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 ping-pong, and I don't remember ever beating Bohr at that game.” 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.” 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 Oresund, 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.” But the Oresund 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
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.”
What Bohr understood about the nucleus he embodied in a landmark lecture to the Danish Academy on January 27, 1936, subsequently published in
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 present 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.
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. 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.[2]
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 nu-cleons 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 Hquid 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. 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. 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.
