Inside the Centre: The Life of J. Robert Oppenheimer (36 page)

This close collaboration between theorists and experimentalists at Berkeley was to be mutually beneficial in the mid- to late 1930s, but in the crucial year of 1932 there are signs that communication was not all it should have been: not only was Oppenheimer wrong-footed by experimental developments, but Lawrence seemed to be held back by a lack of appreciation of certain crucial theoretical points.

Most of Lawrence’s efforts were directed at building bigger and bigger cyclotrons and accelerating protons to greater and greater energies. A key moment in this development came in February 1932, when his latest cyclotron succeeded in accelerating protons to an energy of one million volts. ‘I wrote the figure on the blackboard,’ Livingston later remembered. ‘Lawrence came in late one evening. He saw the board, looked at the
microammeter to check the resonance current and literally danced around the room.’ Ever the publicist, Lawrence lost no time in spreading the news, and the next day Livingston recalls: ‘We were busy all that day demonstrating million-volt protons to eager viewers.’ The
San Francisco Examiner
sent a reporter to Berkeley to see what was going on, and announced excitedly that Lawrence and Livingston ‘are setting about trying to break up the atom and release its terrific energy’. Reporting on plans to build an even bigger cyclotron, the
Examiner
went on: ‘With the greater magnet, they hope to shatter the atom completely with an ultimate 25,000,000-volt impact.’

But, as quantum theorists had known for many years before this, and as Oppenheimer must surely have been aware, there was a very good chance of splitting the atom with much lower energies than these. Indeed, the key theoretical point in understanding this centres on a phenomenon that Oppenheimer himself had been among the first to consider: the mysterious process known as ‘quantum tunnelling’. Due to the fact that, according to quantum theory, protons, along with electrons, and so on, are
both
particles and waves, there is a significant possibility that the particles that make up a nucleus can suddenly appear
outside
the electrical barrier (the ‘Coulomb barrier’) that surrounds the nucleus. Rutherford had known about the phenomenon, but had pictured it as electrons pulling protons out of their shell, like tugboats pulling liners out to sea. It was the Russian physicist George Gamow who first realised that it was a direct consequence of the wave-like nature of subatomic particles, and that quantum mechanics offered the means to quantify and predict the probability of such ‘tunnelling’ occurring.

Gamow’s analysis appeared in print twice in quick succession in the autumn of 1928: in German in the
Zeitschrift für Physik
on 12 October, and in English in
Nature
on 24 November. Actually, in terms of being the first to publish such an analysis, Gamow had narrowly been beaten to it by an article by Edward Condon and Ronald Gurney that appeared in
Nature
in September, but, from the point of view of the history of splitting the atom, Gamow’s account is more important for two reasons: first, unlike Condon and Gurney, Gamow was less concerned with how protons might get out of the nucleus than how they might get
in
; and second, it was Gamow’s account that stimulated the work that led to the world’s first splitting of the atom.

Shortly before it was published, an advance copy of Gamow’s
Zeitschrift
article was sent to John Cockcroft at the Cavendish Laboratory, who saw immediately its implication that atoms might be split with relatively low-voltage protons. Rutherford, when he made a public appeal for progress in artificial acceleration of particles, had thought that one needed eight
million volts or more. However, in a series of calculations that Cockcroft wrote out, showed to Rutherford, but did not publish, he demonstrated that, if Gamow’s analysis was correct, a ‘mere’ 300,000 volts would probably be enough.

Encouraged by Rutherford, Cockcroft worked together with his Cavendish colleague Ernest Walton to design and build a machine capable of accelerating protons to the required 300,000 volts. By May 1930, they had a machine capable of 280,000 volts and felt able the following August to publish an account of their work in the
Proceedings of the Royal Society
, in which they took the possibly risky step of making public Cockcroft’s calculation that 300,000 volts would be enough to penetrate the nucleus and so split the atom. Around the same time, however, Rutherford gave a speech in which he claimed: ‘What we require is an apparatus to give us a potential of the order of ten million volts.’ Whether Rutherford believed this, or whether these words were a smokescreen, is difficult to say, but, knowing perfectly well that Lawrence and his team were constructing machines capable of more than one million volts, Rutherford continued to encourage Cockcroft and Walton in their endeavours using more modest machinery.

