The Making of the Atomic Bomb

older European scientists still thought his inconsistent hypotheses ad hoc and the idea of a quantized atom repugnant. The war itself stalled advance.

Yet he persisted, groping his way forward in the darkness. “Only a rare and uncanny intuition,” writes the Italian physicist Emilio Segre, “saved Bohr from getting lost in the maze.” He guided himself delicately by what he called the correspondence principle. As Robert Oppenheimer once explained it, “Bohr remembered that physics was physics and that Newton described a great part of it and Maxwell a great part of it.” So Bohr assumed that his quantum rules must approximate, “in situations where the actions involved were large compared to the quantum, to the classical rules of Newton and of Maxwell.” That correspondence between the reliable old and the unfamiliar new gave him an outer limit, a wall to feel his way along.

Bohr built his Institute for Theoretical Physics with support from the University of Copenhagen and from Danish private industry, occupying it on January 18, 1921, after more than a year of delay — he struggled with the architect's plans as painfully as he struggled with his scientific papers. The city of Copenhagen ceded land for the institute on the edge of the Faelled-park, broad with soccer fields, where a carnival annually marks the Danish celebration of Constitution Day. The building itself was modest gray stucco with a red tile roof, no larger than many private homes, with four floors inside that looked like only three outside because the lowest floor was built partly below grade and the top floor, which served the Bohrs at first as an i apartment, extended into the space under the peaked roof (later, as Bohr's family increased to five sons, he built a house next door and the apartment served as living quarters for visiting students and colleagues). The institute included a lecture hall, a library, laboratories, offices and a popular Ping-Pong table where Bohr often played. “His reactions were very fast and accurate,” says Otto Frisch, “and he had tremendous will power and stamina. In a way those qualities characterized his scientific work as well.”

In 1922, the year his Nobel Prize made him a Danish national hero, Bohr accomphshed a second great theoretical triumph: an explanation of the atomic structure that underlies the regularities of the periodic table of the elements. It linked chemistry irrevocably to physics and is now stan-i dard in every basic chemistry text. Around the nucleus, Bohr proposed, atoms are built up of successive orbital shells of electrons — imagine a set of nested spheres — each shell capable of accommodating up to a certain number of electrons and no more. Elements that are similar chemically are similar because they have identical numbers of electrons in their outermost shells, available there for chemical combination. Barium, for example, an alkaline earth, the fifty-sixth element in the periodic table, atomic weight 137.34, has electron shells filled successively by 2, 8, 18, 18, 8 and 2 electrons. Radium, another alkaline earth, the eighty-eighth element, atomic weight 226, has electron shells filled successively by 2, 8, 18, 32, 18, 8 and 2 electrons. Because the outer shell of each element has two valence electrons, barium and radium are chemically similar despite their considerable difference in atomic weight and number. “That [the] insecure and contradictory foundation [of Bohr's quantum hypotheses],” Einstein would say, “was sufficient to enable a man of Bohr's unique instinct and perceptive-ness to discover the major laws of spectral lines and of the electron shells of the atom as well as their significance for chemistry appeared to me like a miracle… This is the highest form of musicality in the sphere of thought.”

Confirming the miracle, Bohr predicted in the autumn of 1922 that element 72 when discovered would not be a rare earth, as chemists expected and as elements 57 through 71 are, but would rather be a valence 4 metal like zirconium. George de Hevesy, now settled in at Bohr's institute, and a newly arrived young Dutchman, Dirk Coster, went to work using X-ray spectroscopy to look for the element in zircon-bearing minerals. They had not finished their checking when Bohr went off with Margrethe in early December to claim his Nobel Prize. They called him in Stockholm the night before his Nobel lecture, only just in time: they had definitely identified element 72 and it was chemically almost identical to zirconium. They named the new element hafnium after Hafnia, the old Roman name for Copenhagen. Bohr announced its discovery with pride at the conclusion of his lecture the next day.

Despite his success with it, quantum theory needed a more solid foundation than Bohr's intuition. Arnold Sommerfeld in Munich was an early contributor to that work; after the war the brightest young men, searching out the growing point of physics, signed on to help. Bohr remembered the period as “a unique cooperation of a whole generation of theoretical physicists from many countries,” an “unforgettable experience.” He was lonesome no more.

