by Ray Monk
As it turned out, cosmic-ray research would indeed confirm Yukawa’s hypothesis of a particle bigger than an electron but smaller than a proton, and, thus confirmed, that hypothesis would in turn provide the solution to the puzzle about the penetrative power of cosmic rays that had prompted Oppenheimer to talk about their having a ‘less absorbable component’. However, as would become gradually clear (it took about twelve years, beginning in 1935) amid much confusion and controversy, the penetrative particle in cosmic rays is not the carrier of the ‘strong nuclear force’ – that is another particle somewhat like it. How confused the initial picture of these particles was can be gleaned from their changing nomenclature: to begin with, before it was realised they were different, they were called ‘mesotrons’, which was changed to ‘mesons’ for reasons of linguistic probity (the Greek word for ‘middle’ being ‘mesos’ rather than ‘mesotros’); then, to distinguish them from each other, the one that is a component of cosmic rays was called a ‘µ-meson’ (mu-meson), and the one that is the carrier of the strong nuclear force was called the ‘π-meson’ (pi-meson). Then it was decided that mesons are by definition carriers of the strong nuclear force and therefore that the mu-meson is not a meson at all. It was accordingly renamed the ‘muon’, while the other was renamed the ‘pion’. Playing a leading role in both the creation and the clearing up of these confusions, and thus being there at the birth of what has grown into the (for most people) utterly bewildering new discipline of particle physics, were Oppenheimer and his students.
Much of the key observational evidence of cosmic rays that led to the discovery of the ‘mesotron’ was collected by Anderson and Neddermeyer at the summit of Pikes Peak in the Rocky Mountains, where they went in the summer of 1935. There, at an altitude of about 14,000 feet, they set up the equipment they had brought with them and took thousands of photographs of extremely high-energy cosmic-ray collisions. Without knowing anything about Yukawa’s article, Anderson wrote to Millikan at Caltech from the top of Pikes Peak to say he thought he had evidence of a particle intermediate in mass between electrons and protons. He was a cautious man, however, and did not want to publish this result until he was completely sure of it. It was therefore not until the summer of 1936 that Anderson and Neddermeyer published a scholarly account of their trip to Pikes Peak in the Physical Review. Entitled ‘Cloud Chamber Observations of Cosmic Rays at 4,300 Meters and Near Sea Level’, their article reproduced some of their more dramatic photographs, taken both at Pikes Peak and in Pasadena, and in a very modest and hesitant way tried to make sense of them. Without actually declaring that they had found a new particle, they gave good reasons why the penetrative particle they had photographed could not be either a proton or an electron. They also made the important announcement that their observations refuted something that had been widely believed by physicists, namely that the theory of quantum electrodynamics broke down when applied to particles of extremely high energy. They had observed particles at more than one billion volts, they recorded, and the theory had stood up very well.
It was this last aspect of the article that most interested Oppenheimer. He had been saying for years that the theory broke down at high energies, but he did not seem to mind being proved wrong on that account. On the contrary, he seemed delighted with the findings reported by Anderson and Neddermeyer, not least because they allowed him to pick a theoretical fight with Heisenberg, a fight he felt confident of winning. Heisenberg had recently been drawn into the analysis of cosmic rays, thinking that he had a new insight that would shed light on their nature. The insight in question was one derived from recent work published by Enrico Fermi on the subject of beta decay. Taking up the issue that had prompted Pauli to suggest the neutrino (a name conferred on the as-yet-undiscovered particle by Fermi), Fermi had proposed a completely new analysis of beta radiation that bore a striking analogy to Yukawa’s new analysis of nuclear forces. The analogy was no accident; Fermi’s theory was published at the beginning of 1934, about ten months before Yukawa’s, and was one of the main inspirations for Yukawa’s theory. Like Yukawa, what Fermi proposed was the introduction of a new basic force into physics – what is now called the ‘weak nuclear force’ – to explain beta emissions. This new force would act upon electrons, neutrinos and nucleons (protons and neutrons) and would explain the process whereby a neutron decays into a proton, emitting as it does so beta radiation (electrons) and neutrinos.
