The Making of the Atomic Bomb

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. 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.”

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.”

“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.”

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.” 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 Gottingen 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.” 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.”

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.”

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.” 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.” 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 N0rland, 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,

Newnham, Cambridge

Oh Harald!

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.” 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