The Pope of Physics

by Gino Segrè

  Frisch and Meitner were familiar with Bohr’s recent conjectures about a new picture of the nucleus. Far from visualizing it as a compact solid sphere, he was describing it in August 1938 as “a drop of fluid, and the states of excitation can be compared with the oscillations in volume and shape of a sphere under the influence of its elasticity and surface tension.” The two asked themselves, What might be further consequences of viewing the uranium nucleus as a drop? Being struck by a neutron might split the drop into two smaller ones. Were that to happen, the electric force between them would rapidly drive the two drops away from each other, gaining energy as they receded.

  Aunt and nephew interrupted their walk, sitting down on the trunk of a fallen tree. They began to calculate the amount of energy the drops would accumulate in flying apart. It seemed immense, more than twenty times the energy needed to detach a neutron from a typical nucleus and more than twenty million times as much as the typical energy of a chemical bond. They conjectured this might be possible, but before believing it, they knew they had another problem to solve.

  Even though Einstein’s famous E=mc2 asserts that mass and energy are interchangeable, the overall conservation of energy is a bedrock physics principle that cannot be violated. This principle was nothing short of gospel in the bible of physics. The picture of drops separating could only be right if there were a source for the energy of motion the drops acquired. Meitner’s thirty years of experience with nuclei became relevant. She realized that the sum of the two fragments’ masses was less than the uranium mass. A rapid estimate showed her that the difference in mass was roughly equal to the energy of motion needed. Everything checked out.

  Meitner and Frisch understood that a uranium nucleus bombarded by slow neutrons can split into two smaller nuclei. Frisch would soon give the process a name suggested to him by a biology colleague: nuclear fission. Fermi, the other Boys, and several scientists after them had all been wrong about seeing transuranics. What the Boys had actually produced was far more important. They had failed, however, to recognize it.

  Frisch and Meitner went over the details of their estimates during the next week. It repeatedly seemed correct, but they thought it would be wise to consult Bohr before either writing to Hahn or submitting anything for publication. On the first of January, Frisch went back to Copenhagen; on the third he spoke to Bohr. The Dane was preparing for a trip to the United States and didn’t have much time, but not a lot was needed. As soon as Frisch began to explain what he and his aunt had concluded, Bohr slapped his forehead with his hand, exclaiming, “Oh what idiots we all have been! Oh but this is wonderful! This is just as it must be! Have you and Meitner written a paper about it?”

  The third had been a Tuesday. Over the next three days, aunt and nephew, he in Copenhagen and she back in Stockholm, conferred by telephone on the contents of the paper. Frisch wrote an outline and on the evening of the sixth of January brought it to Bohr, who approved its contents. The next morning, Frisch handed Bohr a typed version of the paper’s first two pages just as Bohr and his nineteen-year-old son were catching the train for Goteborg to board an ocean liner for the transatlantic crossing.

  When saying goodbye to Frisch, Bohr told him that he would not speak of these new results until he had heard that the Frisch-Meitner paper was in press. He wanted to make sure they received full credit. This wasn’t merely a question of protecting priority. Despite her prestige, Meitner’s position in Stockholm was precarious; she had been coolly received, and she had little funding and almost no laboratory equipment. As for Frisch, he had only a temporary appointment in Copenhagen. With Europe on the brink of war, the ever-supportive Bohr was hoping to enhance their chances for more secure positions.

  The two pages Frisch handed Bohr included a description of the experiment he intended to perform to establish that fission was taking place. It was relatively easy to conduct since he had the key components: a uranium sample, a neutron beam, and knowledge of what to look for. All he needed were two pieces of inexpensive and readily available equipment: an ionization counter and a linear amplifier.

  Frisch had confirmation of the expected result by the end of the week. The uranium nucleus could be split. Frisch proceeded to write two letters to Nature, a short one on the actual experiment and a more thorough one with Meitner on the whole idea of fission. In the longer letter to Nature, an extension of the draft given to Bohr, Frisch and his aunt stated in no uncertain terms that if fission was correct, transuranic element detection needed to be revisited—thus bringing into question a four-year history of scientific research and its conclusions.

