“There came into the room a slight-looking boy,” Ernest Rutherford’s McGill colleague and biographer A. S. Eve recalls of Manchester days, “whom Rutherford at once took into his study.190, 191 Mrs. Rutherford explained to me that the visitor was a young Dane, and that her husband thought very highly indeed of his work. No wonder, it was Niels Bohr!” The memory is odd. Bohr was an exceptional athlete. The Danes cheered his university soccer exploits. He skied, bicycled and sailed; he chopped wood; he was unbeatable at Ping-Pong; he routinely took stairs two at a time. He was also physically imposing: tall for his generation, with “an enormous domed head,” says C. P. Snow, a long, heavy jaw and big hands.192 He was thinner as a young man than later and his shock of unruly, combed-back hair might have seemed boyish to a man of Eve’s age, twelve years older than Rutherford. But Niels Bohr was hardly “slight-looking.”
Something other than Bohr’s physical appearance triggered Eve’s dissonant memory: probably his presence, which could be hesitant. He was “much more muscular and athletic than his cautious manner suggested,” Snow confirms. “It didn’t help that he spoke with a soft voice, not much above a whisper.” All his life Bohr talked so quietly—and yet indefatigably—that people strained to hear him. Snow knew him as “a talker as hard to get to the point as Henry James in his later years,” but his speech differed dramatically between public and private and between initial exploration of a subject and eventual mastery.193 Publicly, according to Oskar Klein, a student of Bohr’s and then a colleague, “he took the greatest care to get the most accurately shaded formulation of the matter.” Albert Einstein admired Bohr for “uttering his opinions like one perpetually groping and never like one who [believed himself to be] in the possession of definite truth.”194 If Bohr groped through the exploratory phases of his deliberations, with mastery “his assurance grew and his speech became vigorous and full of vivid images,” Lise Meitner’s physicist nephew Otto Frisch noted.195, 196 And privately, among close friends, says Klein, “he would express himself with drastic imagery and strong expressions of admiration as well as criticism.”197
Bohr’s manner was as binary as his speech. Einstein first met Bohr in Berlin in the spring of 1920. “Not often in life,” he wrote to Bohr afterward, “has a human being caused me such joy by his mere presence as you did,” and he reported to their mutual friend Paul Ehrenfest, an Austrian physicist at Leiden, “I am as much in love with him as you are.”198 Despite his enthusiasm Einstein did not fail to observe closely his new Danish friend; his verdict in Bohr’s thirty-fifth year is similar to Eve’s in his twenty-eighth: “He is like an extremely sensitive child who moves around the world in a sort of trance.” At first meeting—until Bohr began to speak—the theoretician Abraham Pais thought the long, heavy face “gloomy” in the extreme and puzzled at that momentary impression when everyone knew “its intense animation and its warm and sunny smile.”199
Bohr’s contributions to twentieth-century physics would rank second only to Einstein’s. He would become a scientist-statesman of unmatched foresight. To a greater extent than is usually the case with scientists, his sense of personal identity—his hard-won selfhood and the emotional values he grounded there—was crucial to his work. For a time, when he was a young man, that identity was painfully divided.
* * *
Bohr’s father, Christian Bohr, was professor of physiology at the University of Copenhagen. In Christian Bohr’s case the Bohr jaw extended below a thick mustache and the face was rounded, the forehead not so high. He may have been athletic; he was certainly a sports enthusiast, who encouraged and helped finance the Akademisk Boldklub for which his sons would one day play champion soccer (Niels’ younger brother Harald at the 1908 Olympics). He was progressive in politics; he worked for the emancipation of women; he was skeptical of religion but nominally conforming, a solid bourgeois intellectual.
Christian Bohr published his first scientific paper at twenty-two, took a medical degree and then a Ph.D. in physiology, studied under the distinguished physiologist Carl Ludwig at Leipzig. Respiration was his special subject and he brought to that research the practice, still novel in the early 1880s, of careful physical and chemical experiment. Outside the laboratory, a friend of his explains, he was a “keen worshipper” of Goethe; larger issues of philosophy intrigued him.200
One of the great arguments of the day was vitalism versus mechanism, a disguised form of the old and continuing debate between those, including the religious, who believe that the world has purpose and those who believe it operates automatically and by chance or in recurring unprogressive cycles. The German chemist who scoffed in 1895 at the “purely mechanical world” of “scientific materialism” that would allow a butterfly to turn back into a caterpillar was disputing the same issue, an issue as old as Aristotle.
In Christian Bohr’s field of expertise it emerged in the question whether organisms and their subsystems—their eyes, their lungs—were assembled to preexisting purpose or according to the blind and unbreathing laws of chemistry and of evolution. The extreme proponent of the mechanistic position in biology then was a German named Ernst Heinrich Haeckel, who insisted that organic and inorganic matter were one and the same. Life arose by spontaneous generation, Haeckel argued; psychology was properly a branch of physiology; the soul was not immortal nor the will free. Despite his commitment to scientific experiment Christian Bohr chose to side against Haeckel, possibly because of his worship of Goethe. He had then the difficult work of reconciling his practice with his views.
Partly for that reason, partly to enjoy the company of friends, he began stopping at a café for discussions with the philosopher Harald Høffding after the regular Friday sessions of the Royal Danish Academy of Sciences and Letters, of which they were both members. The congenial physicist C. Christensen, who spent his childhood as a shepherd, soon added a third point of view. The men moved from café meetings to regular rotation among their homes. The philologist Vilhelm Thomsen joined them to make a formidable foursome: a physicist, a biologist, a philologist, a philosopher. Niels and Harald Bohr sat at their feet all through childhood.
As earnest of his commitment to female emancipation Christian Bohr taught review classes to prepare women for university study. One of his students was a Jewish banker’s daughter, Ellen Adler. Her family was cultured, wealthy, prominent in Danish life; her father was elected at various times to both the lower and upper houses of the Folketing, the Danish parliament. Christian Bohr courted her; they were married in 1881. She had a “lovable personality” and great unselfishness, a friend of her sons would say.201Apparently she submerged her Judaism after her marriage. Nor did she matriculate at the university as she must originally have planned.
Christian and Ellen Bohr began married life in the Adler family townhouse that faced, across a wide street of ancient cobbles, Christianborg Palace, the seat of the Folketing. Niels Bohr was born in that favorable place on October 7, 1885, second child and first son. When his father accepted an appointment at the university in 1886 the Bohr family moved to a house beside the Surgical Academy, where the physiology laboratories were located. There Niels and his brother Harald, nineteen months younger, grew up.
