Read Inside the Centre: The Life of J. Robert Oppenheimer Online
Authors: Ray Monk
Inside the Centre: The Life of J. Robert Oppenheimer (49 page)
Luis W. Alvarez, a colleague of Lawrence’s at the Radiation Laboratory,
seems to have been the first physicist at Berkeley to receive the news. He later recalled:
I remember exactly how I heard about it. I was sitting in the barber chair in Stevens Union having my hair cut, reading the
Chronicle
. I didn’t subscribe to the
Chronicle
, I just happened to be reading it, and in the second section, buried away some place, was an announcement that some German chemists had found that the uranium atom split into two pieces when it was bombarded with neutrons – that’s all there was to it. So I remember telling the barber to stop cutting my hair and I got right out of that barber chair and ran as fast as I could to the Radiation Laboratory where my student Phil Abelson, who is now editor of
Science
, had been working very hard to try and find out what transuranium elements were produced when neutrons hit uranium; he was so close to discovering fission that it was almost pitiful. He would have been there, guaranteed, in another few weeks.
When Alvarez arrived at the laboratory, panting, with his news about fission, Abelson was there, making observations on what he thought were traces of transuranic elements. Alvarez recalls:
I played it kind of dramatically when I saw Phil. I said: ‘Phil, I’ve got something to tell you but want you to lie down first.’ So being a good graduate student he lay down on the table right alongside the control room of the cyclotron. ‘Phil, what you are looking at are not transuranium elements, they are elements in the middle of the periodic table.’ . . . I showed him what was in the
Chronicle
, and of course he was terribly depressed.
Like many other American experimental physicists, Alvarez, on hearing about fission, immediately set up an experiment to confirm it. Only after he had the experiment up and running did Oppenheimer hear the news. His first reaction was ‘That’s impossible’ and, according to Alvarez, he ‘gave a lot of theoretical reasons why fission couldn’t really happen’.
When I invited him over to look at the oscilloscope later, when we saw the big pulses, I would say that in less than fifteen minutes Robert had decided that this was indeed a real effect and, more importantly, he had decided that some neutrons would probably boil off in the reaction, and that you could make bombs and generate power, all inside of a few minutes. He just had a block on the thing because he was so sure that Coulomb barriers wouldn’t permit the nucleus to undergo fission. But
it was amazing to see how rapidly his mind worked, and he came to the right conclusions.
That day, Oppenheimer called Felix Bloch at Stanford. ‘You must come to Berkeley immediately,’ he told him. ‘There is something of the utmost importance I must show you.’ ‘There was a note of urgency in his voice,’ Bloch later said, ‘one I don’t recall ever hearing in Oppenheimer before.’ As soon as Bloch arrived in Berkeley, Oppenheimer’s first words to him were: ‘They have discovered fission.’ Glenn T. Seaborg, a chemist at Berkeley, remembers that, very soon after the news hit the West Coast, a seminar was held to discuss uranium fission. ‘I do not recall ever seeing Oppie so stimulated and so full of ideas.’
Oppenheimer’s almost feverish excitement is clear from the letter he wrote to Willie Fowler, a day or two after hearing about fission, both in its content and in its breathless style. It reads as if it were written in an enormous hurry:
The U business is unbelievable. We first saw it in the papers, wired for more dope and have had a lot of reports since. You know it started with Hahn’s finding that what he had taken for Ra in one of the U activities fractionally crystallized with Ba. And then the recognition that the ekauranium [transuranic] series was chemically compatible with a series starting with Ma, running on through Rhe and Os and Pd. And then understanding suddenly why there were such long chains of beta decay, to get rid of the neutron excess with which half a U nucleus would start . . . Many points are still unclear: where are the short lived high energy betas one would expect? Are there strong gammas as one would think from the big dipole moments of the pieces? In how many ways does the U come apart? At random, as one might guess, or only in certain ways? And most of all, are there many neutrons that come off during the splitting, or from the excited pieces? If there are then a 10 cm cube of U would be quite something.
‘What do you think?’ he asked Fowler. ‘It is, I think, exciting, not in the rare way of positrons and mesotrons, but in a good honest practical way.’
As it turned out, Fowler was not very interested. When he was later asked about when and how he had first heard of fission, he replied airily: ‘I remember very vaguely about fission. I guess we got the word from Oppenheimer. I would be hard put to say for sure.’
