Inside the Centre: The Life of J. Robert Oppenheimer (35 page)

In their ‘letter to the editor’, Oppenheimer and Carlson dismissed both Millikan’s view that cosmic rays were photons and also Compton’s view that they were made up of protons. Perhaps, they suggested, cosmic rays might consist of the ‘neutrons’ posited by Pauli. At the end of their note, Oppenheimer and Carlson promised that the results of their calculations of the collisions between electrons and Pauli’s neutrons would be ‘published very shortly’.

Oppenheimer and Carlson’s letter was dated 9 October 1931. Three days after that, Oppenheimer was in New York to be at his mother’s bedside as she lay dying. ‘I found my mother terribly low,’ he wrote to Lawrence, ‘almost beyond hope.’

Four days later, Oppenheimer wrote again to Lawrence, thanking him for his ‘sweet message’ and ‘lovely roses’. ‘Things are pretty bad here,’ he told him. ‘Mother, after a short reprise, has been growing rapidly very much worse; she is comatose, now; and death is very near.’

Ella died the following day. Oppenheimer’s old friend and teacher Herbert Smith spent that afternoon with him, and remembers him saying immediately after his mother’s death: ‘I am the loneliest man in the world.’

Oppenheimer’s letters to Lawrence from New York show how much he disliked being away from Berkeley, from physics and from the school of young theorists that he was developing. ‘I feel pretty awful to be away so long,’ he told Lawrence. ‘You will do what you can for the fatherless theoretical children, won’t you?’ In the following letter he insisted: ‘You must let me know if there is anything that I can do for you here; and if a word from me can be of any help to my deserted students, do not, please, hesitate to ask for it.’

As soon as he decently could, Oppenheimer returned to his ‘deserted students’ in Berkeley. Before he did so, he arranged to meet his father,
together with Frank, in New Orleans in December. The plan was for the three of them to spend ten days together there over the Christmas period, before Oppenheimer, Frank and, no doubt, a number of Oppenheimer’s students, including Frank Carlson, attended the meeting of the American Physical Society, which was held in New Orleans on 29 and 30 December.

Coming so soon after Ella’s death, it is hard to imagine the family holiday being anything other than mournful, and, as it happened, the American Physical Society meeting was also quite an ordeal. Robert Millikan – who as president of Caltech was, in some sense, Oppenheimer’s boss – chose these meetings to publicly, vociferously and belligerently defend his theological understanding of cosmic rays against non-believers, including Oppenheimer and Carlson, whose letter to the editor of the
Physical Review
dismissing Millikan’s views had appeared in print in November. Millikan evidently felt that he had much to lose if his views on cosmic rays were publicly discredited, since he had devoted a good deal of time and energy to trying to persuade not only his fellow physicists, but also the general public, of those views. He had been interviewed by the
New York Times
on the subject, and had given many public presentations of his claim that cosmic rays were evidence of God’s existence and His beneficence. In the New Year of 1932,
Time
magazine carried an interview with him in which he made the same claim. It was evidently not a view that he was willing to give up lightly

Clearly shaken by the vehemence of Millikan’s attack in New Orleans, Oppenheimer, in a letter to Lawrence written on the way back to California, thanked him for the ‘comforting words’ he had whispered to him during Millikan’s onslaught. ‘I was pretty much in need of them,’ he told Lawrence, ‘feeling ashamed of my report, and distressed rather by Millikan’s hostility and his lack of scruple.’ He also told Lawrence that he had received a call from a news reporter, saying that he had been sent by Lawrence and asking him his views on the controversy, but ‘I did not give him anything; I hope that in that I did not offend your wishes.’

If Oppenheimer was hoping that, by not parading his dispute with Millikan before the general public, he could soften Millikan’s attitude, he was mistaken. For the rest of his life Millikan treated Oppenheimer with unremitting hostility. ‘Millikan loathed Oppenheimer,’ Birge recalls, ‘wouldn’t match the promotions we gave him here, and harassed him maliciously.’ At Berkeley, Oppenheimer had been promoted to an associate professorship at the start of the 1931–2 academic year, but it would be another three years before Caltech followed suit. ‘Millikan just left his name in the faculty register,’ Birge remarked, ‘and made him miserable when the chance came.’