* * *
As far back as Niels Bohr could remember, he liked to dream of great interrelationships. His father was fond of speaking in paradoxes; Niels may have discovered his dreaming in that paternal habit of mind.202, 203 At the same time the boy was profoundly literal-minded, a trait often undervalued that became his anchoring virtue as a physicist. Walking with him when he was about three years old, his father began pointing out the balanced structure of a tree—the trunk, the limbs, the branches, the twigs—assembling the tree for his son from its parts. The literal child saw the wholeness of the organism and dissented: if it wasn’t like that, he said, it wouldn’t be a tree. Bohr told that story all his life, the last time only days before he died, seventy-eight years old, in 1962. “I was from first youth able to say something about philosophical questions,” he summarized proudly then. And because of that ability, he said, “I was considered something of a different character.”204
Harald Bohr was bright, witty, exuberant and assumed at first to be the smarter of the two brothers. “At a very early stage, however,” says Niels Bohr’s later collaborator and biographer Stefan Rozental, “Christian Bohr took the opposite view; he realized Niels’ great abilities and special gifts and the extent of his imagination.” The father phrased his realization in what would have been a cruel comparison if the brothers had been less devoted.205 Niels, he pronounced, was “the special one in the family.”206
Assigned in the fifth grade to draw a house, Niels produced a remarkably mature drawing but counted the fence pickets first. He liked carpentry and metalworking; he was household handyman from an early age. “Even as a child [he] was considered the thinker of the family,” says a younger colleague, “and his father listened closely to his views on fundamental problems.” He almost certainly had trouble learning to write and always had trouble writing.207, 208 His mother served loyally as his amanuensis: he dictated his schoolwork to her and she copied it down.
He and Harald bonded in childhood close as twins. “There runs like a leitmotif above all else,” Rozental notices, “the inseparability that characterized the relationship between the two brothers.” They spoke and thought “à deux” recalls one of their friends.209,210“In my whole youth,” Bohr reminisced, “my brother played a very large part. . . . I had very much to do with my brother. He was in all respects more clever than I.” Harald in his turn told whoever asked that he was merely an ordinary person and his brother pure gold, and seems to have meant it.211, 212
Speech is a clumsiness and writing an impoverishment. Not language but the surface of the body is the child’s first map of the world, undifferentiated between subject and object, coextensive with the world it maps until awakening consciousness divides it off. Niels Bohr liked to show how a stick used as a probe—a blind man’s cane, for example—became an extension of the arm.213 Feeling seemed to move to the end of the stick, he said. The observation was one he often repeated—it struck his physicist protégés as wondrous—like the story of the boy and the tree, because it was charged with emotional meaning for him.
He seems to have been a child of deep connection. That is a preverbal gift. His father, with his own Goethesque yearnings for purpose and wholeness—for natural unity, for the oceanic consolations of religion without the antique formalisms—especially sensed it. His overvalued expectation burdened the boy.
Religious conflict broke early. Niels “believed literally what he learnt from the lessons on religion at school,” says Oskar Klein. “For a long time this made the sensitive boy unhappy on account of his parents’ lack of faith.” Bohr at twenty-seven, in a Christmastime letter to his fiancée from Cambridge, remembered the unhappiness as paternal betrayal: “I see a little boy in the snow-covered street on his way to church.214 It was the only day his father went to church. Why? So the little boy would not feel different from other little boys. He never said a word to the little boy about belief or doubt, and the little boy believed with all of his heart.”215
The difficulty with writing was a more ominous sign. The family patched the problem over by supplying him with his mother’s services as a secretary. He did not compose mentally while alone and then call in his helper. He composed on the spot, laboriously. That was the whispering that reminded C. P. Snow of the later Henry James. As an adult Bohr drafted and redrafted even private letters. His reworking of scientific papers in draft and then repeatedly in proof became legendary.216 Once after continued appeals to Zurich for the incomparable critical aid of the Austrian theoretical physicist Wolfgang Pauli, who knew Bohr well, Pauli responded warily, “If the last proof is sent away, then I will come.” Bohr collaborated first with his mother and with Harald, then with his wife, then with a lifelong series of younger physicists.217 They cherished the opportunity of working with Bohr, but the experience could be disturbing. He wanted not only their attention but also their intellectual and emotional commitment: he wanted to convince his collaborators that he was right. Until he succeeded he doubted his conclusions himself, or at least doubted the language of their formulation.
Behind the difficulty with writing lay another, more pervasive difficulty. It took the form of anxiety that without the extraordinary support of his mother and his brother would have been crippling. For a time, it was.218
It may have emerged first as religious doubt, which appeared, according to Klein, when Niels was “a young man.” Bohr doubted as he had believed, “with unusual resolution.” By the time he matriculated at the University of Copenhagen in the autumn of 1903, when he was eighteen, the doubt had become pervasive, intoxicating him with terrifying infinities.219
Bohr had a favorite novel. Its author, Poul Martin Møller, introduced En Dansk Students Eventyr (The Adventures of a Danish Student) as a reading before the University of Copenhagen student union in 1824. It was published posthumously. It was short, witty and deceptively lighthearted. In an important lecture in 1960, “The Unity of Human Knowledge,” Bohr described Møller’s book as “an unfinished novel still read with delight by the older as well as the younger generation in [Denmark].” It gives, he said, “a remarkably vivid and suggestive account of the interplay between the various aspects of our position [as human beings].”220 After the Great War the Danish government helped Bohr establish an institute in Copenhagen. The most promising young physicists in the world pilgrimaged to study there.221 “Every one of those who came into closer contact with Bohr at the Institute,” writes his collaborator Léon Rosenfeld, “as soon as he showed himself sufficiently proficient in the Danish language, was acquainted with the little book: it was part of his initiation.”222
What magic was contained in the little book? It was the first Danish novel with a contemporary setting: student life, and especially the extended conversations of two student cousins, one a “licentiate”—a degree candidate—the other a “philistine.” The philistine is a familiar type, says Bohr, “very soberly efficient in practical affairs”; the licentiate, more exotic, “is addicted to remote philosophical meditations detrimental to his social activities.”223 Bohr quotes one of the licentiate’s “philosophical meditations”:
[I start] to think about my own thoughts of the situation in which I find myself. I even think that I think of it, and divide myself into an infinite retrogressive sequence of “I’s” who consider each other. I do not know at which “I” to stop as the actual, and in the moment I stop at one, there is indeed again an “I” which stops at it. I become confused and feel a dizziness as if I were looking down into a bottomless abyss.224
“Bohr kept coming back to the different meanings of the word ‘I,’ ” Robert Oppenheimer remembered, “the ‘I’ that acts, the ‘I,’ that thinks, the ‘I,’ that studies itself.”225
Other conditions that trouble the licentiate in Møller’s novel might be taken from a clinical description of the conditions that troubled the young Niels Bohr. This disability, for example:
Certainly I have seen thoughts put on paper before; but since I have come distinctly to perceive the contradiction implied in such an action, I feel completely incapable of forming a single written sentence. . . . I torture myself to solve the unaccountable puzzle, how one can think, talk, or write. You see, my friend, a movement presupposes a direction. The mind cannot proceed without moving along a certain line; but before following this line, it must already have thought it. Therefore one has already thought every thought before one thinks it. Thus every thought, which seems the work of a minute, presupposes an eternity. This could almost drive me to madness.226
Or this complaint, on the fragmentation of the self and its multiplying duplicity, which Bohr in later years was wont to quote:
Thus on many occasions man divides himself into two persons, one of whom tries to fool the other, while a third one, who is in fact the same as the other two, is filled with wonder at this confusion. In short, thinking becomes dramatic and quietly acts the most complicated plots with itself and for itself; and the spectator again and again becomes actor.227
“Bohr would point to those scenes,” Rosenfeld notes, “in which the licentiate describes how he loses the count of his many egos, or [discourses] on the impossibility of formulating a thought, and from these fanciful antinomies he would lead his interlocutor . . . to the heart of the problem of unambiguous communication of experience, whose earnestness he thus dramatically emphasized.”228 Rosenfeld worshiped Bohr; he failed to see, or chose not to report, that for Bohr the struggles of the licentiate were more than “fanciful antinomies.”