In January 1932, Cockcroft received a letter from his old friend Joseph Boyce, who told him: ‘I have just been on a very brief visit in California and thought you might be interested in a brief report on high-voltage work there.’ The ‘place on the coast where things are really going on,’ reported Boyce, ‘is Berkeley’:

Lawrence, Boyce added, ‘is a very able director, has many graduate students, adequate financial backing, and his work so far . . . has achieved sufficient success to justify great confidence in his future’.

In the light of such reports, Rutherford’s encouragement became more urgent. On 14 April 1932, after they had been told by Rutherford to ‘stop messing about and wasting their time’, Cockcroft and Walton, without any great hopes of success, fired some accelerated protons at a sample of lithium, a very light metal with an atomic mass of 7. The result was so astonishing that Rutherford and Chadwick were called to the laboratory to verify that there was no mistake in their observations. What they all saw were the familiar scintillations that told of the emission of alpha
particles.
^fn33
There was only one conclusion to draw: the protons had caused the lithium nucleus to break up, forming two alpha particles (that is, helium nuclei, which have an atomic mass of 4, which makes perfect mathematical sense, since the combined atomic masses of the lithium nucleus plus the proton is 8, equivalent to two helium nuclei). In other words, what Cockcroft and Walton had achieved was the world’s first splitting of the atom by artificial means. What is more, they had done it with protons accelerated to a voltage significantly below the 300,000 that Cockcroft had calculated. In subsequent tests, they discovered that lithium nuclei could be disintegrated at 125,000 volts, which was far below what anybody had thought possible.

When Cockcroft and Walton measured the energy of the alpha particles emitted from the reaction, the results provided both dramatic confirmation of the most famous equation in science – E = mc
²
– and a startling illustration of the kind of energy that can be released by an atomic reaction. For the answer was eight million volts. Since from each lithium nucleus there emerged two alpha particles, this means that, from the collision of a single proton, travelling with an energy of 125,000 volts, with a single lithium nucleus, sixteen million volts of energy had been released (two alpha particles, each with an energy of eight million volts). Little wonder, then, that people immediately started to wonder how such tremendous energy releases might be used in explosives.

Partly because he was acutely conscious that this kind of speculation would inevitably follow the announcement of their achievement, Rutherford – after helping Cockcroft and Walton write up their experiment for
Nature
– urged upon them the importance of keeping quiet about it until a sober account had appeared in print. But, as Walton spelled out in a letter written to his girlfriend, Freda, this was not Rutherford’s only reason for not letting the news leak out. ‘We know,’ Walton wrote, ‘that people in the States are working along similar lines and Rutherford would like to see any credit going to the Cavendish. He is not fond of American physicists in general on account of their tendency to do a great deal of boasting about very little.’

And yet Rutherford was evidently itching to announce the news. On 28 April 1932, two days before the report for
Nature
appeared, he chaired a meeting at the Royal Society in London on ‘The Structure of Atomic Nuclei’, which had been organised primarily to allow discussion of Chadwick’s discovery of the neutron. Rutherford arranged for Cockcroft and Walton to
be present at this occasion, and then, before introducing Chadwick, announced their achievement in disintegrating the lithium nucleus.

Two days later the
Nature
piece appeared, but, meanwhile, Rutherford’s announcement at the Royal Society had attracted the attention of the press, and on Sunday 1 May 1932, the
Reynold’s Illustrated News
, under the heading ‘
SCIENCE
’
S GREATEST DISCOVERY
’, reported:

The same day the
Sunday Express
went with: ‘The Atom Split. But World Still Safe’, while the
Daily Mirror
pleaded: ‘Let it be split, so long as it does not explode.’ The idea that atom-splitting would lead to extremely powerful bombs had been around since the 1920s. Bertrand Russell mentioned it in his 1923 best-seller
The ABC of Atoms
, and it formed the central idea in a play called
Wings over Europe
, in which scientists threaten world leaders that they will use atom bombs to destroy the major cities of the world unless an international policy is agreed to use the tremendous energy released by nuclear reactions. The play had premiered in New York in 1928, but, by a strange coincidence, was showing in London at the very time that news broke about the splitting of the atom by Cockcroft and Walton.

