about the problem of cognition, the stages of life, the nature of the good. The experience meant much to him as he could still relate it in accurate detail many years later. ‘[Kierkegaard] made a powerful impression on me when I wrote my dissertation at a parsonage on Funen, and I read his works day and night … His honesty and willingness to think the problems through to their very limit is what is great. And his language is wonderful, often sublime.’ Bohr sent his copy of Stages(page 16)p. 16to Harald as a birthday present from Funen. ‘It is the only thing I have to send; nevertheless, I don’t think I could easily find anything better. … I think absolutely that it is about the most beautiful thing that I have ever read.’

C1.P24Kierkegaard’s Stages employs six personae to convey his insights into the human condition. The earliest stage, the aesthetic, which for some people lasts a lifetime, is a period of carefree experimentation. Kierkegaard depicts it through speeches given by four of his avatars at a symposium on love, life, and the universe. Each says something true, though his statement conflicts with what the others say. Another avatar, a self-satisfied judge, sets forth the merits of a good marriage, the essence of the second or ethical stage. The judge’s wife was patient, understanding, supportive, protective; neither he nor she could achieve as much apart as they did by pooling their complementary qualities; each contributed an equal share to the truths of married life. Bohr needed such a partner more than most men. Margrethe fitted the pattern perfectly. The third and final stage, the religious, can be reached only by a leap of faith, a quantum jump that, as we know, Bohr could not make.

C1.P25Another of Kierkegaard’s personae made a perfect model for a romantic young critic walking in solitude around a country personage. This was Johannes Climacus, who had a passion for thinking so intense that he could not think about girls. ‘In love he was, madly in love, but with thought, or rather with thinking.’ Having a ‘romantic soul which always looked for difficulties’, that is, being a consummate critic like Bohr, Climacus managed to prove that the foundational principle, ‘modern philosophy begins with doubt’, to which philosophers since Descartes had ascribed some meaning, meant nothing at all. Poor Climacus never advanced even to the threshold of received philosophy. ‘He became more and more retiring, fearing that thinkers of distinction might smile at him when they heard that he too wanted to think.’(page 17)p. 17

C1.P26The master’s thesis Bohr was writing when he consulted Kierkegaard daily reviewed the electron theory of metals, which proposed to explain their properties of radiation, magnetism, and conduction on the implausible assumption that electrons flow through a wire as gas molecules move down a pipe. The assumption had enabled physicists to transfer the elaborate statistical methods they used with gases to the situation in a conducting wire. Bohr continued with his critical review of the electron theory in his doctoral dissertation, which he defended without opposition in 1911, as no one in Denmark had enough physics to judge it. Among other valuable results Bohr proved that the theory could not give an account of the radiation of heat from metals or their behaviour under a magnet in agreement with experiment.

C1.P27The failure to account for radiation did not surprise him. A decade earlier a simpler case of heat radiation had resisted an aggressive attack by Max Planck, the leading theoretical physicist in Germany. Planck’s simpler but more recondite problem was to determine the energy spectrum of the radiation in a hot oven whose walls are maintained at a constant temperature. By ‘energy spectrum’ he understood the relative intensities of the radiation at different frequencies, so much in the red region, so much in the yellow, so much in the blue, and so on. The goal was to specify a number ρ (the Greek letter ‘rho’) for the intensity of every detectable frequency ν (Greek ‘nu’) at the constant temperature T of the oven. Or, as physicists wrote the object of his quest, Planck sought a mathematical expression, ρ(ν, T), that summarized the state of the radiation.

C1.P28Planck’s oven problem attracted him because ordinary physics could prove that ρ(ν, T) did not depend on the material of the oven walls and so must have significance well beyond a description of (page 18)p. 18the radiation in a particular heated cavity. Besides the quantity defining equilibrium (T) and the variable designating colour (ν), ρ could contain only constants characteristic of the two great departments of physics at whose intersection Planck’s problem stood. These departments dealt with ‘aether’ (the medium in which light, heat radiation, and electromagnetic forces had their seat) and ‘matter’ (ponderable atoms in the various species itemized in the periodic table of elements). Up-to-date theorists in 1900 recognized the then recently detected electron, supposed to consist of a minute mass inseparably bound to an electric charge, as the likely coupler between aether and matter. In an extravagant extrapolation of this idea, J. J. Thomson had proposed to make the electron the building block of atoms, the key to the peculiar properties of metals and the periodic character of the elements, and, when in motion, the cause of most processes in the aether including radiation.