Ratiocination—that is the technical term for what the licentiate does, the term for what the young Bohr did as well—is a defense mechanism against anxiety. Thought spirals, panicky and compulsive. Doubt doubles and redoubles, paralyzing action, emptying out the world. The mechanism is infinitely regressive because once the victim knows the trick, he can doubt anything, even doubt itself. Philosophically the phenomenon could be interesting, but as a practical matter ratiocination is a way of stalling. If work is never finished, its quality cannot be judged. The trouble is that stalling postpones the confrontation and adds that guilt to the burden. Anxiety increases; the mechanism accelerates its spiraling flights; the self feels as if it will fragment; the multiplying “I” dramatizes the feeling of impending breakup. At that point madness reveals its horrors; the image that recurred in Bohr’s conversation and writing throughout his life was the licentiate’s “bottomless abyss.”229 We are “suspended in language,” Bohr liked to say, evoking that abyss; and one of his favorite quotations was two lines from Schiller:230
Nur die Fülle führt zur Klarheit,231
Und im Abgrund wohnt die Wahrheit
Only wholeness leads to clarity,
And truth lies in the abyss.
But it was not in Møller that Bohr found solid footing. He needed more than a novel, however apposite, for that. He needed what we all need for sanity: he needed love and work.
“I took a great interest in philosophy in the years after my [high school] examination,” Bohr said in his last interview. “I came especially in close connection with Høffding.”232 Harald Høffding was Bohr’s father’s old friend, the other charter member of the Friday-night discussion group.233 Bohr had known him from childhood. Born in 1843, he was twelve years older than Christian Bohr, a profound, sensitive and kindly man. He was a skillful interpreter of the work of Søren Kierkegaard and of William James and a respected philosopher in his own right: an anti-Hegelian, a pragmatist interested in questions of perceptive discontinuity. Bohr became a Høffding student. It seems certain he also turned personally to Høffding for help. He made a good choice. Høffding had struggled through a crisis of his own as a young man, a crisis that brought him, he wrote later, near “despair.”234
Høffding was twelve years old when Søren Kierkegaard died of a lung infection in chill November 1855, old enough to have heard of the near-riot at the grave a somber walk outside the city walls, old enough for the strange, awkward, fiercely eloquent poet of multiple pseudonyms to have been a living figure. With that familiarity as a point of origin Høffding later turned to Kierkegaard’s writings for solace from despair. He found it especially in Stages on Life’s Way, a black-humorous dramatization of a dialectic of spiritual stages, each independent, disconnected, bridgeable only by an irrational leap of faith. Høffding championed the prolific and difficult Dane in gratitude; his second major book, published in 1892, would help establish Kierkegaard as an important philosopher rather than merely a literary stylist given to outbursts of raving, as Danish critics had first chosen to regard him.
Kierkegaard had much to offer Bohr, especially as Høffding interpreted him. Kierkegaard examined the same states of mind as had Poul Martin Møller. Møller taught Kierkegaard moral philosophy at the university and seems to have been a guide.235 After Møller’s death Kierkegaard dedicated The Concept of Dread to him and referred to him in a draft of the dedication as “my youth’s enthusiasm, my beginning’s confidant, mighty trumpet of my awakening, my departed friend.”236 From Møller to Kierkegaard to Høffding to Bohr: the line of descent was direct.
Kierkegaard notoriously suffered from a proliferation of identities and doubts. The doubling of consciousness is a central theme in Kierkegaard’s work, as it was in Møller’s before him. It would even seem to be a hazard of long standing among the Danes. The Danish word for despair, Fortvivlelse, carries lodged at its heart the morpheme tvi, which means “two” and signifies the doubling of consciousness.237 Tvivl in Danish means “doubt”; Tvivlesyg means “skepticism”; Tvetydighed, “ambiguity.” The self watching itself is indeed a commonplace of puritanism, closely akin to the Christian conscience.
But unlike Møller, who jollies the licentiate’s Tvivl away, Kierkegaard struggled to find a track through the maze of mirrors. Høffding, in his History of Modern Philosophy, which Bohr would have read as an undergraduate, summarizes the track he understood Kierkegaard to have found: “His leading idea was that the different possible conceptions of life are so sharply opposed to one another that we must make a choice between them, hence his catchword either-or; moreover, it must be a choice which each particular person must make for himself, hence his second catchword, the individual.” And, following: “Only in the world of possibilities is there continuity; in the world of reality decision always comes through a breach of continuity.”238, 239 Continuity in the sense that it afflicted Bohr was the proliferating stream of doubts and “I’s” that plagued him; a breach of that continuity—decisiveness, function—was the termination he hoped to find.
