Inside the Centre: The Life of J. Robert Oppenheimer (50 page)

On 7 February, Bohr sent the
Physical Review
an initial paper that did not go into all the theoretical details that he and Wheeler had discussed, but did announce the important conclusion they had reached: that fission by slow neutrons was possible only in less than 1 per cent of naturally occurring uranium. Oppenheimer must have read this, and must have made use of it in his lectures on fission at Caltech, but there is no mention of it in his correspondence.

The next important question to answer was: are neutrons – so-called ‘secondary’ neutrons – emitted during the fission process (‘If there are then a 10 cm cube of U would be quite something’)? The answer to this was provided by Leo Szilard early in March 1939, after some experiments he conducted at Columbia seemed to establish that ‘the number of neutrons emitted per fission [is] about two’ – a result quickly confirmed by experiments in other laboratories, including Fermi’s. This meant that the chain reaction imagined by Szilard when he pondered Rutherford’s lecture on the splitting of the atom some years earlier was indeed a possibility. ‘That night,’ Szilard said later, ‘there was little doubt in my mind that the world was headed for grief.’

In the light of these results, Szilard, accompanied by Fermi and Eugene Wigner, tried to warn the US government about the danger. Through George Pegram, dean of physics at Columbia, they managed to secure a meeting between Fermi and Admiral Stanford C. Hooper, technical
assistant to the Chief of Naval Operations. At the meeting, however, Fermi made the mistake of lecturing the admiral on neutron physics rather than talking about bombs. His lecture therefore failed to inspire the required sense of urgency. ‘Couldn’t you arouse the admiral’s interest in the atomic bomb?’ Fermi’s wife asked him many years later. ‘You are using big words,’ Fermi replied. ‘You forget that in March 1939, there was little likelihood of an atomic bomb.’

Szilard and Fermi had both written papers reporting on the neutron-emission experiments, but Szilard persuaded Fermi that, for the moment at least, they should not be published. At Princeton, Szilard and Wigner met Bohr in order to persuade him not to publish any further research on fission. At this meeting they were joined by yet another Hungarian, and a man who would subsequently play a major role in Oppenheimer’s life: Edward Teller. Four years younger than Oppenheimer, Teller had left Hungary for Germany when he was just eighteen. He took his PhD at Leipzig, with Heisenberg as his supervisor, and then worked with Bohr in Copenhagen, before taking up a position at George Washington University, Washington, DC, in 1935. By 1939, he was an established part of the American scientific world, known equally for his scientific brilliance and his personal pugnaciousness.

On this occasion the three Hungarians failed to persuade Bohr of the necessity to keep fission reseach under wraps. Bohr hated secrecy, believing openness to be essential to the progress of science. He was also not convinced that the secondary emission of neutrons alone guaranteed the possibility of an atomic bomb. It would make the bomb possible
if
one could get hold of a sufficiently large lump of pure (or fairly pure) U-235, but such are the difficulties of isotope separation that Bohr was convinced it would never happen. ‘It can never be done,’ he insisted, ‘unless you turn the United States into one huge factory.’

In March and April, the Paris group led by Frédéric Joliot-Curie published two papers (in English) reporting their own experiments on secondary neutrons and giving their own conclusion that, in a sufficiently large amount of uranium, a chain reaction was indeed possible. After this, Szilard has recalled, ‘Fermi was adamant that withholding publication made no sense.’ He might equally have drawn the opposite conclusion. As soon as these papers came to the attention of the Reich Ministry of Education, the German government imposed a ban on the export of uranium and set up a conference that initiated a research programme on nuclear fission.