  Frisch was in an excellent mood on the sixteenth of January as he mailed the two letters to Nature. He knew this was a significant piece of research, though he had no sense of how pivotal it would become. In addition, two days earlier he had received the news that his father, who had been imprisoned in Dachau after Kristallnacht, was going to be released. He felt enormous relief. All that remained was to sit down and write Bohr a report. That would wait a few days, because he was totally exhausted.

  It may seem implausible that fission hadn’t been detected previously since, as Frisch had just demonstrated, it was relatively simple to observe. There was, however, a justification as to why nobody had looked for fission. They were following a wise, but not always correct, scientific precept: don’t go chasing something revolutionary unless there is a good reason for doing so. In 1934 the notion of a nucleus splitting in two seemed much too far-fetched to contemplate. Four years later, thanks to Bohr’s picture of the nucleus and the Hahn-Strassmann result, it had a hint of logical pursuit.

  Not having discovered fission embarrassed Fermi, the discomfort heightened by his trip to Stockholm that coincided with Hahn’s announcement and with the Frisch-Meitner discovery. The timing was such that he even had to add an addendum to his Nobel Prize lecture, stating how the recent Hahn-Strassmann experiment made it necessary to reexamine the subject of transuranic element production.

  The matter was made even more painful by the fact that four years earlier, the Boys had performed the same experiment as Frisch recently had, with one small but crucial modification. Wanting to make sure they counted only the alpha particles produced by neutron bombardment of uranium and not those due to the natural decay of the uranium sample, they had covered the uranium with a very thin foil of aluminum. It was thick enough to absorb the sample’s natural decays but not so thick as to mask the expected signal from neutron absorption.

  Unfortunately, the foil absorbed the massive fragments produced by fission. Had the Boys left the foil off even once, they would have seen the unmistakable pulses that Frisch observed four years later. But they may not have suspected the significance of what they were seeing without Bohr’s picture of the nucleus as a drop. They might simply have decided it was an experimental error. Speculating many years later on the discovery of fission, both Amaldi and Segrè were hesitant about whether they would have recognized the meaning of the pulses. Current information, erroneous about the chemical behavior of elements near uranium on the periodic table, had helped convince the Boys they were seeing transuranics. Many physicists conjecture that if Fermi had seen a large pulse, he would have suspected something and not rested until he had found the explanation.

  The oversight continued to rankle Fermi. He always prided himself on not jumping to conclusions, on preparing for all possible eventualities, and on never leaving things to chance. But in this case he felt he had erred dramatically. Later, when a University of Chicago colleague pondered the derivation of an innocuous bas-relief figure over the entrance door to the Institute for Nuclear Studies, Fermi halfheartedly joked that the figure was “probably a scientist not discovering fission.”

  How would the world have been different had fission been discovered in early 1935? The most frightening scenario, and not an unreasonable one, is that Hitler’s Germany would have recognized fission’s potential, mobilized its scientists, and embarked on a crash program to develop
the atom bomb. And they might have been successful. If so, how would Europe have responded to such a threat? Perhaps Fermi’s not discovering fission is one of the world’s greatest gifts of good fortune.

  One cannot leave the story of fission’s discovery without asking why Lise Meitner did not share the Nobel Prize in Chemistry awarded to Otto Hahn in 1944 “for his discovery of the fission of heavy nuclei.” Looking at the history of women scientists, there is a serious concern that Meitner was not fairly recognized. Prejudice against women may have played a part, although Madame Curie had already been honored with two Nobels. Antagonism by Manne Siegbahn, an influential Swedish physicist who also directed the institute where Meitner worked, could also have been a factor. He was very conscious of his own prestige and perhaps jealous of her recognition. In addition, Sweden may have wanted to underscore its wartime neutrality. It had awarded the physics prize to an American Jew, Isidor Isaac Rabi. By awarding—in the same year—the chemistry prize to a German unassociated with fleeing the Third Reich because of racial laws, the Nobel Committee perhaps felt it achieved balance.