Sommerfeld brought with him to Gottingen in the early summer of 1922 his most promising student, a twenty-year-old Bavarian named Werner Heisenberg, to hear Bohr as visiting lecturer there. “I shall never forget the first lecture,” Heisenberg wrote fifty years later, the memory still textured with fine detail. “The hall was filled to capacity. The great Danish physicist… stood on the platform, his head slightly inclined, and a friendly but somewhat embarrassed smile on his lips. Summer light flooded in through the wide-open windows. Bohr spoke fairly softly, with a slight Danish accent… Each one of his carefully formulated sentences revealed a long chain of underlying thoughts, of philosophical reflections, hinted at but never fully expressed. I found this approach highly exciting.”

Heisenberg nevertheless raised pointed objection to one of Bohr's statements. Bohr had learned to be alert for bright students who were not afraid to argue. “At the end of the discussion he came over to me and asked me to join him that afternoon on a walk over the Hain Mountain,” Heisenberg remembers. “My real scientific career only began that afternoon.” It is the memory of a conversion. Bohr proposed that Heisenberg find his way to Copenhagen eventually so that they could work together. “Suddenly, the future looked full of hope.” At dinner the next evening Bohr was startled to be challenged by two young men in the uniforms of the Gottin-gen police. One of them clapped him on the shoulder: “You are arrested on the charge of kidnapping small children!” They were students, genial frauds. The small child they guarded was Heisenberg, boyish with freckles and a stiff brush of red hair.

Heisenberg was athletic, vigorous, eager — “radiant,” a close friend says. “He looked even greener in those days than he really was, for, being a member of the Youth Movement… he often wore, even after reaching man's estate, an open shirt and walking shorts.” In the Youth Movement young Germans on hiking tours built campfires, sang folk songs, talked of knighthood and the Holy Grail and of service to the Fatherland. Many were idealists, but authoritarianism and anti-Semitism already bloomed poisonously among them. When Heisenberg finally got to Copenhagen at Eastertime in 1924 Bohr took him off on a hike through north Zealand and asked him about it all. “'But now and then our papers also tell us about more ominous, anti-Semitic, trends in Germany, obviously fostered by demagogues,’” Heisenberg remembers Bohr questioning. “'Have you come across any of that yourself?’” That was the work of some of the old officers embittered by the war, Heisenberg said, “but we don't take these groups very seriously.”

Now, as part of the “unique cooperation” Bohr would speak of, they went freshly to work on quantum theory. Heisenberg seems to have begun with a distaste for visualizing unmeasurable events. As an undergraduate, for example, he had been shocked to read in Plato's Timaeus that atoms had geometric forms: “It saddened me to find a philosopher of Plato's critical acumen succumbing to such fancies.” The orbits of Bohr's electrons were similarly fanciful, Heisenberg thought, and Max Born and Wolfgang Pauli, his colleagues at Gottingen, concurred. No one could see inside an atom. What was known and measurable was the light that came out of the atomic interior, the frequencies and amplitudes associated with spectral lines. Heisenberg decided to reject models entirely and look for regularities among the numbers alone.

He returned to Gottingen as a Privatdozent working under Born. Toward the end of May 1925 his hay fever flared; he asked Born for two weeks' leave of absence and made his way to Heligoland, a stormy sliver of island twenty-eight miles off the German coast in the North Sea, where very little pollen blew. He walked; he swam long distances in the cold sea; “a few days were enough to jettison all the mathematical ballast that invariably encumbers the beginning of such attempts, and to arrive at a simple formulation of my problem.” A few days more and he glimpsed the system he needed. It required a strange algebra that he cobbled together as he went along where numbers multiplied in one direction often produced different products from the same numbers multiplied in the opposite direction. He worried that his system might violate the basic physical law of the conservation of energy and he worked until three o'clock in the morning checking his figures, nervously making mistakes. By then he saw that he had “mathematical consistency and coherence.” And so often with deep physical discovery, the experience was elating but also psychologically disturbing:

At first, I was deeply alarmed. I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior, and felt almost giddy at the thought that I now had to probe this wealth of mathematical structures nature had so generously spread out before me. I was far too excited to sleep, and so, as a new day dawned, I made for the southern tip of the island, where I had been longing to climb a rock jutting out