Making of the Atomic Bomb

by Richard Rhodes

  I daresay you remember my ringing you up about a man working here who had a patent which he thought ought to be kept secret. I enclose a letter from him on the subject as you suggested. I am naturally somewhat less optimistic about the prospects than the inventor, but he is a very good physicist and even if the chances were a hundred to one against it seems to me it might be worth keeping the thing secret as it is not going to cost the Government anything.846

  The patent, Szilard explained in the letter Lindemann enclosed, “contains information which could be used in the construction of explosive bodies . . . very many thousand times more powerful than ordinary bombs.”847 He was concerned about “the disasters which could be caused by their use on the part of certain Powers which might attack this country.” Wisely and withal inexpensively the Admiralty accepted the patent into its safekeeping.

  * * *

  Eight months in Copenhagen had suited Edward Teller. He met George Gamow on the Odessan’s last visit there, after the Solvay Conference of the previous autumn; the two of them roared across Denmark and back during Easter vacation on Gamow’s motorcycle, working over a problem in quantum mechanics. The Rockefeller Foundation did not approve of marriage during a fellowship period, but James Franck had interceded on his behalf and Teller had married his childhood sweetheart, Mici Harkanyi, in Budapest on February 26. He had also written an important paper. He returned to London with Mici in the summer of 1934 with his reputation enhanced and again took up his lectureship at University College. Assuming they would settle in England, the Tellers signed a nine-year lease just before Christmas on a pleasant three-room flat.

  Two offers arrived in January, one of which changed Teller’s mind. The first was from Princeton: a lectureship. The second was from Gamow: a full professorship at George Washington University. GWU wanted to strengthen its physics department; Gamow wanted company and liked Teller’s verve.

  Teller was twenty-six years old and a newlywed. He was less than sure about living in the United States, but a full professorship was not something he could sensibly refuse. His wife found someone to sublet the flat. The U.S. State Department refused nonquota immigration visas because Teller had only taught for one year—the Copenhagen time counted merely as a fellowship—and was required to have taught for two. He had not, however, tried for visas on the Hungarian immigration quota because he assumed the quota was full. In fact there was room. The Tellers followed the Gamows across the Atlantic in August 1935.

  * * *

  Niels Bohr celebrated his fiftieth birthday on October 7. “Bohr in those days seemed at the height of his powers, bodily and mentally,” Otto Frisch observes. “When he thundered up the steep staircase [of the institute], two steps at a time, there were few of us younger ones that could keep pace with him. The peace of the library was often broken by a brisk game of pingpong, and I don’t remember ever beating Bohr at that game.”848 To honor Denmark’s leading physicist, George de Hevesy organized a fund-raising campaign; the Danish people contributed 100,000 kroner to buy Bohr 0.6 gram of radium for his birthday. De Hevesy divided the radium, in liquid solution, into six equal parts, mixed each with beryllium powder and allowed them to dry, making six potent neutron sources. He had them mounted on the ends of long rods and stored them in a dry well in the basement of the institute that had been dug originally to supply vibration-free housing for a spectrograph.

  The institute’s annual Christmas party continued to be held in the well room, Stefan Rozental recalls: “The lid of the well served as a table, a Christmas tree stood in the middle, and all the personnel were gathered, from the chief down to the youngest apprentice in the workshop, and served a modest meal of sausages and beer. During the party Niels Bohr used to make a speech in which he gave a sort of survey of the past year.”849 Safely below the sausages, stuck in a gallon flask of carbon disulphide, the neutron sources silently transmuted sulfur to radioactive phosphorus for de Hevesy’s biological radioisotope studies.

  Bohr had won national distinction for his work and the enduring gratitude of refugees for his aid; he had also faced personal pain. In 1932 the Danish Academy offered him lifetime free occupancy of the Danish House of Honor, a palatial estate in Pompeiian style built originally for the founder of Carlsberg Breweries and subsequently reserved for Denmark’s most distinguished citizen (Knud Rasmussen, the polar explorer, was its previous occupant). By then the institute buildings included a modest director’s house, but the Bohrs shared it with five handsome sons. They moved to the mansion beside the brewery, the best address in Denmark after the King’s.