Instead of giving his response to Millikan’s attack to a news reporter, Oppenheimer no doubt wanted to return to the issues involved in a dignified and properly academic fashion, by fulfilling the promise he and Carlson had made in their note to the
Physical Review
to publish ‘very shortly’ their calculations concerning the collisions between electrons and Pauli’s ‘neutrons’. However, almost as soon as he returned to Berkeley from New Orleans, the work he and Carlson were planning to undertake was overtaken by a series of momentous experimental discoveries that has led to the year of 1932 being described as a miraculous year in physics.

The first of these was announced in a paper published in the 1 January 1932 issue of the
Physical Review
entitled ‘A Hydrogen Isotope of Mass 2’. The paper had three authors: Ferdinand Brickwedde, G.N. Murphy and Harold Urey, the last of whom, after gaining his PhD at Berkeley under Gilbert Lewis, was an associate professor at Columbia University. What Urey and his colleagues had to report was the discovery of deuterium, an isotope of hydrogen that is twice as heavy, having an atomic mass of 2, rather than 1.
^fn31
Physicists had long wanted to find a chemical with an atomic mass of two because of the potential such a thing would offer for the investigation of the structure of nuclei. The nucleus of hydrogen, with an atomic mass of just one, has no structure, while all other known chemicals, prior to the discovery of deuterium, had three or more particles in their nuclei and so had a structure too complicated to investigate in detail. Deuterium, however – the ‘hydrogen atom of nuclear physics’, as the physicist Victor Weisskopf once called it – allowed physicists to bring to the study of nuclei everything they knew about two-body systems, thus making extremely detailed calculations possible.

The nucleus of deuterium is a perfect example of the kind of thing that had persuaded Rutherford back in 1920 that there must be such a thing as a neutral particle with the same mass as a proton. In fact, with remarkable prescience, Rutherford had explicitly predicted ‘the possible existence of an atom of mass nearly 2 carrying one charge, which is to be regarded as an isotope of hydrogen’. The fact that, as Rutherford had predicted, deuterium has twice the mass, but the same charge as normal hydrogen, would seem to indicate that the additional mass has no charge, which in turn would be perfectly explained if one were to imagine the deuterium nucleus to consist, as Rutherford had imagined, of one proton and one neutron. The only thing blocking this way of picturing it was
that neutrons had not yet been discovered. However, this barrier was removed little more than a month after Urey’s announcement of the discovery of deuterium, when, on 27 February 1932, there appeared in
Nature
a letter to the editor by James Chadwick of the Cavendish Laboratory, which, with undue but characteristic restraint, Chadwick entitled ‘Possible Existence of a Neutron’.

What Chadwick presented in this short communication was what almost every physicist who read it agreed to be conclusive evidence of the existence of neutrons. Chadwick had collected this evidence from a series of experiments that he performed, working day and night,
^fn32
over a period of three weeks in the first two months of 1932. His inspiration came from a piece published in the journal
Comptes rendus
on 18 January 1932, in which the French physicists Frédéric Joliot and his wife, Irène Curie, described a puzzling phenomenon they had witnessed in an experiment in which they bombarded beryllium with very energetic alpha particles emitted from polonium. What they recorded was that this produced extremely powerful radiation from the beryllium, which they assumed to be gamma radiation – that is, photons. So powerful was this ‘gamma radiation’ that when they placed paraffin wax in front of it, it knocked protons out of the wax with an energy of 4.5 million volts. In order to achieve this, the supposed gamma radiation would have required an energy of about fifty-five million volts, an energy previously encountered only by those studying cosmic rays.

When Chadwick read this report, he realised immediately that a more likely explanation of the phenomenon recorded by Joliot and Curie was that the protons were being knocked out of the paraffin wax by
neutrons
, which, being roughly the same size as protons, would need a kinetic energy only slightly larger than that of the protons they set in motion. Using what, by today’s standards, looks a makeshift and unimpressive piece of equipment, Chadwick was able to repeat the experiment conducted by Joliot and Curie, and to extend it by showing that the radiation from beryllium ejects particles from hydrogen, helium, lithium, carbon, oxygen and argon. His results, he stated in his letter to
Nature
, were difficult to explain if one assumed, as Joliot and Curie had done, that the emissions from beryllium were gamma radiation, adding: ‘The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons.’