On 12 July 1939, Szilard and Wigner made their now-famous trip to Peconic, Long Island, to meet Einstein. Their initial reason for wanting to talk to Einstein was that they knew he was on good terms with the King and Queen of the Belgians. The Congo, then a Belgian colony, owned
the world’s largest supply of uranium, and Szilard and Wigner wanted, through Einstein, to alert the Belgians to the global importance of their uranium supplies. First they had to explain to Einstein what had been discovered about fission and secondary neutrons, all of which (despite the fact that some of the key ideas had been discovered in his room at Princeton and written on his blackboard) was new to him. Then the three of them drafted a letter to the Belgian ambassador in Washington, a copy of which they sent to the US State Department. Back in New York, Szilard, worrying about whether they had done the right thing, sought the counsel of a prominent banker, Alexander Sachs, who had served as advisor to the Roosevelt government. Sachs’s advice was that the letter should not have gone to a government department; rather, it should go directly to the President.

So on 2 August, this time accompanied by Teller, Szilard returned to Long Island to see Einstein again, and the three of them worked on a letter to Roosevelt, which, after going through several drafts, ended up warning the President that, in light of the experiments on secondary neutrons conducted by Joliot, Fermi and Szilard, ‘it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and large quantities of new radium-like elements would be generated’. This, they went on, ‘would also lead to the construction of bombs. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory.’ They recommended that the President set up a permanent contact between the government and the physicists working on chain reactions. The letter was signed ‘Yours very truly, Albert Einstein’. After several delays, on 11 October 1939, it was delivered in person to the President by Sachs. ‘What you are after,’ Roosevelt is reported to have said to Sachs, ‘is to see that the Nazis don’t blow us up.’ ‘Precisely,’ replied Sachs. Ten days later, the US bomb project, in its initial form as the Advisory Committee on Uranium, was born.

Despite his evident excitement about fission and its possible application to weaponry when the news first broke, there is nothing to indicate that, throughout the months that led up to Sachs’s crucial meeting with Roosevelt, Oppenheimer was in any way concerned with or involved in – or even particularly interested in – the scientific breakthroughs and political manoeuvres that culminated in the establishment of the US research programme. Other than his lectures on the Bohr–Wheeler theory of fission in the spring and summer of 1939, he seems to have concentrated on other things, principally his paper with Snyder on black holes. It is thus a curious fact that while Szilard, Fermi, Wheeler, Bohr and many other physicists (both theoretical and experimental) were establishing the relevant facts about the fission of uranium and laying down
the fundamental principles upon which the physics of the atom bomb was built, Oppenheimer – the ‘father of the atom bomb’ – was contemplating the gravitational collapse of stars in outer space, and in the process making his greatest ever contribution to science.

Why
Oppenheimer seemed so uninterested in fission during these crucial months is something of a mystery. It is possible, of course, that he remained intensely interested in fission from February 1939 until his involvement in the US bomb project two and a half years later, but that the conversations in which this interest was discussed went unrecorded and unrecalled, and the writings that expressed it have not survived. However, given how thoroughly this period has been researched, how many interviews with the relevant people have been conducted, and what an intense spotlight has been shone by historians on these months, this seems unlikely. Nor is this apparent lack of interest in fission confined to Oppenheimer; it extends also to his friends and students. At both Berkeley and Caltech his friends and colleagues seemed reluctant to pursue the science of nuclear fission, while his students, though they had seemed to share his initial enthusiasm, were writing their PhD theses on other things: Volkoff and Snyder on astrophysics, Christy on cosmic rays, Kusaka on the mesotron, and Dancoff, Morrison and Weinberg on quantum electrodynamics.

In January 1940, a very thorough review of the literature on fission by the Princeton physicist Louis Turner was published in
Reviews of Modern Physics
. In his introductory paragraph Turner notes that: ‘Although less than a year had passed since the discovery by Hahn and Strassmann that the capture of neutrons by uranium nuclei may lead to their disruption to form lighter nuclei, nearly one hundred papers on this subject have already appeared.’ Turner then summarises the findings of those papers under such headings as ‘Neutrons Produced in Fission’, ‘Theory of Fission’, ‘Secondary Neutrons’, and so on. At the end of his review article Turner lists all the papers that he had discussed, among which are strikingly few by scientists at Berkeley and Caltech. There is nothing by Oppenheimer or any of his students, nothing by Lawrence or any of his students, and only three by people working at the Rad Lab: one by Abelson, one by Ed McMillan and one by a new arrival, the Jewish Italian physicist Emilio Segrè.