  Two years later an accident took the Bohrs’ eldest son, Christian, nineteen years old. Father, son and two friends were sailing on the Öresund, the sea passage between Denmark and Sweden, when a squall blew up. Christian “was drowned by falling over[board] in a very rough sea from a sloop,” Robert Oppenheimer reports, “and Bohr circled as long as there was light, looking for him.”850 But the Öresund is cold. For a time Bohr retreated into grief. Exhausting as it was, the refugee turmoil helped him.

  Everyone at the institute followed Fermi’s neutron work with fascination. Frisch, the only physicist on hand who knew Italian, was drafted to translate the successive papers aloud as soon as each issue of the Ricerca Scientifica arrived. The Copenhagen group was puzzled that slow neutrons affected some elements more intensely than others; on the one-particle model of the nucleus even a slow neutron should almost always shoot completely through a nucleus without capture.

  From Cornell Hans Bethe published a paper calculating the slim odds of neutron capture. They conflicted squarely with observation. Frisch remembers the colloquium in Copenhagen in 1935 when someone reported on Bethe’s paper:

  On that occasion Bohr kept interrupting, and I was beginning to wonder, with some irritation, why he didn’t let the speaker finish. Then, in the middle of a sentence, Bohr suddenly stopped and sat down, his face completely dead. We looked at him for several seconds, getting anxious. Had he been taken unwell? But then he suddenly got up and said with an apologetic smile, “Now I understand it.”851

  What Bohr understood about the nucleus he embodied in a landmark lecture to the Danish Academy on January 27, 1936, subsequently published in Nature. “Neutron capture and nuclear constitution” exploited the phenomenon of neutron capture to propose a new model of the nucleus; once again, as he had with Rutherford’s planetary model of the atom, Bohr stood on the solid ground of experiment to argue for radical theoretical change.852

  He visualized a nucleus made up of neutrons and protons closely packed together—a model now familiar—rather than a single particle. (Nuclear particles collectively are known as nucleons.) A neutron entering such a crowded nucleus would not pass through; it would collide with the nearest nucleons, surrender its kinetic energy (as a cue ball does at break in billiards) and be captured by the strong force that holds the nucleus together. The energy added by the neutron would agitate the nearby nucleons; they would collide in turn with other nucleons beyond; the net effect would be a more generally agitated, “hotter” nucleus but one where no single component could quickly acquire enough energy to push through the electrical barrier and escape. If the nucleus then radiated its excess energy by ejecting a gamma photon, “cooling off,” none of its nucleons could accrue enough energy to escape. The result, already confirmed by Fermi’s experiments, would be the creation of a heavier isotope of the original element being bombarded.

  More violent assaults on the nucleus, Bohr thought, would still disperse their energies throughout the compound nucleus created by their capture. Subsequent reconcentration of the energy might allow the nucleus to eject several charged or uncharged particles. Bohr did not think his compound model of the nucleus boded well for harnessing nuclear energy:

  For still more violent impacts, with particles of energies of about a thousand million volts, we must even be prepared for the collision to lead to an explosion of the whole nucleus. Not only are such energies, of course, at pr
esent far beyond the reach of experiments, but it does not need to be stressed that such effects would scarcely bring us any nearer to the solution of the much discussed problem of releasing the nuclear energy for practical purposes. Indeed, the more our knowledge of nuclear reactions advances the remoter this goal seems to become.853

  Thus by the mid-1930s the three most original living physicists had each spoken to the question of harnessing nuclear energy. Rutherford had dismissed it as moonshine; Einstein had compared it to shooting in the dark at scarce birds; Bohr thought it remote in direct proportion to understanding. If they seem less perceptive in their skepticism than Szilard, they also had a better grasp of the odds. The essential future is always unforeseen. They were experienced enough not to long for it.