By the time he wrote up his results in full for the June 1932 issue of
Proceedings of the Royal Society
, Chadwick had overcome any doubts and his paper was published under the less tentative title ‘The Existence of
a Neutron’. Oppenheimer and Carlson seem to have waited for this full version of Chadwick’s report to appear before returning to the subject of neutrons, since it was not until 18 July 1932 that they fulfilled the promise they had made the previous October and sent their own detailed paper on ‘The Impacts of Fast Electrons and Magnetic Neutrons’ to the
Physical Review
. Perhaps somewhat oddly, Oppenheimer and Carlson do not mention Chadwick or cite his work (though they are surely alluding to it when they mention experimental evidence for the existence of neutrons). One might suppose that this is because they were concerned with
Pauli’s
neutron, which has a mass thousands of times smaller than that of Chadwick’s, but this is not borne out by the paper, which shows, rather, a confusion between the two.

On the one hand, Oppenheimer and Carlson speak of the neutron as ‘a hypothetical elementary neutral particle’ whose existence was ‘tentatively proposed by Pauli’. On the other hand, though they point out that Pauli had supposed this hypothetical particle would have a mass ‘not much greater than the electron’, they say: ‘One may, however, assume that the neutron has a mass close to that of the proton’ – an assumption surely based on Chadwick’s calculations. The notion of a ‘magnetic neutron’ employed by Oppenheimer and Carlson is, then, an uneasy mixture of the very different notions of Pauli and Chadwick. It is therefore perhaps not surprising that their paper should end with the conclusion that it is unlikely that cosmic rays consist of such ‘magnetic neutrons’, since ‘there is no experimental evidence for the existence of a particle like the magnetic neutron’.

That Oppenheimer and Carlson were not the only ones confused about the relation between Pauli’s hypothetical particle and the discoveries of Chadwick is suggested by a witty pastiche of Goethe’s play
Faust
that was performed at Bohr’s institute in Copenhagen in April 1932. In this version of the Faustian legend, the role of Mephistopheles is played by Pauli, who is trying to tempt ‘Faust’ – Ehrenfest – into believing in a particle that has no mass and no charge (and is therefore practically undetectable). Oppenheimer is given a very brief part in the play, in a scene that takes place at ‘Mrs Ann Arbor’s Speakeasy’ (the Ann Arbor summer school). There, Mephisto/Pauli tempts the drunken ‘American physicists sitting sadly at the Bar’, including Oppenheimer, into accepting the existence of the neutron. In the finale of the play, Chadwick appears ‘and says, with pride’:

To which Mephisto/Pauli replies:

In 1934, the Italian physicist Enrico Fermi proposed the name ‘neutrino’ (little neutron) to distinguish Pauli’s hypothetical particle from Chadwick’s ‘heavyweight’, and it would not be until 1955 that experimental confirmation of the existence of neutrinos would be produced. Oppenheimer and Carlson’s idea that Pauli’s neutrino might hold the key to understanding the nature of cosmic rays was sunk by the confusion between the two neutral particles, and, in any case, turned out to be wrong –
whichever
‘neutron’ it concerned.

Having his ideas overtaken by experimental developments seems to have had a salutary effect on Oppenheimer, who henceforth made it his business to know
everything
going on in experimental physics. Raymond Birge has recalled: ‘In our seminars Oppenheimer knew more experimental physics than even the experimental physicists did. He could reel off figures and equations relating to experiments better than any experimental physicist in the room.’ Among the seminars in question were the weekly Wednesday-afternoon colloquia, where experimentalists and theorists met for discussion, and the Journal Club, which met every Tuesday evening to go through recent work, both experimental and theoretical. Lawrence’s assistant, Milton Stanley Livingston, remembers that at these meetings the experimentalists ‘sat afraid to ask Oppenheimer anything’, with the exception of Lawrence himself, who won their admiration for his willingness to ‘pop up and ask something silly’.