In a paper that he published in June 1936, Heisenberg suggested that this new force field postulated by Fermi held the key to understanding cosmic rays. In particular, he believed that the phenomenon that had earlier attracted the attention of Oppenheimer – the ‘showers’ of electron/positron production – might be explained by Fermi’s new field. At the root of Heisenberg’s analysis was the belief that the accepted theory of quantum electrodynamics broke down at the extremely high energies observed in cosmic radiation. As Anderson and Neddermeyer had shown this belief to be false, the motivation for Heisenberg’s theory had disappeared. This was pointed out in a confrontational manner by Oppenheimer in a paper called ‘On Multiplicative Showers’, which he co-wrote with Frank Carlson (for whom he had not yet managed to find an academic position and who therefore remained at Berkeley and Caltech), and sent to the Physical Review at the end of 1936. ‘It would seem,’ declared Oppenheimer and Carlson, that Heisenberg’s theory ‘is without cogent experimental foundation; and we believe that in fact it rests on an abusive extension of the formalism of the theory of the electron neutrino field’.
Oppenheimer and Carlson ended their paper with the suggestion that the observations of Anderson and Neddermeyer pointed to ‘another cosmic ray component’ that would explain the showers that seem unlikely to have been made by electrons or protons. In this, of course, they were simply echoing what Anderson and Neddermeyer had said. The latter pair had been convinced that they had evidence of a new particle ever since they went to Pikes Peak in the summer of 1935, and in May 1937 they finally committed themselves in print to the claim that their observations showed ‘the presence of some particles less massive than protons but more penetrating than electrons’. Their caution, however, proved costly, in that, in the same volume of the Physical Review that their article was published, there appeared a short report of experiments conducted by two physicists at Harvard, J.C. Street and E.C. Stevenson, which seemed to establish the existence of exactly the same new particle. It is therefore not uncommon, much to Anderson’s later chagrin, for it to be said and written that the particle was discovered by Street and Stevenson.
In a letter to the editor of the Physical Review, dated 1 June 1937 and published under the heading ‘Note on the Nature of Cosmic-Ray Particles’, Oppenheimer and Serber drew attention to the fact that Yukawa had seemingly predicted this new particle. Many years later, Serber noted proudly that this was the first time anyone had referred to Yukawa’s paper and that ‘a very conscious purpose of our paper was to call attention to Yukawa’s idea’. This was not their only purpose, however. Their paper was also, though it did not mention Heisenberg by name, part of Oppenheimer’s ongoing campaign against Heisenberg, drawing attention as it did to Yukawa’s suggestion that ‘the possibility of exchanging such particles of intermediate mass would offer a more natural explanation of the range and magnitude of the exchange forces between proton and neutron than the Fermi theory of the electron-neutrino field’.
In the same issue of the Physical Review appeared an article that Oppenheimer and Serber had written a little earlier, in March 1937, together with Lothar Nordheim and his wife, Gertrude. Lothar Nordheim was a German physicist, who, because he was Jewish, had had to leave his position in Göttingen. Through the Emergency Committee for Displaced Scholars, he had been given a visiting professorship at Purdue University in Indiana. Nordheim was considered an expert on Fermi’s theory of beta decay, so his appearance in the United States provided Oppenheimer with a good opportunity to publish an authoritative refutation of Heisenberg’s use o
f that theory, which is what their joint paper, called ‘The Disintegration of High-Energy Protons’, amounts to. The argument the four authors outline is that, when Fermi’s theory is brought to bear on observations of the disintegration of protons (by this time it was known that protons were the main component of cosmic rays), it completely fails to explain the phenomenon of cosmic-ray showers. ‘The point of view adopted by Heisenberg in his theory of showers,’ Oppenheimer and his fellow authors write, is to regard as valid ‘just those implications of present theory which would at first seem most subject to suspicion’. In the light of their analysis, they conclude bluntly, Heisenberg’s theory ‘no longer affords any explanation of showers’.