At that time, we weren’t doing very much with neutrons and didn’t have any strong neutron sources, so I frankly can’t remember wanting
to do anything in that area, and I don’t believe Charlie [Lauritsen] did. We were so busy with the things we were doing ourselves at that time that we did not respond to the fission discovery in the way that many other labs did. There was always the joke, ‘Well, that’s heavy element physics, we’re in the business of bombarding light elements, nothing heavier than neon around here.’ So I never did any experiments in fission and I’m pretty sure that Charlie didn’t.
Oppenheimer’s excitement about the discovery seems, from the very beginning, to have had its roots not in pure science, but in the possibility that it would lead to extremely powerful explosives. That, presumably, is what he meant when he said to Fowler that it was interesting ‘in a good honest practical way’. It is certainly what he meant when he mentioned how ‘interesting’ a 10-cm cube of uranium would be, as is made clear in a letter he wrote to Uhlenbeck on 5 February. If it turned out, Oppenheimer said, that a significant number of neutrons were released with the fission reaction, then a chain reaction could occur, in which case: ‘I think it really not too improbable that a ten cm cube of uranium deuteride (one should have something to slow the neutrons without capturing them) might very well blow itself to hell.’
Robert Serber had in the summer of 1938 left Berkeley to take up an assistant professorship at the University of Illinois in Urbana, but, Serber says in his autobiography, ‘Oppie would write me every Sunday.’
From one of those Sunday letters, which I received in January 1939, I learned of the discovery of fission. In that first letter Oppie mentioned the possibility of nuclear power and of an explosive. My immediate reaction, and I’m sure that of most other nuclear theorists, was that I should have thought of fission myself.
Oppenheimer no doubt missed Serber badly during those frenetic opening months of 1939. There is nobody with whom he would rather have discussed the implications of fission than Serber, nobody whose help he would rather have had in thinking through all the questions that he had listed in his letter to Willie Fowler. However, with Serber miles away in the Midwest, and Fowler (like, it seems, the rest of Caltech) uninterested, Oppenheimer turned to his students.
As students completed their PhD theses and moved on – the lucky ones to academic appointments – the group of graduates that surrounded Oppenheimer and followed him between Berkeley and Pasadena was continually changing, their adoration and unconscious imitation of him the only thing that stayed constant. In the summer of 1938, Willis Lamb graduated and moved to New York, where, as mentioned earlier, he worked
at Columbia University. George Volkoff and Hartland Snyder, however, were still at Berkeley, and so were two other students, Philip Morrison and Sidney Dancoff. Following in Volkoff’s footsteps, two other University of British Columbia graduates had joined Oppenheimer: Robert Christy in 1936 and the Japanese-born Shuichi Kusaka in 1937. Two new students in the academic year 1938–9 were Bernard Peters and Joseph Weinberg.
In Berkeley, even among other students, Oppenheimer’s graduates were considered a bohemian crowd. Morrison has been described as ‘a scrappy little man on fire with his science’, who ‘hitch-hiked from Pittsburgh and lunched on cat meat to stay near Oppenheimer’. Joe Weinberg, meanwhile, ‘had originally started from the lower east side of New York and eventually found his way to the mecca with the clothes he wore and a spare pair of shoes in a paper sack’. Raymond Birge became concerned about the class of people Oppenheimer was attracting. ‘New York Jews flocked out here to him and some were not as nice as he was,’ Birge said. ‘Lawrence and I were very concerned to have people here who were nice people as well as good students.’
Indeed, it had been Birge’s concern to have ‘nice people’ at Berkeley that had prevented Oppenheimer from securing a job for Robert Serber there. When he urged Birge to appoint Serber, Birge is reputed to have said (not to Oppenheimer, but in a letter to someone else): ‘One Jew in the department is enough.’ Birge and Lawrence laid down two rules governing appointments: 1. no one with a PhD from Berkeley; and 2. no bohemians. The first ruled out Oppenheimer’s graduate students, the second his NRC fellowship students like Serber.
To Lawrence’s great displeasure, almost all of Oppenheimer’s students were left-wing and many of them already were (or later became) members of the Communist Party. Of them all, Bernard Peters had the most colourful past. He was a German Jew who had escaped Dachau and arrived in the US with his wife, Hannah, in 1934. They settled in New York, where, while Hannah trained as a doctor, Peters worked in an import business. In 1937, when Hannah finished her medical degree, they bought a car and drove out west, where Hannah became a research fellow at Stanford and Peters worked as a longshoreman. Through Jean Tatlock they met Oppenheimer, who encouraged Peters to come to Berkeley as a graduate student in physics.