As it turned out, of course, the fear of the energy that might be released from within a nucleus was, in general, well founded. However, for the moment it was premature; the kind of energy release observed in the disintegration of lithium, though extremely impressive, could not be used to make explosives. This is for two reasons. First, though there is a dramatic difference between the energy of the penetrating protons and the energy of the released alpha particles, one has to bear in mind that, at 125,000 volts, only one proton in about ten million will penetrate the nucleus. Thus, the total energy needed to release the sixteen million volts of the two alpha particles is about 1.25 billion volts. Second, an explosive requires a chain reaction, which had not yet been witnessed and which could not possibly occur with the disintegration of a light element like lithium. Rutherford’s famous dismissal as ‘moonshine’ of the idea that atomic-splitting might be a source of energy in the future was, therefore, perfectly reasonable in the light of what was then known. However, it was while pondering a report of that remark in
The Times
that it suddenly occurred to the Hungarian scientist Leo Szilard, that, if one could find an element that disintegrated when bombarded with neutrons, and if that element emitted two neutrons for every one it absorbed, then a chain reaction could occur that
would
be a source of enormous energy.

It would take a few years for the rest of the world to catch up with Szilard. Meanwhile, the story of Cockcroft and Walton’s achievement was picked up by newspapers all over the world, including the
New York Times
, which in its Sunday edition on 8 May, under the heading ‘The Atom Is Giving Up Its Mighty Secrets’, described the experiment and commented: ‘Never was a result more unexpected obtained.’ This was no doubt read by Ernest Lawrence, who was at that time preparing to get married to Mary Blumer in New Haven, Connecticut. The wedding took place on 14 May, by which time, it is certain, Lawrence knew that he had been pre-empted as the first person to smash the atom. It is therefore not true, as legend has it, that he heard the news while on honeymoon. It is possible, however, that the telegram his assistant James Brady remembers receiving – ‘Cockcroft and Walton have disintegrated the lithium atom. Get lithium from chemistry department and start preparations to repeat with cyclotron’ – was sent by him while still on honeymoon.

The news from the Cavendish did not stop Lawrence from building bigger and bigger machines, nor did it damage his ability to attract huge funds for these projects – indeed, if anything, it helped by stirring up interest – but it must have made him aware of the importance of being well informed about theory, just as Chadwick’s discovery of the neutron had made Oppenheimer aware of the importance of keeping up with experimental developments.

The fourth and final major development in experimental physics during the
annus mirabilis
of 1932 – the discovery of the positron by Carl Anderson of Caltech – exhibited a lack of communication between theorists and experimentalists that seems nothing short of bizarre. As Graham Farmelo, Paul Dirac’s biographer, has said: ‘Many of the characters in this strange denouement, including Dirac, behaved in ways that are now barely comprehensible.’

Among those characters whose behaviour seems inexplicable was Oppenheimer himself. There are several reasons why one might have expected Oppenheimer to have been in close contact with Anderson while he conducted the research that led to the discovery of the positron. In the first place, Anderson, as we have seen, had been a student of his. Indeed, during his first lecture course at Caltech in 1930, Anderson had been his
only
student. Second, though Anderson’s postdoctoral research was conducted with Millikan rather than with Oppenheimer, it was on a subject in which Oppenheimer had a deep interest: the nature of cosmic rays. Third, the hypothetical existence of the positron, a particle with the same mass as an electron but with a positive rather than negative charge, had been discussed by Oppenheimer in print, when he showed that Dirac’s theory of quantum electrodynamics – the very theory he had tried to explain in a lecture attended by Anderson – demanded it. And yet, despite
all this, when Anderson discovered the positron, he did so apparently unaware that the existence of such a particle had been predicted by Dirac, or indeed that the possibility of its existence had been discussed at all by anybody.