Immediately before the publication of these new discoveries and these attacks on his theory, Heisenberg was, as he wrote to Pauli on 26 April 1937, ‘optimistic concerning the discovery of Fermi processes in cosmic radiation’. Three days later, Heisenberg, still professor of physics at Leipzig, got married. After his honeymoon he was due to take up a new, more prestigious appointment in Munich, as Arnold Sommerfeld’s successor. His appointment to this coveted position, however, was delayed by the manoeuvring of hard-line Nazis, distrustful of the ‘white Jew’ that they accused Heisenberg of being. Heisenberg wanted to travel, but it was not until the spring of 1938 that he was allowed to leave Germany to visit Britain, from where, he wrote to his wife that ‘it is important to me now to lose myself entirely in physics’. At both Cambridge and Manchester he spent many hours discussing cosmic rays and the new Yukawa particle with people who knew what they were talking about, such as Patrick Blackett. From what he learned from these discussions, Heisenberg was able to marshal new arguments for his view on cosmic radiation. Or at least part of that view. He gave up (because it had been effectively destroyed) the claim that Fermi theory could explain cosmic-ray showers, but retained the view that had motivated that claim, namely that quantum-electrodynamic theory ceased to be valid above a certain energy, and that the showers associated with the newly discovered particle were proof of that fact.
In May 1938, Heisenberg wrote a paper called ‘The Limits of Applicability of the Present Quantum Theory’, which he sent to Bohr in Copenhagen and delivered as a lecture to several audiences made up of those German physicists who had chosen to stay in Hitler’s Germany. In the early summer of 1939, Heisenberg was again allowed to leave Germany and this time went to the United States, where he spent a month visiting old colleagues and friends at various American universities. The highlight of his visit was a symposium on cosmic rays at Chicago, where a dispirited Heisenberg listened as one paper after another poured scorn on his latest theory. Most scornful of all was Oppenheimer. ‘According to Heisenberg’s recollection of the meeting,’ writes David Cassidy in his biography of Heisenberg, ‘the animated discussion following his session soon degenerated into a shouting match between himself and J. Robert Oppenheimer.’ At the beginning of August, Heisenberg set off to return to Germany. Scarcely more than a month later, his country would be at war with Britain.
It was clearly very important to Oppenheimer to win his argument with Heisenberg and one senses that, for him at least, there was more at stake than the cogency or otherwise of certain views about the nature of cosmic-ray showers. It was important to him not only to win that argument, but also to defeat Heisenberg. Winning the argument would be a triumph of American science over German science, striking a blow against the Nazi regime, whose ridiculous views about the dangers of ‘Jewish science’ and the superiority of ‘deutsche Physik’ Heisenberg now represented, not because those were his views but because, as a patriotic German, he had decided to work for the Nazis rather than leave the country.
What Oppenheimer later called his ‘smouldering fury about the treatment of Jews in Germany’ had served to awaken in him a sense of comradeship with the German Jews in general and, in particular, with his own family. After his father died on 20 September 1937, Oppenheimer arranged for Julius’s youngest sister, Hedwig Stern, together with her son, Alfred, and his family to emigrate to the United States. Hedwig had originally planned to live in New York, but Oppenheimer persuaded her and her family to settle instead in Berkeley. She lived there for the rest of her life and remained very fond of her nephew. When she died in 1966, Alfred wrote to Oppenheimer to say that she had led a full and active life and that: ‘Your closeness has made it richer still.’
Alfred has recalled a significant and revealing conversation he had with Oppenheimer soon after he and his mother arrived in the United States. When, he remembered, he asked Oppenheimer how they would fare in America and what America was really like, Oppenheimer replied:
It is big here, not just geographically, but in thinking and spirit. You can move with ease from place to place and among people of all social rank and economic standing. And all people have the possibility to a high degree of influencing their destiny because they have the democratic means. There is a direction for the people and for the country, but this is re-evaluated all the time. You have seen atrocities in Europe and you wonder, can it happen here? I would reply that there is a lack of coercion here, a depressurizing safety valve built into the very nature of a democracy like America’s. Totalitarianism is far less likely here than in Europe.
These comments were made at the very time, towards the end of 1937, that Oppenheimer was undergoing a political awakening, which would later lead him to face charges of being ‘un-American’.