For some reason, Joe Weinberg did not start in September 1938, but arrived rather in February 1939. He had, it seems, been sent there mid-term by his physics professor at Wisconsin, Gregory Breit, who told him that Berkeley was one of the few places in the world where ‘a person as crazy as you could be acceptable’. As soon as he arrived, Weinberg went to Oppenheimer’s room, to find a meeting in full flow. After being introduced to, among others, Lawrence, Snyder, Morrison and Dancoff, he
joined Morrison and Dancoff for lunch at the student-union restaurant. The conversation was dominated by fission and a telegram that had recently arrived from Bohr. ‘On the basis of the data,’ Weinberg remembered, ‘we designed a bomb.’ Morrison, however, was convinced that it would not work, that the chain reaction would peter out before leading to an explosion. Nevertheless, Morrison recalls that, within a week of them all learning about fission, ‘there was on the blackboard in Robert Oppenheimer’s office a drawing – a very bad, an execrable drawing – of a bomb’.
Remarkably, despite all this excitement, there is no evidence that, in the months that followed, Oppenheimer did any serious scientific work on the theory of nuclear fission; no evidence, for example, of any sustained attempt to answer the questions that he told Fowler urgently needed addressing. There is instead a rather conspicuous silence on the subject. Although in February 1939 his students were apparently designing bombs in the student-union canteen and he was leaving drawings of explosives on his blackboard for all to see, after that, until the autumn of 1941 when he was invited to contribute to the US bomb project, one searches in vain for the word ‘bomb’, or indeed the word ‘fission’, in his letters and in the recollections of conversations with friends. The only recorded exception to this silence that I know of is Fowler’s recollection that at Caltech ‘Oppie gave some lectures on what was essentially the Bohr–Wheeler theory of fission’.
Assuming this was in the summer of 1939, then Oppenheimer, characteristically, was lecturing on a brand-new theory, one that had yet to appear in print. It was, moreover, one that was directly applicable to the questions Oppenheimer had raised upon hearing the news of fission, and therefore to the question of whether fission could lead to the construction of an atomic bomb. The theory was worked out by Bohr and Wheeler in Princeton in the spring of 1939, written out by Wheeler in June and appeared in the
Physical Review
on 1 September, in the same issue that contained Oppenheimer and Snyder’s seminal article on black holes.
The origins of the Bohr–Wheeler theory lie in conversations the two had in Princeton in the first week of February 1939, after they returned from the Washington conference. With the entire community of American physicists still buzzing with the news of fission, Bohr asked Wheeler if he would like to work on a more detailed theory of the phenomenon. ‘It was an exciting time,’ says Wheeler in his autobiography, though he emphasises that, for them at least, the excitement was to do with pure science, not explosives: ‘Bombs and reactors were only in the backs of our minds as we worked together. We were trying to understand a new nuclear phenomenon, not design anything.’
To inform their theoretical deliberations, Bohr and Wheeler asked the
experimental physicists at Princeton to conduct some experiments to determine how the probability of fission in uranium varies with the energy of the incoming neutrons. The results were puzzling: the probability is high for high-energy neutrons and diminishes as the energy diminishes, until, at very low energies, it becomes high again. Why should this be? Taking a walk to ponder this question, Bohr, accompanied by Rosenfeld, went from the faculty club to Einstein’s office (which he was borrowing at the time), where he rushed to the blackboard, saying: ‘Now listen: I have it all.’
What Bohr had realised was that the high probability of fission at low energies was due to a rare isotope of uranium, U-235, present only in 0.7 per cent of natural uranium. The more common form of uranium, U-238, being more stable, requires higher-energy neutrons to split it. Low-energy neutrons stand a higher chance of hitting the nucleus, because their wavelengths are longer, but upon impact they will only split the more unstable U-235 nuclei. So, high-energy neutrons stand a good chance of splitting any uranium nuclei they happen to hit, including those of U-238, while low-energy neutrons stand a good chance of hitting all nuclei, and of splitting those of U-235, but the neutrons in between are not travelling fast enough to split U-238, or slow enough to stand a good chance of hitting anything.