He turned first to mathematics. He learned in a university lecture about Riemannian geometry, a type of non-Euclidean geometry developed by the German mathematician Georg Riemann to represent the functions of complex variables. Riemann showed how such multivalued functions (a number, its square root, its logarithm and so on) could be represented and related on a stack of coincident geometric planes that came to be called Riemann surfaces. “At that time,” Bohr said in his last interview, “I really thought to write something about philosophy, and that was about this analogy with multivalued functions.240 I felt that the various problems in psychology—which were called the big philosophical problems, of the free will and such things—that one could really reduce them when one considered how one really went about them, and that was done on the analogy to multivalued functions.” By then he thought the problem might be one of language, of the ambiguity—the multiple values, as it were—between different meanings of the word “I.” Separate each different meaning on a different plane and you could keep track of what you were talking about. The confusion of identities would resolve itself graphically before one’s eyes.
The scheme was too schematic for Bohr. Mathematics was probably too much like ratiocination, leaving him isolated within his anxiety. He thought of writing a book about his mathematical analogies but leapt instead to work that was far more concrete. But notice that the mathematical analogy begins to embed the problem of doubt within the framework of language, identifying doubt as a specialized form of verbal ambiguity, and notice that it seeks to clarify ambiguities by isolating their several variant meanings on separate, disconnected planes.
The solid work Bohr took up, in February 1905, when he was nineteen years old, was a problem in experimental physics.241 Each year the Royal Danish Academy of Sciences and Letters announced problems for study against a two-year deadline, after which the academy awarded gold and silver medals for successful papers. In 1905 the physics problem was to determine the surface tension of a number of liquids by measuring the waves produced in those liquids when they were allowed to run out through a hole (the braided cascade of a garden hose demonstrates such waves). The method had been proposed by the British Nobelist John William Strutt, Lord Rayleigh, but no one had yet tried it out. Bohr and one other contestant accepted the challenge.
Bohr went to work in the physiology laboratory where he had watched and then assisted his father for years, learning the craft of experiment. To produce stable jets he decided to use drawn-out glass tubes. Because the method required large quantities of liquid he limited his experiment to water. The tubes had to be flattened on the sides to make an oval cross section; that gave the jet of water the shape it needed to evolve braidlike waves. All the work of heating, softening and drawing out the tubes Bohr did himself; he found it hypnotic. Rosenfeld says Bohr “took such delight in this operation that, completely forgetting its original purpose, he spent hours passing tube after tube through the flame.”242
Each separate experimental determination of the surface-tension value took hours. It had to be done at night, when the lab was unoccupied, because the jets were easily disturbed by vibration. Slow work, but Bohr also dawdled. The academy had allowed two years. Toward the end of that time Christian Bohr realized his son was procrastinating to the point where he might not finish his paper before the deadline. “The experiments had no end,” Bohr told Rosenfeld some years later on a bicycle ride in the country; “I always noticed new details that I thought I had first to understand. At last my father sent me out here, away from the laboratory, and I had to write up the paper.”243
“Out here” was Naerumgaard, the Adler country estate north of Copenhagen. There, away from the temptations of the laboratory, Niels wrote and Harald transcribed an essay of 114 pages. Niels submitted it to the academy on the day of deadline, but even then it was incomplete; three days later he turned in an eleven-page addendum that had been accidentally left off.
The essay, Bohr’s first scientific paper, determined the surface tension only of water but also uniquely extended Rayleigh’s theory. It won a gold medal from the academy. It was an outstanding achievement for someone so young and it set Bohr’s course for physics. Unlike mathematicized philosophy, physics was anchored solidly in the real world.
In 1909 the Royal Society of London accepted the surface-tension paper in modified form for its Philosophical Transactions. Bohr, who was still only a student working toward his master’s degree when the essay appeared, had to explain to the secretary of the society, who had addressed him by his presumed academic title, that he was “not a professor.”244
Retreating to the country had helped him once. It might help again. Naerumgaard ceased to be available when the Adler family donated it for use as a school. When the time came to study for his master’s degree examinations, between March and May 1909, Bohr traveled to Vissenbjerg, on the island of Funen, the next island west from Copenhagen’s Zealand, to stay at the parsonage of the parents of Christian Bohr’s laboratory assistant. Niels procrastinated on Funen by reading Stages on Life’s Way. The day he finished it he enthusiastically mailed the book to Harald. “This is the only thing I have to send,” he wrote his younger brother; “nevertheless, I don’t think I could easily find anything better. . . . It is something of the finest I have ever read.”245 At the end of June, back in Copenhagen, again on deadline day, Bohr turned in his master’s thesis, copied out in his mother’s hand.
Harald had sprinted ahead of him by then, having won his M.Sc. in April and gone off to the Georgia-Augusta University in Gottingen, Germany, the center of European mathematics, to study for his Ph.D. He received that degree in Göttingen in June 1910. Niels wrote his younger brother tongue-in-cheek that his “envy would soon be growing over the rooftops,” but in fact he was happy with his progress on his own doctoral dissertation despite having spent “four months speculating about a silly question about some silly electrons and [succeeding] only in writing circa fourteen more or less divergent rough drafts.”246, 247 Christensen had posed Bohr a problem in the electron theory of metals for his master’s thesis; the subject interested Bohr enough to continue pursuing it as his doctoral work. He was specializing in theoretical studies now; to try to do experimental work too, he explained, was “unpractical.”248
He returned to the parsonage at Vissenbjerg in the autumn of 1910. His work slowed. He may have recalled the licentiate’s dissertation problems, for he again turned to Kierkegaard. “He made a powerful impression on me when I wrote my dissertation in a parsonage in Funen, and I read his works night and day,” Bohr told his friend and former student J. Rud Nielsen in 1933. “His honesty and his willingness to think the problems through to their very limit is what is great. And his language is wonderful, often sublime. There is of course much in Kierkegaard that I cannot accept. I ascribe that to the times in which he lived. But I admire his intensity and perseverance, his analysis to the utmost limit, and the fact that through these qualities he turned misfortune and suffering into something good.”249
He finished his Ph.D. thesis, “Studies in the electron theory of metals,” by the end of January 1911. On February 3, suddenly, at fifty-six, his father died. He dedicated his thesis “in deepest gratitude to the memory of my father.”250 He loved his father; if there had been a burden of expectation he was free of that burden now.
As was customary, he publicly defended his thesis in Copenhagen on May 13. “Dr. Bohr, a pale and modest young man,” the Copenhagen newspaper Dagbladet reported under a crude drawing of the candidate standing in white tie and tails at a heavy lectern, “did not take much part in the proceedings, whose short duration is a record.”251 The small hall was crowded to overflowing. Christiansen, one of the two examiners, said simply that hardly anyone in Denmark was well enough informed on the subject to judge the candidate’s work.