  In his lecture Bohr preferred to state only general principles, but to trace “the consequences of the general argument here developed” he had a specific mathematical model in mind.854 He published a discussion of that model the following year, in 1937. It reached all the way back to his doctoral dissertation on the surface tension of fluids to demonstrate the usefulness of treating the atomic nucleus as if it were a liquid drop.1

  The tendency of molecules to stick together gives liquids a “skin” of surface tension. A falling raindrop thus rounds itself into a small perfect sphere. But any force acting on a liquid drop deforms it (think of the wobbles of a water-filled balloon thrown into the air and caught). Surface tension and deforming forces work against each other in complex ways; the molecules of the liquid bump and collide; the drop wobbles and distorts. Eventually the added energy dissipates as heat, and the drop steadies again.

  The nucleus, Bohr proposed, was similar. The force that stuck the nucleons together was the nuclear strong force. Counteracting that strong force was the common electrical repulsion of the positively charged nuclear protons. The delicate balance between the two fundamental forces made the nucleus liquidlike. Energy added from the outside by particle bombardment deformed it; it wobbled like a liquid drop, oscillating complexly just as the braided streams of water Bohr had studied for his dissertation had oscillated. Which meant he could use Rayleigh’s classical formulae for the surface tension of liquids to understand the complex nuclear energy levels and exchanges that Fermi’s work had revealed. “This 1937 paper had to close with many issues not cleared up,” writes the American theoretical physicist John Archibald Wheeler, who helped Bohr clear up more of them later.855, 856 The liquid-drop model proved useful, however, and Frisch in Copenhagen and Meitner in Berlin, among others, took it to heart.

  * * *

  One fine October Thursday in 1937 Ernest Rutherford, a vigorous sixty-six, went out into the garden of his house on the green Cambridge Backs to trim a tree. He took a bad fall. He was “seedy” later in the day, Mary Rutherford said—nausea and indigestion—and she arranged for a masseur.857 Rutherford vomited that night. In the morning he called his family doctor. He suffered from a slight umbilical hernia, which he confined with a truss; his doctor found a possible strangulation, consulted with a specialist and directed the Rutherfords to the Evelyn Nursing Home for emergency surgery. Rutherford told his wife along the way that his business and financial affairs were all in order. She said his illness wasn’t serious and asked him not to worry.

  Surgery that evening confirmed a partial strangulation, released the imprisoned portion of the small intestine and restored its circulation. Saturday Rutherford seemed to be recovering but he began vomiting again on Sunday and there were signs of infection, deadly in those days before antibiotics. Monday he was worse; his doctors consulted the surgeon, a Melbourne man, who advised against a second operation given the patient’s age and symptoms. Rutherford was made comfortable with intravenous saline, six pints by Tuesday, and a stomach tube. Tuesday morning, October 19, he was slightly improved, but though his wife judged him “a wonderful patient [who] bears his discomforts splendidly” and believed she discovered “just a thread of hope,” he began that afternoon to weaken.858 A bequest he decided late in the day suggests he found gratitude in those last hours reviewing his life. “I want to leave a hundred pounds to Nelson College,” he told Mary Rutherford.859 “You can see to it.” And again loudly a little later: “Remember, a hundred to Nelson College.” He died that evening. “Heart and circulation failed” because of massive infection, his doctor wrote, “and the end came peacefully.”

  An international gathering of physicists in Bologna that week celebrated the 200th anniversary of the birth of Luigi Galvani; Cambridge cabled the news of Rutherford’s death on the morning of October 20. Bohr was on hand and accepted the grim duty of announcement. “When the meeting scheduled for that morning assembled,” writes Mark Oliphant, “Bohr went to the front, and with faltering voice and tears in his eyes informed the gathering of what had happened.”860 They were shocked at the abruptness of the loss. Bohr had visited Rutherford at Cambridge a few weeks earlier; the Cavendish men had seen their leader in fine fettle only days ago.