Segrè, who had worked with Fermi in Italy, joined the Rad Lab in the autumn of 1938. He was drawn there by the possibilities of the cyclotron, but, as he describes in his autobiography, soon after he arrived he began to understand why those possibilities had so far failed to result in important discoveries and fundamental scientific breakthroughs. ‘The more familiar I became with the Rad Lab,’ Segrè writes, ‘the more surprised I was; it operated very differently from any other laboratory I had been
in. There were many students, but they seemed to me to be left to themselves, without scientific guidance.’

The ‘areas outside of physics’ that Segrè mentions here are primarily areas of medical research. Though Lawrence always stressed in public how important the cyclotron was to fundamental physics, to others, particularly to those with money to donate, he emphasised the value of the cyclotron for producing radioactive isotopes that had applications in both medical research and practical medicine. There was no doubt that the machine had proved its worth for that purpose; Segrè’s concern was evidently that students attracted to the Rad Lab with the intention of pursuing fundamental physical research were instead used to keep the cyclotron going as a kind of factory for producing isotopes.

Segrè’s view that Lawrence’s knowledge of, and interest in, nuclear physics was limited was held by many other scientists, but many too would have agreed with the following summary of Lawrence’s strengths and weaknesses given by Hans Bethe:

Bethe seems right on all counts. Despite being wrong-footed time and time again by new discoveries in physics, despite, as Heilbron and Seidel put it in their history of the Rad Lab, ‘the disagreeable fact that no major discovery had yet been made in any cyclotron laboratory’, Lawrence somehow managed to turn each lost opportunity into a successful case
for pouring more and more money into his ambitions of building bigger and bigger machines.

Though he had at his disposal the world’s most powerful accelerator, and a budget that other laboratories could only dream of, Lawrence had missed every major discovery in physics since 1932: deuterium and the neutron, the splitting of the lithium nucleus, the positron, the artificial creation of radioactive isotopes, the mesotron and, finally, nuclear fission – all of them had been discovered either by using much less powerful equipment than the Berkeley cyclotrons or by analysing cosmic rays, the high energies of which are provided by nature free of charge.

And yet, despite his conspicuous lack of scientific achievement, in the 1930s Lawrence was by far America’s most famous scientist. In 1937
Time
magazine put him on their cover, calling him ‘the cyclotron man, foremost US destroyer and creator of atoms’. His lecture tours were a great success, he had honorary degrees conferred on him by South Dakota, Princeton and Yale, and he was showered with grants, prizes and donations. He was an extremely successful promoter of his own product, and, in the face of much evidence to the contrary, was remarkably good at persuading people, especially those with funds, that what was needed for scientific breakthroughs in nuclear physics were bigger and bigger cyclotrons. Measured by the diameter of their magnets, the inexorable progress was this: the 11-inch was followed by the 27-inch, and then a 37-inch.

When news of fission broke, Lawrence’s mind was concentrated on getting his newest and biggest cyclotron to date – a 60-inch – up and running. In the wake of the announcement of fission, and with a brass neck that had got him where he was, Lawrence sent letters to physicists around the world, detailing the ‘successes’ of the Rad Lab, and claiming that curiosity about fission among his colleagues was so overwhelming that many of them were committing the ‘heresy’ of suspending work on the planned new cyclotron in order to study fission. ‘For obvious reasons,’ Lawrence pointed out, ‘we want to find out whether neutrons are given off in the splitting process.’ On that point, too, the Rad Lab was scooped; while Alvarez gained inconclusive results from the cyclotron, Fermi, Szilard and Joliot provided the affirmative answer that set the US bomb project in motion. Meanwhile, Lawrence went back to the task of setting up the 60-inch cyclotron, and when, in the summer of 1939, it was up and running, he started thinking about his next machine, which he said would have a 120-inch magnet, weigh 2,000 tons and be capable of energies of 100 million volts.