Before he died Christian Bohr had helped arrange a fellowship from the Carlsberg Foundation for his son for study abroad. Niels spent the summer sailing and hiking with Margrethe Nørland, the sister of a friend, a beautiful young student whom he had met in 1910 and to whom, shortly before his departure, he became engaged. Then he went off in late September to Cambridge. He had arranged to study at the Cavendish under J. J. Thomson.
29 Sept. 1911
Eltisley Avenue 10,
Things are going so well for me. I have just been talking to J. J. Thomson and have explained to him, as well as I could, my ideas about radiation, magnetism, etc. If you only knew what it meant to me to talk to such a man. He was extremely nice to me, and we talked about so much; and I do believe that he thought there was some sense in what I said. He is now going to read [my dissertation] and he invited me to have dinner with him Sunday at Trinity College; then he will talk with me about it. You can imagine that I am happy. . . . I now have my own little flat. It is at the edge of town and is very nice in all respects. I have two rooms and eat all alone in my own room. It is very nice here; now, as I am sitting and writing to you, it blazes and rumbles in my own little fireplace.
Niels Bohr was delighted with Cambridge. His father’s Anglophilia had prepared him to like English settings; the university offered the tradition of Newton and Clerk Maxwell and the great Cavendish Laboratory with its awesome record of physical discovery. Bohr found that his schoolboy English needed work and set out reading David Copperfield with an authoritative new dictionary at hand, looking up every uncertain word. He discovered that the laboratory was crowded and undersupplied. On the other hand, it was amusing to have to go about in cap and gown (once he was admitted to Trinity as a research student) “under threat of high fines,” to see the Trinity high table “where they eat so much and so first-rate that it is quite unbelievable and incomprehensible that they can stand it,” to walk “for an hour before dinner across the most beautiful meadows along the river, with the hedges flecked with red berries and with isolated windblown willow trees—imagine all this under the most magnificent autumn sky with scurrying clouds and blustering wind.”253, 254 He joined a soccer club; called on physiologists who had been students of his father; attended physics lectures; worked on an experiment Thomson had assigned him; allowed the English ladies, “absolute geniuses at drawing you out,” to do their duty by him at dinner parties.255
But Thomson never got around to reading his dissertation. The first meeting had not, in fact, gone so well. The new student from Denmark had done more than explain his ideas; he had shown Thomson the errors he found in Thomson’s electron-theory work. “I wonder,” Bohr wrote Margrethe soon after, “what he will say to my disagreement with his ideas.”256 And a little later: “I’m longing to hear what Thomson will say. He’s a great man. I hope he will not get angry with my silly talk.”257
Thomson may or may not have been angry. He was not much interested in electrons anymore. He had turned his attention to positive rays—the experiment he assigned Bohr concerned such rays and Bohr found it distinctly unpromising—and in any case had very little patience with theoretical discussions. “It takes half a year to get to know an Englishman,” Bohr said in his last interview. “ . . . It was the custom in England that they would be polite and so on, but they wouldn’t be interested to see anybody. . . .258 I went Sundays to the dinner in Trinity College. . . . I was sitting there, and nobody spoke to me ever in many Sundays. But then they understood that I was not more eager to speak to them than they were to speak to me. And then we were friends, you see, and then the whole thing was different.” The insight is generalized; Thomson’s indifference was perhaps its first specific instance.
Then Rutherford turned up at Cambridge.
He “came down from Manchester to speak at the annual Cavendish Dinner,” says Bohr. “Although on this occasion I did not come into personal contact with [him], I received a deep impression of the charm and power of his personality by which he had been able to achieve almost the incredible wherever he worked. The dinner”—in December—“took place in a most humorous atmosphere and gave the opportunity for several of Rutherford’s colleagues to recall some of the many anecdotes which already then were attached to his name.”259 Rutherford spoke warmly of the recent work of the physicist C. T. R. Wilson, the inventor of the cloud chamber (which made the paths of charged particles visible as lines of water droplets hovering in supersaturated fog) and a friend from Cambridge student days. Wilson had “just then,” says Bohr, photographed alpha particles in his cloud chamber scattering from interactions with nuclei, “the phenomenon which only a few months before had led [Rutherford] to his epoch-making discovery of the atomic nucleus.”260
Bohr had matters on his mind that he would soon relate to the problem of the nucleus and its theoretically unstable electrons, but it was Rutherford’s enthusiastic informality that most impressed him at the annual dinner.261 Remembering this period of his life long afterward, he would single out for special praise among Rutherford’s qualities “the patience to listen to every young man when he felt he had any idea, however modest, on his mind.”262 In contrast, presumably, to J. J. Thomson, whatever Thomson’s other virtues.
Soon after the dinner Bohr went up to Manchester to visit “one of my recently deceased father’s colleagues who was also a close friend of Rutherford,” whom Bohr wanted to meet.263 The close friend brought them together. Rutherford looked over the young Dane and liked what he saw despite his prejudice against theoreticians. Someone asked him later about the discrepancy. “Bohr’s different,” Rutherford roared, disguising affection with bluster. “He’s a football player!” Bohr was different in another regard as well; he was easily the most talented of all Rutherford’s many students—and Rutherford trained no fewer than eleven Nobel Prize winners during his life, an unsurpassed record.264, 265
Bohr held up his decision between Cambridge and Manchester until he could go over everything with Harald, who visited him in Cambridge in January 1912 for the purpose. Then Bohr eagerly wrote Rutherford for permission to study at Manchester, as they had discussed in December. Rutherford had advised him then not to give up on Cambridge too quickly—Manchester is always here, he told him, it won’t run away—and so Bohr proposed to arrive for spring term, which began in late March.266 Rutherford gladly agreed. Bohr felt he was being wasted at Cambridge. He wanted substantial work.
His first six weeks in Manchester he spent following “an introductory course on the experimental methods of radioactive research,” with Geiger and Marsden among the instructors.267 He continued pursuing his independent studies in electron theory. He began a lifelong friendship with a young Hungarian aristocrat, George de Hevesy, a radiochemist with a long, sensitive face dominated by a towering nose. De Hevesy’s father was a court councillor, his mother a baroness; as a child he had hunted partridge in the private game park of the Austro-Hungarian emperor Franz Josef next to his grandfather’s estate. Now he was working to meet a challenge Rutherford had thrown at him one day to separate radioactive decay products from their parent substances. Out of that work he developed over the next several decades the science of using radioactive tracers in medical and biological research, one more useful offspring of Rutherford’s casual but fecund paternity.