  After a quarter century of major contributions by Hahn, there was also a general feeling that his time had come for the award. However, the same was true of Meitner, his longtime collaborator. The most equitable resolution would probably have been the one Bohr suggested: a Nobel Prize in Chemistry to Hahn and Strassmann, and one in physics to Frisch and Meitner.

  20

  NEWS TRAVELS

  On January 2, 1939, the RMS Franconia, the ship carrying the Fermis to America, came within sight of the low coast of Long Island. Soon afterward, despite the cold and a harsh blowing wind, almost all the passengers stood on deck to admire the Statue of Liberty, their symbolic welcome to the new country. For many of them, it would become their permanent home. But for the RMS Franconia, New York was a temporary port. In eight months she would be carrying war troops and in six years she would serve as the headquarters ship for Winston Churchill and the British delegation at Yalta. For now, in 1939, she had provided safe passage to Italian refugees: a Nobel Prize winner and his family.

  Nella and Giulio, respectively seven and two, were bundled up as they were led down the gangplank by their parents and by the nursemaid the Fermis had brought with them. Laura had been resistant to leaving her “gracious way of life.” Her attachment to Italy was to a life-style that included, in part, household help. The bigger issue was that Rome had always been her home, and the thought of leaving her family and the culture of the Eternal City pained her deeply. She had strong doubts that America could live up to her standards.

  Enrico, in contrast, was grateful to be leaving. Science in Italy had been compromised by Fascism and he, more than Laura, realized how her Judaism made life precarious. As Laura confessed, “Enrico had often suggested that we leave … and each time I had raised objections.” With extensive anti-Semitic laws promulgated in Italy in the fall of 1938, even Laura had known it was time to go. The offer from Columbia was a relief. Disembarking in the New World, Fermi turned with a grin to his wife and children and announced, “We have founded the American branch of the Fermi family.”

  George Pegram, the dean of the college as well as chairman of the Columbia physics department, greeted the Fermis warmly at the pier. His recruitment skills and organizational acumen had led this astute Southern gentleman to build one of the foremost departments in the United States. He had also established a strong research program in neutron physics. The addition of Fermi was a personal triumph for him.

  Smiling as he struggled to understand Laura’s somewhat limited English, Pegram ushered the Fermis to the King’s Crown Hotel, located on 116th Street, the very heart of Columbia University. He made sure they were housed comfortably and provided information about nearby apartment rentals. Having spent the summer of 1936 teaching at Columbia, Fermi was acquainted with the neighborhood, but this was different. He had then come alone and only for a few months.

  Exactly two weeks later, on the sixteenth of January, Enrico and Laura were back at the pier, this time to greet another physicist from Europe. The Drottningholm docked at the Swedish-American Line’s Fifty-Seventh Street pier at one in the afternoon. Even before the ship reached its berth, the Fermis recognized Niels Bohr at the railing, scanning the reception crowd gathered on the dock. The fifty-four-year-old physicist’s plan was to spend four and a half months at Princeton’s Institute for Advanced Study, along with his son Erik, and then return to Copenhagen. Accompanying him on the voyage was Leon Rosenfeld, a Belgian physicist and a longtime collaborator of Bohr’s.

  It had been a rough crossing of the North Atlantic and Bohr had suffered from seasickness on the nine-day crossing. But he worked constantly. With Rosenfeld’s assistance, Bohr had gone over and over the Frisch-Meitner arguments for fission. By the time they landed in New York he was certain they were correct. However, he wanted this revolutionary discovery to remain secret, waiting until his friends’ articles on the subject were submitted. Unfortunately, he neglected to let Rosenfeld know of his promise to Frisch not to speak of their work until then.

  In addition to the Fermis, a twenty-eight-year-old Princeton physicist named John Wheeler greeted Bohr at the pier that Monday afternoon. Wheeler, who had spent a year at Bohr’s Institute in Copenhagen from 1934 to 1935, had come to New York on a morning train to greet his former mentor, expecting to return to Princeton that day with Bohr and Erik. But they decided to stay in New York for a night. Wheeler took only Rosenfeld back with him.