Anger at the Nazi persecution of the Jews and a feeling of identity with that Jewish community were important factors in Oppenheimer’s shift in the 1930s from someone who demanded to know ‘what has politics to do with truth, goodness and beauty?’ to someone deeply and actively committed to radical politics. But there were other factors compelling him in that direction. He himself mentioned the importance of seeing ‘what the depression was doing to my students’. That is, they could not get jobs, or the jobs they could get were inadequate, which made Oppenheimer, who had taken such a conspicuous lack of interest in the stock-market crash of 1929, begin, as he put it, ‘to understand how deeply political and economic events could affect men’s lives’. When, after his father’s death, he came into his inheritance, he made a will leaving his money to Berkeley to provide fellowships for graduate students.
Another important factor in his involvement with left-wing groups, again emphasised by Oppenheimer himself, was his need for comradeship. ‘I began,’ he said, ‘to feel the need to participate more fully in the life of the community.’ When he started to join with others to pursue political goals, he felt something he often longed for, but very rarely achieved: a sense of belonging: ‘I liked the new sense of companionship, and at the time felt that I was coming to be part of the life of my time and country.’ Robert Serber’s account of what seems to have been Oppenheimer’s very first act of engagement with the political tumult of the 1930s – his participation in the rally in support of the longshoremen’s strike in 1934, which ended with the improbable image of Oppenheimer shouting ‘Strike! Strike! Strike!’ along with the crowd – seems to bear this out.
‘The matter which most engaged my sympathies and interests,’ Oppenheimer wrote, ‘was the war in Spain.’
This was not a matter of understanding and informed convictions. I had never been to Spain: I knew a little of its literature; I knew nothing of its history or politics or contemporary problems. But like a great many other Americans I was emotionally committed to the Loyalist cause. I contributed to various organizations for Spanish relief. I went to, and helped with, many parties, bazaars, and the like. Even when the war in Spain was manifestly lost, these activities continued. The end of the war and the defeat of the Loyalists caused me great sorrow.
The Spanish Civil War began on 17 July 1936, when a group of generals led by the fascist General Franco, and supported by various conservative and nationalist groups, attempted to remove by force the elected leftist government. It ended in April 1939 with a victory for the fascists. As soon
as the war broke out, it became a cause célèbre among left-leaning people, especially those in Britain and the United States, who saw it as the front line in the battle between fascism and democracy. The governments of the democratic countries, however, were prepared to do little to help the Loyalist cause, leaving the Soviet Union as the one country prepared to help.
This state of affairs meant that the international effort to support the Loyalist government and oppose the spread of fascism was led by the international Communist Party, which thus seized the opportunity to increase its influence. So dominant was the Communist Party in the various campaigns and projects designed to provide help to the Loyalists that it was more or less impossible to contribute to those campaigns and projects without coming into contact with, and working alongside, communists. More than 3,000 US citizens went to Spain to serve the Loyalist cause, some as soldiers (two entire battalions, with the patriotic names Abraham Lincoln and George Washington, were made up of volunteers from America), others as doctors, nurses, ambulance drivers, and so on. Not all of them were communists by any means, but simply to get to Spain they would all have had to work with communists, since it was the Communist Party that was organising the operation: arranging boats to Spain, trains across the country and helping to place the volunteers where they might be of most help to the cause.
There is some question about whether – and, if so, in what sense – Oppenheimer ever became a member of the Communist Party, but there is no question that he became what is known as a ‘fellow traveller’. The first sign of his being interested in communism (rather than merely supportive of left-wing causes such as the longshoremen’s strike in 1934) occured in the summer of 1936, when he astonished a friend by claiming to have read all three volumes of Marx’s Das Kapital on the train from Berkeley to New York. At around the same time he claimed to have bought and read the complete works of Lenin. There is little in his recorded conversations, his writings or his correspondence to suggest a detailed familiarity with Marx’s Kapital and the complete works of Lenin, so perhaps one should treat these claims with some scepticism, but the mere fact that he made them suggests a fairly radical break with the past.