Bohr learned about radiochemistry from de Hevesy.268 He began to see connections with his electron-theory work. His sudden burst of intuitions then was spectacular. He realized in the space of a few weeks that radioactive properties originated in the atomic nucleus but chemical properties depended primarily on the number and distribution of electrons. He realized—the idea was wild but happened to be true—that since the electrons determined the chemistry and the total positive charge of the nucleus determined the number of electrons, an element’s position on the periodic table of the elements was exactly the nuclear charge (or “atomic number”): hydrogen first with a nuclear charge of 1, then helium with a nuclear charge of 2 and so on up to uranium at 92.
De Hevesy remarked to him that the number of known radio elements already far outnumbered the available spaces on the periodic table and Bohr made more intuitive connections. Soddy had pointed out that the radio elements were generally not new elements, only variant physical forms of the natural elements (he would soon give them their modern name, isotopes). Bohr realized that the radio elements must have the same atomic number as the natural elements with which they were chemically identical. That enabled him to rough out what came to be called the radioactive displacement law: that when an element transmutes itself through radioactive decay it shifts its position on the periodic table two places to the left if it emits an alpha particle (a helium nucleus, atomic number 2), one place to the right if it emits a beta ray (an energetic electron, which leaves behind in the nucleus an extra positive charge).
Periodic table of the elements. The lanthanide series (“rare earths”), beginning with lanthanum (57), and the actinide series, which begins with actinium (89) and includes thorium (90) and uranium (92), are chemically similar. Other families of elements read vertically down the table—at the far right, for example, the noble gases: helium, neon, argon, krypton, xenon, radon.
All these first rough insights would be the work of other men’s years to anchor soundly in theory and experiment. Bohr ran them in to Rutherford. To his surprise, he found the discoverer of the nucleus cautious about his own discovery. “Rutherford . . . thought that the meagre evidence [so far obtained] about the nuclear atom was not certain enough to draw such consequences,” Bohr recalled.269 “And I said to him that I was sure that it would be the final proof of his atom.” If not convinced, Rutherford was at least impressed; when de Hevesy asked him a question about radiation one day Rutherford responded cheerfully, “Ask Bohr!”270
Rutherford was well prepared for surprises, then, when Bohr came to see him again in mid-June. Bohr told Harald what he was on to in a letter on June 19, after the meeting:
It could be that I’ve perhaps found out a little bit about the structure of atoms. You must not tell anyone anything about it, otherwise I certainly could not write you this soon. If I’m right, it would not be an indication of the nature of a possibility . . . but perhaps a little piece of reality. . . . You understand that I may yet be wrong, for it hasn’t been worked out fully yet (but I don’t think so); nor do I believe that Rutherford thinks it’s completely wild; he is the right kind of man and would never say that he was convinced of something that was not entirely worked out. You can imagine how anxious I am to finish quickly.271
Bohr had caught a first glimpse of how to stabilize the electrons that orbited with such theoretical instability around Rutherford’s nucleus. Rutherford sent him off to his rooms to work it out. Time was running short; he planned to marry Margrethe Nørland in Copenhagen on August 1. He wrote Harald on July 17 that he was “getting along fairly well; I believe I have found out a few things; but it is certainly taking more time to work them out than I was foolish enough to believe at first.272 I hope to have a little paper ready to show to Rutherford before I leave, so I’m busy, so busy; but the unbelieveable heat here in Manchester doesn’t exactly help my diligence. How I look forward to talking to you!” By the following Wednesday, July 22, he had seen Rutherford, won further encouragement, and was making plans to meet Harald on the way home.273
Bohr married, a serene marriage with a strong, intelligent and beautiful woman that lasted a lifetime. He taught at the University of Copenhagen through the autumn term. The new model of the atom he was struggling to develop continued to tax him. On November 4 he wrote Rutherford that he expected “to be able to finish the paper in a few weeks.”274 A few weeks passed; with nothing finished he arranged to be relieved of his university teaching and retreated to the country with Margrethe. The old system worked; he produced “a very long paper on all these things.”275 Then an important new idea came to him and he broke up his original long paper and began rewriting it into three parts. “On the constitution of atoms and molecules,” so proudly and bravely titled—Part I mailed to Rutherford on March 6, 1913, Parts II and III finished and published before the end of the year—would change the course of twentieth-century physics. Bohr won the 1922 Nobel Prize in Physics for the work.
* * *
As far back as Bohr’s doctoral dissertation he had decided that some of the phenomena he was examining could not be explained by the mechanical laws of Newtonian physics. “One must assume that there are forces in nature of a kind completely different from the usual mechanical sort,” he wrote then.276 He knew where to look for these different forces: he looked to the work of Max Planck and Albert Einstein.
Planck was the German theoretician whom Leo Szilard would meet at the University of Berlin in 1921; born in 1858, Planck had taught at Berlin since 1889. In 1900 he had proposed a revolutionary idea to explain a persistent problem in mechanical physics, the so-called ultraviolet catastrophe. According to classical theory there should be an infinite amount of light (energy, radiation) inside a heated cavity such as a kiln. That was because classical theory, with its continuity of process, predicted that the particles in the heated walls of the cavity which vibrated to produce the light would vibrate to an infinite range of frequencies.
Obviously such was not the case. But what kept the energy in the cavity from running off infinitely into the far ultraviolet? Planck began his effort to find out in 1897 and pursued it for three hard years. Success came with a last-minute insight announced at a meeting of the Berlin Physical Society on October 19, 1900. Friends checked Planck’s new formula that very night against experimentally derived values. They reported its accuracy to him the next morning. “Later measurements, too,” Planck wrote proudly in 1947, at the end of his long life, “confirmed my radiation formula again and again—the finer the methods of measurement used, the more accurate the formula was found to be.”277
Planck solved the radiation problem by proposing that the vibrating particles can only radiate at certain energies. The permitted energies would be determined by a new number—“a universal constant,” he says, “which I called h. Since it had the dimension of action (energy X time), I gave it the name, elementary quantum of action.”278 (Quantum is the neuter form of the Latin word quantus, meaning “how great.”) Only those limited and finite energies could appear which were whole-number multiples of hv: of the frequency ν times Planck’s h. Planck calculated h to be a very small number, close to the modern value of 6.63 × 10−27 erg-seconds. Universal h soon acquired its modern name: Planck’s constant.
Planck, a thoroughgoing conservative, had no taste for pursuing the radical consequences of his radiation formula. Someone else did: Albert Einstein. In a paper in 1905 that eventually won for him the Nobel Prize, Einstein connected Planck’s idea of limited, discontinuous energy levels to the problem of the photoelectric effect. Light shone on certain metals knocks electrons free; the effect is applied today in the solar panels that power spacecraft. But the energy of the electrons knocked free of the metal does not depend, as common sense would suggest, on the brightness of the light. It depends instead on the color of the light—on its frequency.