C1.P29And how does an electron stir up the aether? This was one of the two ‘clouds’, or dark problems, that the dean of British mathematical physicists, Lord Kelvin, saw looming over physics at the start of the 20th century. The pith of the problem lay in the difficulty, widely acknowledged by 1900 as the impossibility, of describing the behaviour of aether with the concepts developed for ponderable matter. Kelvin pointed to experiments that had failed to detect any of the effects that should have arisen if aether had the least inclination to react mechanically to the motion of a ponderable body through it. Einstein dispelled this cloud in 1905 but, as Kelvin had foreseen, at a high cost; for the theory of relativity demands the surrender of intuitions of space and time. Kelvin was no less prescient in identifying his second cloud. It washed out Planck’s attempt to subjugate cavity radiation to ordinary physics.

C1.P30This second cloud was a democratic principle known as ‘equipartition of energy’. An apparently inescapable conclusion of the representation of a gas sample as a huge assemblage of (page 19)p. 19bouncing molecules, the principle required that at equilibrium and over time every molecule should enjoy the same average energy proportional to the equilibrium temperature. Equipartition proved catastrophic (that is how physicists described it) when applied to the aether in Planck’s oven because it drove all the radiant energy into high frequencies. Planck’s physics made his oven glow even when cold.

C1.P31The reason that the upper modes in cavity radiation should batten on energy at the expense of the lower even with the equal suffrage of equipartition is easy to see. When equilibrium sets up in the cavity, the aether may be compared, despite its refusal to obey the laws of mechanics, to a gigantic collection of guitar strings each of which has its ends fixed to a wall. The fixing allows only vibrations whose wavelengths are integral subdivisions of the strings’ lengths. The shorter the wavelength (and therefore the higher the frequency) the larger the number of waves that can fit along the string. Since equipartition assigns the same average energy to every mode, the upper end of the spectrum has almost all the energy when the radiation comes to equilibrium. To avoid this theoretical catastrophe, some of Thomson’s colleagues, notably James Jeans, had the ingenious idea that the world has not existed long enough for equilibrium to take hold. Theirs was physics on God’s scale.

C1.P32Planck reversed the prejudice in favour of higher modes by arbitrarily placing a threshold on the energy needed to activate them. In effect he required the ‘resonators’ that made up the walls of the cavity to have a minimum activation energy ε proportional to their frequency, whence the expression $ε=hν$⁠, which would become as famous in physics as $E=mc2$⁠. (A resonator is an electron fixed to a perfect weightless infinitesimal spring, available in all good physics laboratories, which emits precisely homogeneous radiation when vibrating.) The finite (though very small) value of ‘Planck’s constant’ h makes resonators at higher frequencies struggle harder than those at lower frequencies to (page 20)p. 20obtain their minimum ‘quantum’ hν. Its social equivalent would be to make it harder for rich people than for poor people to make money—an apt measure of the oddity of Planck’s quantum theory.

C1.P33Planck’s resonators could take up and give off energy only in jumps and lumps hν. That violated concepts of continuity on which ordinary physics relied. And it left the sub-puzzles whether a resonator that accumulates more than one quantum can dispose of it all at once, or only in single quanta one at a time; and whether the emitted quantum causes waves in the aether as a rock does by falling into a pond, or sails through the pond as if it were not there. In 1905 Einstein gave reasons for believing that in some instances radiant energy traverses the aether like projectiles shot through a vacuum. (Einstein did not banish the medium to which physicists of his time gave the title ‘aether’: the ‘space’ or ‘vacuum’ of modern physics performs even more services.) Bohr rejected Einstein’s ‘light quanta’ for some twenty years before experiment, forcing him to admit them, prompted the creation of his definitive mixture of philosophy and physics.

C1.P34Fortunately, Bohr did not need to know the nature of light to finish his doctoral thesis of 1911. He worried rather about equipartition, which, as he discovered, destroyed a major success of the electron theory of metals. This was the ascription of para- and diamagnetism to the orientation of electron orbits in a magnetic field. Bohr demonstrated that equipartition did not allow the orientation to persist if the electrons interacted. To save the phenomena a mechanism foreign to the theory, something, Bohr hinted, analogous to Planck’s restriction on resonators, had to be invoked to prevent promiscuous sharing of energy. Since Bohr expected all theories to break down somewhere, he no doubt was delighted to discover so clear a case of failure on his own. The problem with magnetism was one of the serious flaws in the theory of metals that Bohr planned to discuss with Thomson during the postdoctoral year he arranged to spend at Cambridge.

(page 21)p. 21C1.P35Bohr arrived in Cambridge in the autumn of 1911, detained, after sustaining his thesis in the spring, by the sudden death of his father in February and the revision of his thesis for the press. He had also to collect some money for a year’s study abroad, which he did by a request, amounting in total to some twenty words, to the Carlsberg Foundation, and to secure, in many more words, an English translation of his thesis that he hoped to print in England.