  That evening Wheeler and Rosenfeld went to the weekly informal physics gathering for Princeton graduate students and faculty. In the course of the meeting Wheeler asked Rosenfeld for news from Copenhagen. The Belgian innocently reported both the Frisch-Meitner conjecture and Bohr’s impression of their work.

  Arriving in Princeton a day later, Bohr was aghast to discover the physics community abuzz about the phenomenon that would soon be known as fission. He could hardly blame Rosenfeld for the leak. Still determined to protect the scoop of Frisch and Meitner, Bohr’s anxieties heightened when he found no mail. Frisch had told him that he would contact him after conducting a small experiment in Copenhagen to confirm his and Meitner’s findings. Had something gone amiss? Yet Bohr was confident enough about their discovery to pen a short letter to Nature designed to protect the precedence of the Frisch and Meitner results. On January 20, with no news from Copenhagen, Bohr wrote to Frisch inquiring about the status of things.

  In a postscript, Bohr added that the Hahn-Strassmann article in Naturwissenschaften about the appearance of barium after neutron bombardment of uranium had just reached Princeton and was stimulating a great deal of discussion. So were Frisch and Meitner’s findings on fission. On January 24, with still no word from Frisch, Bohr wrote him again. He did so with even greater urgency regarding publication of their results. An event was about to take place that would spread the news everywhere about the possibility of fission.

  The event was the brainchild of George Gamow, a tall, fun-loving, and imaginative Russian, who in 1928 had been one of the first scientists to apply quantum mechanics to a nuclear physics problem. After a series of adventures, he had arrived in the United States in 1934 and accepted a faculty position at George Washington University in the nation’s capital.

  Having spent three years at Bohr’s Copenhagen Institute, Gamow had come to appreciate the stimulation of its annual weeklong gatherings and decided he wanted to start something akin to them in the United States. Though maintaining the informality, he envisioned a different format: announce the topic ahead of time and then gather a group of thirty or so experts for three days of free-ranging exchanges. With the support of the university’s administration and the Carnegie Institution as cosponsor, the first such meeting was held in April 1935. It was an enormous success and subsequently instituted on a yearly basis.

  Gamow and his fellow refugee Edward Teller, who had joined him on the George Washington University physics faculty in the fal
l of 1935, were planning to hold the fifth such conference on January 26, 27, and 28, 1939. A topic had been chosen and an invitation list finalized. When Gamow heard that Bohr and Fermi were both on the East Coast, they were quickly added to the list. Eager to meet American physicists and other refugee scientists, many of whom were friends, the two recent arrivals accepted.

  Bohr reached Washington on the evening of the twenty-fifth and had dinner with Gamow. After dropping him at his hotel, the spirited Russian immediately called Teller. His first words to his colleague were “Bohr has gone crazy. He says the uranium nucleus splits.” Gamow and Teller decided to throw out the previous schedule: the conference’s first two talks would instead be given by Bohr and Fermi, each explaining what it means that a nucleus can split.

  That afternoon, before Bohr and Fermi went to Washington, there had been a somewhat comic wild goose chase between them. Bohr had stopped by the Columbia physics department looking for Fermi but hadn’t found him in his office. Having heard the news from Princeton about fission, Fermi was talking to his colleague John Dunning about conducting an experiment like the one that, unbeknownst to Fermi, Frisch had already performed. Meanwhile, still looking for Fermi, Bohr had gone down to the department’s basement, where the cyclotron was housed, hoping to find him at work there. The only person he met there was a graduate student, Herbert Anderson.

  Bohr was not one to make distinctions between young and old or students and senior faculty when he felt the need to talk about physics, which seemed to be most of the time, albeit with various degrees of clarity. As Anderson later remembered, Bohr walked right up to him and started out, “‘Young man, I’d like to tell you about fission.’” So I said ‘Fine.’ I was stupefied. Here was the greatest physicist in the world and he comes over to you … And Bohr, when he talks to you, he talks in a whisper. So what he does is he practically hugs you and speaks in your ear, so it’s a very close encounter. And I was overwhelmed by all that.” Having finished talking to Anderson, Bohr left to catch the train for Washington, never having connected with Fermi but happy to have had with Anderson what he thought was a meaningful exchange on the topic of fission.