Einstein saw a quantum condition in this odd fact. He proposed the heretical possibility that light, which years of careful scientific experiment had demonstrated to travel in waves, actually traveled in small individual packets—particles—which he called “energy quanta.” Such photons (as they are called today), he wrote, have a distinctive energy hv and they transfer most of that energy to the electrons they strike on the surface of the metal. A brighter light thus releases more electrons but not more energetic electrons; the energy of the electrons released depends on hv and so on the frequency of the light. Thus Einstein advanced Planck’s quantum idea from the status of a convenient tool for calculation to that of a possible physical fact.
With these advances in understanding Bohr was able to confront the problem of the mechanical instability of Rutherford’s model of the atom. In July, at the time of the “little paper ready to show to Rutherford,” he already had his central idea. It was this: that since classical mechanics predicted that an atom like Rutherford’s, with a small, massive central nucleus surrounded by orbiting electrons, would be unstable, while in fact atoms are among the most stable of systems, classical mechanics was inadequate to describe such systems and would have to give way to a quantum approach. Planck had introduced quantum principles to save the laws of thermodynamics; Einstein had extended the quantum idea to light; Bohr now proposed to lodge quantum principles within the atom itself.
Through the autumn and early winter, back in Denmark, Bohr pursued the consequences of his idea. The difficulty with Rutherford’s atom was that nothing about its design justified its stability. If it happened to be an atom with several electrons, it would fly apart. Even if it were a hydrogen atom with only one (mechanically stable) electron, classical theory predicted that the electron would radiate light as it changed direction in its orbit around the nucleus and therefore, the system losing energy, would spiral into the nucleus and crash. The Rutherford atom, from the point of view of Newtonian mechanics—as a miniature solar system—ought to be impossibly large or impossibly small.
Bohr therefore proposed that there must be what he called “stationary states” in the atom: orbits the electrons could occupy without instability, without radiating light, without spiraling in and crashing. He worked the numbers of this model and found they agreed very well with all sorts of experimental values. Then at least he had a plausible model, one that explained in particular some of the phenomena of chemistry. But it was apparently arbitrary; it was not more obviously a real picture of the atom than other useful models such as J. J. Thomson’s plum pudding.
Help came then from an unlikely quarter. A professor of mathematics at King’s College, London, J. W. Nicholson, whom Bohr had met and thought a fool, published a series of papers proposing a quantized Saturnian model of the atom to explain the unusual spectrum of the corona of the sun. The papers were published in June in an astronomy journal; Bohr didn’t see them until December. He was quickly able to identify the inadequacies of Nicholson’s model, but not before he felt the challenge of other researchers breathing down his neck—and not without noticing Nicholson’s excursion into the jungle of spectral lines.
Oriented toward chemistry, communicating back and forth with George de Hevesy, Bohr had not thought of looking at spectroscopy for evidence to support his model of the atom. “The spectra was a very difficult problem,” he said in his last interview. “ . . . One thought that this is marvelous, but it is not possible to make progress there. Just as if you have the wing of a butterfly, then certainly it is very regular with the colors and so on, but nobody thought that one could get the basis of biology from the coloring of the wing of a butterfly.”279
Taking Nicholson’s hint, Bohr now turned to the wings of the spectral butterfly.
Spectroscopy was a well-developed field in 1912. The eighteenth-century Scottish physicist Thomas Melvill had first productively explored it. He mixed chemical salts with alcohol, lit the mixtures and studied the resulting light through a prism. Each different chemical produced characteristic patches of color. That suggested the possibility of using spectra for chemical analysis, to identify unknown substances. The prism spectroscope, invented in 1859, advanced the science. It used a narrow slit set in front of a prism to limit the patches of light to similarly narrow lines; these could be directed onto a ruled scale (and later onto strips of photographic film) to measure their spacing and calculate their wavelengths. Such characteristic patterns of lines came to be called line spectra. Every element had its own unique line spectrum. Helium was discovered in the chromosphere of the sun in 1868 as a series of unusual spectral lines twenty-three years before it was discovered mixed into uranium ore on earth. The line spectra had their uses.
But no one understood what produced the lines. At best, mathematicians and spectroscopists who liked to play with wavelength numbers were able to find beautiful harmonic regularities among sets of spectral lines. Johann Balmer, a nineteenth-century Swiss mathematical physicist, identified in 1885 one of the most basic harmonies, a formula for calculating the wavelengths of the spectral lines of hydrogen. These, collectively called the Balmer series, look like this:
It is not necessary to understand mathematics to appreciate the simplicity of the formula Balmer derived that predicts a line’s location on the spectral band to an accuracy of within one part in a thousand, a formula that has only one arbitrary number:
(the Greek letter λ, lambda, stands for the wavelength of the line; η takes the values 3, 4, 5 and so on for the various lines). Using his formula, Balmer was able to predict the wavelengths of lines to be expected for parts of the hydrogen spectrum not yet studied. They were found where he said they would be.
A Swedish spectroscopist, Johannes Rydberg, went Balmer one better and published in 1890 a general formula valid for a great many different line spectra. The Balmer formula then became a special case of the more general Rydberg equation, which was built around a number called the Rydberg constant. That number, subsequently derived by experiment and one of the most accurately known of all universal constants, takes the precise modern value of 109,677 cm−1.
Bohr would have known these formulae and numbers from undergraduate physics, especially since Christensen was an admirer of Rydberg and had thoroughly studied his work. But spectroscopy was far from Bohr’s field and he presumably had forgotten them. He sought out his old friend and classmate, Hans Hansen, a physicist and student of spectroscopy just returned from Göttingen. Hansen reviewed the regularity of line spectra with him. Bohr looked up the numbers. “As soon as I saw Balmer’s formula,” he said afterward, “the whole thing was immediately clear to me.”280
What was immediately clear was the relationship between his orbiting electrons and the lines of spectral light. Bohr proposed that an electron bound to a nucleus normally occupies a stable, basic orbit called a ground state. Add energy to the atom—heat it, for example—and the electron responds by jumping to a higher orbit, one of the more energetic stationary states farther away from the nucleus. Add more energy and the electron continues jumping to higher orbits. Cease adding energy—leave the atom alone—and the electrons jump back to their ground states, like this:
With each jump, each electron emits a photon of characteristic energy. The jumps, and so the photon energies, are limited by Planck’s constant. Subtract the value of a lower-energy stationary state W2 from the value of a higher energy stationary state W1 and you get exactly the energy of the light as hv. So here was the physical mechanism of Planck’s cavity radiation.