  Bohr “spoke from the heart,” says Oliphant, recalling “the debt which science owed so great a man whom he was privileged to call both his master and his friend.” For Oliphant it was “one of the most moving experiences of my life.” Remembering Rutherford in a letter to Oppenheimer on December 20 Bohr balanced loss with hope, complementarily: “Life is poorer without him; but still every thought about him will be a lasting encouragement.”861 And in 1958, in a memorial lecture, Bohr said simply that “to me he had almost been as a second father.”862

  The sub-dean of Westminster immediately approved interment of Rutherford’s ashes in the nave of Westminster Abbey, just west of Newton’s tomb and in line with Kelvin’s. Eulogizing Rutherford at a conference in Calcutta the following January, James Jeans identified his place in the history of science:

  Voltaire said once that Newton was more fortunate than any other scientist could ever be, since it could fall to only one man to discover the laws which governed the universe. Had he lived in a later age, he might have said something similar of Rutherford and the realm of the infinitely small; for Rutherford was the Newton of atomic physics.863

  Ernest Rutherford unknowingly wrote his own more characteristic epitaph in a letter to A. S. Eve from his country cottage on the first day of that last October. He reported of his garden what he had also done for physics, vigorous and generous work: “I have made a still further clearance of the blackberry patch and the view is now quite attractive.”864

  * * *

  In September 1934, in the wake of Fermi’s June Nature article “Possible production of elements of atomic number higher than 92,” a curious paper appeared in a publication seldom read by physicists, the Zeitschrift für Angewandte Chemie—the Journal of Applied Chemistry. Its author was a respected German chemist, Ida Noddack, co-discoverer with her husband (in 1925) of the hard, platinum-white metallic element rhenium, atomic number 75. The paper was titled simply “On element 93” and it severely criticized Fermi’s work.865 His “method of proof” was “not valid,” Noddack wrote bluntly. He had demonstrated that “his new beta emitter” was not protactinium and then distinguished it from several other elements descending down the periodic table to lead, but it was “not clear why he chose to stop at lead.” The old view that the radioactive elements form a continuous series beginning at uranium and ending at lead, wrote Noddack, was exactly what the Joliot-Curies’ discovery of artificial radioactivity had disproved. “Fermi therefore ought to have compared his new radioelement with all known elements.”

  The fact was, Noddack went on, any number of elements could be precipitated out of uranium nitrate with manganese. Instead of assuming the production of a new transuranic element, “one could assume equally well that when neutrons are used to produce nuclear disintegrations, some distinctly new nuclear reactions take place which have not been observed previously.” In the past, elements have transmuted only into their near neighbors. But “when heavy nuclei are bombarded by neutrons, it is conceivable tha
t the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors.” They would be, rather, much lighter elements farther down the periodic table than lead.

  Segrè remembers reading the Noddack paper.866 He knows, because he asked them, that Hahn in Berlin and Joliot in Paris read it. It made very little sense to anyone. “I think whatever chemists read it,” Frisch reminisces, “probably thought that this was quite pointless, carping criticism, and the physicists possibly even more so if they read it, because they would say, ‘What’s the use of criticizing unless you give some reason why that criticism would be valid?’ Nobody had ever found a nuclear disintegration creating far-removed elements.”867 Which was a point Noddack had carefully addressed, but was clearly one reason for the paper’s neglect. The summary report for Nature on artificial radioactivity that Amaldi and Segrè had delivered to Rutherford in midsummer 1934 makes the assumption explicit: “It is reasonable to assume that the atomic number of the active element should be close to the atomic number . . . of the bombarded element.”868

  But Fermi seldom left anything to assumption, however reasonable. He would certainly not have left to assumption this issue, about which he was already acutely sensitive because of Corbino’s ill-timed speech (Noddack rubbed salt into that wound by referring to “the reports found in the newspapers”). He sat down and performed the necessary calculations. He later told at least Teller, Segrè and his American protégé Leona Woods that he had done so.869 Teller is quite sure he knows what those calculations were:

  Fermi refused to believe [Noddack]. . . .870 He knew how to calculate whether or not uranium could break in two. . . . He performed the calculation Mrs. Noddack suggested, and found that the probability was extraordinarily low. He concluded that Mrs. Noddack’s suggestion could not possibly be correct. So he forgot about it. His theory was right . . . but . . . it was based on the . . . wrong experimental information.