From this elegant simplification, W1—W2 = hv, Bohr was able to derive the Balmer series. The lines of the Balmer series turn out to be exactly the energies of the photons that the hydrogen electron emits when it jumps down from orbit to orbit to its ground state.
Then, sensationally, with the simple formula
(where m is the mass of the electron, e the electron charge and h Planck’s constant—all fundamental numbers, not arbitrary numbers Bohr made up) Bohr produced Rydberg’s constant, calculating it within 7 percent of its experimentally measured value! “There is nothing in the world which impresses a physicist more,” an American physicist comments, “than a numerical agreement between experiment and theory, and I do not think that there can ever have been a numerical agreement more impressive than this one, as I can testify who remember its advent.”281
“On the constitution of atoms and molecules” was seminally important to physics. Besides proposing a useful model of the atom, it demonstrated that events that take place on the atomic scale are quantized: that just as matter exists as atoms and particles in a state of essential graininess, so also does process. Process is discontinuous and the “granule” of process—of electron motions within the atom, for example—is Planck’s constant. The older mechanistic physics was therefore imprecise; though a good approximation that worked for large-scale events, it failed to account for atomic subtleties.
Bohr was happy to force this confrontation between the old physics and the new. He felt that it would be fruitful for physics. Because original work is inherently rebellious, his paper was not only an examination of the physical world but also a political document. It proposed, in a sense, to begin a reform movement in physics: to limit claims and clear up epistemological fallacies. Mechanistic physics had become authoritarian. It had outreached itself to claim universal application, to claim that the universe and everything in it is rigidly governed by mechanistic cause and effect. That was Haeckelism carried to a cold extreme. It stifled Niels Bohr as biological Haeckelism had stifled Christian Bohr and as a similar authoritarianism in philosophy and in bourgeois Christianity had stifled Søren Kierkegaard.
When Rutherford saw Bohr’s Part I paper, for example, he immediately found a problem. “There appears to me one grave difficulty in your hypothesis,” he wrote Bohr on March 20, “which I have no doubt you fully realise, namely, how does an electron decide what frequency it is going to vibrate at when it passes from one stationary state to the other? It seems to me that you would have to assume that the electron knows beforehand where it is going to stop.”282 Einstein showed in 1917 that the physical answer to Rutherford’s question is statistical—any frequency is possible, and the ones that turn up happen to have the best odds. But Bohr answered the question in a later lecture in more philosophical and even anthropomorphic terms: “Every change in the state of an atom should be regarded as an individual process, incapable of more detailed description, by which the atom goes over from one so-called stationary state to another. . . . We are here so far removed from a causal description that an atom in a stationary state may in general even be said to possess a free choice between various possible transitions.”283 The “catchwords” here, as Harald Høffding might say, are individual and free choice. Bohr means the changes of state within individual atoms are not predictable; the catchwords color that physical limitation with personal emotion.
In fact the 1913 paper was deeply important emotionally to Bohr. It is a remarkable example of how science works and of the sense of personal authentication that scientific discovery can bestow. Bohr’s emotional preoccupations sensitized him to see previously unperceived regularities in the natural world. The parallels between his early psychological concerns and his interpretation of atomic processes are uncanny, so much so that without the great predictive ability of the paper its assumptions would seem totally arbitrary.
Whether or not the will is free, for example, was a question that Bohr took seriously. To identify a kind of freedom of choice within the atom itself was a triumph for his carefully assembled structure of beliefs. The separate, distinct electron orbits that Bohr called stationary states recall Kierkegaard’s stages. They also recall Bohr’s attempt to redefine the problem of free will by invoking separate, distinct Riemann surfaces. And as Kierkegaard’s stages are discontinuous, negotiable only by leaps of faith, so do Bohr’s electrons leap discontinuously from orbit to orbit. Bohr insisted as one of the two “principal assumptions” of his paper that the electron’s whereabouts between orbits cannot be calculated or even visualized.284 Before and after are completely discontinuous. In that sense, each stationary state of the electron is complete and unique, and in that wholeness is stability. By contrast, the continuous process predicted by classical mechanics, which Bohr apparently associated with the licentiate’s endless ratiocination, tears the atom apart or spirals it into radiative collapse.
Bohr may have found his way through his youthful emotional crisis in part by calling up his childhood gift of literal-mindedness. He famously insisted on anchoring physics in fact and refused to carry argument beyond physical evidence. He was never a system-builder. “Bohr characteristically avoids such a word as ‘principle,’ ” says Rosenfeld; “he prefers to speak of ‘point of view’ or, better still, ‘argument,’ i.e. line of reasoning; likewise, he rarely mentions the ‘laws of nature,’ but rather refers to ‘regularities of the phenomena.’ ”285 Bohr was not displaying false humility with his choice of terms; he was reminding himself and his colleagues that physics is not a grand philosophical system of authoritarian command but simply a way, in his favorite phrase, of “asking questions of Nature.”286 He apologized similarly for his tentative, rambling habit of speech: “I try not to speak more clearly than I think.”287
“He points out,” Rosenfeld adds, “that the idealized concepts we use in science must ultimately derive from common experiences of daily life which cannot themselves be further analysed; therefore, whenever any two such idealizations turn out to be incompatible, this can only mean that some mutual limitation is imposed upon their validity.”288 Bohr had found a solution to the spiraling flights of doubt by stepping out of what Kierkegaard called “the fairyland of the imagination” and back into the real world.289In the real world material objects endure; their atoms cannot, then, ordinarily be unstable. In the real world cause and effect sometimes seem to limit our freedom, but at other times we know we choose. In the real world it is meaningless to doubt existence; the doubt itself demonstrates the existence of the doubter. Much of the difficulty was language, that slippery medium in which Bohr saw us inextricably suspended. “It is wrong,” he told his colleagues repeatedly, “to think that the task of physics is to find out how natureis”—which is the territory classical physics had claimed for itself. “Physics concerns what we can say about nature.”290
Later Bohr would develop far more elaborately the idea of mutual limitations as a guide to greater understanding. It would supply a deep philosophical basis for his statecraft as well as for his physics. In 1913 he first demonstrated its resolving power. “It was clear,” he remembered at the end of his life, “and that was the point about the Rutherford atom, that we had something from which we could not proceed at all in any other way than by radical change. And that was the reason then that [I] took it up so seriously.”291