The Basis of Everything

by Andrew Ramsey

  This personal acclaim sat jarringly alongside the pain of bereavement through those bitingly bitter weeks at Celyn. However, while Oliphant noted the melancholy that pervaded the damp, dark house in the early part of that sad stay, he also detected a gradual lightening of Rutherford’s mood as time passed.

  He pretended that the new honour was of secondary importance to him, but he could not help showing how he revelled in this recognition of his standing as a man of science. Despite his sadness, one morning he could be heard singing as he dressed. When the heaps of letters of congratulations, sent on from Cambridge, arrived each morning, he slit the envelopes with obvious pleasure, passing the contents to Miss de Renzie with some bright quip about the sender. Some he answered at once, but others were kept till he returned to Cambridge.27

  * * *

  The shadow of his daughter’s death would forever remain with Rutherford, but gradually the letters he wrote to Mary bore a more familiar theme, as he documented the latest focus of his laboratory work. The single-minded intent that had driven his earlier discoveries was beginning to reappear. While the dawning of 1931 had brought with it darkness and despair, the arrival of 1932 presented a markedly different outlook.

  It was a year that would be retrospectively hailed as nuclear physics’ annus mirabilis.

  10

  THE GOLDEN YEAR

  Cambridge, 1932

  As leader of the first assault on the tightly bound nuclei of light elements such as hydrogen and nitrogen, Rutherford understood more keenly than many that heavy artillery was needed to bombard the atoms at the far end of the periodic table. These materials, culminating in uranium – then the heaviest known naturally occurring element – contained such strong electrical forces of repulsion at their core that they were immune to the peppering of Rutherford’s beloved alpha particle bullets.

  In his 1927 address as Royal Society President, Rutherford had speculated that new tools capable of producing particles that moved in greater quantities and with increased energy were needed if nuclear physics were to continue progressing at the rate it had evolved over the past two decades.

  It has long been my ambition to have available for study a copious supply of atoms and electrons that have an individual energy far transcending that of the alpha and beta particles from radioactive bodies. I am hopeful that I may yet have my wish fulfilled, but it is obvious that many experimental difficulties will have to be surmounted before this can be realised, even on a laboratory scale.1

  A major step towards overcoming that obstacle was taken just over a year later, in January 1929, when Russian theoretical physicist George Gamow visited the Cavendish. Employing a new model utilising the recently postulated wave mechanics theory – a model formulated in conjunction with Niels Bohr at his Copenhagen Institute – Gamow proposed a fresh notion of how particles might penetrate the atomic nucleus. His theory was subsequently taken up by Oliphant’s close friend John Cockcroft, who saw that it raised the prospect of fully disintegrating nuclei of light atoms by bombarding them with artificially accelerated protons.

  Much as Rutherford distrusted the type of complex theory that Gamow and others expounded, and disliked even more a reliance on expensive machinery to solve problems at the expense of table-top experiments and cognitive ingenuity, the Cavendish nevertheless needed to embrace progress. Starting with the massive electromagnetic fields that Kapitza generated in his dedicated laboratory space, the string-and-sealing-wax model began to make way for electrical wiring and metal welds.

  In taking up Rutherford’s challenge to generate faster, more powerful particle bullets, Cockcroft joined forces with Ernest Walton, the Irish scholarship holder who had arrived on that same October morning as Oliphant in 1927. The premise of the pair’s work was to generate an abundant supply of protons by discharging an electrical current through hydrogen, and then using a strong electrical force to direct them.

  It was an aspiration of enormous complexity, and one that would require equally intricate equipment. But, if successful, it would represent a high watermark for the Cavendish on the rapidly changing landscape of nuclear physics.

  At least it would have done, had another seismic discovery not been made at the laboratory early in 1932, a few months before Cockcroft and Walton’s project reached fruition.

  * * *

  Rutherford had first aired his belief that the atomic nucleus might, in addition to charge-carrying protons, house an as yet unidentified neutral particle, in the section of his 1920 Bakerian Lecture that drew little attention. He had speculated that this ‘neutral doublet’, as he called it, would exhibit unique properties, among them a capacity to move freely through matter. It would therefore – in Rutherford’s futuristic vision – be readily able to penetrate the defences of the most intensely fortified heavy elements, because its neutral charge would mean it was immune to electrostatic repulsion. In modern military parlance, it would be a sub-atomic stealth bomber.

  If the broader scientific community, used to dealing only in proven experimental outcomes, gave little heed to the theory, and Rutherford himself rarely revisited it in public as he busied himself with delivering more immediate outcomes, the comments had resonated with James Chadwick. He had been among the audience at that 1920 lecture to the Royal Society in London. Quietly at first, he had begun his hunt for the elusive neutron that would dramatically alter the world’s understanding of matter, and the very future of humankind.

  Across a decade or more, Chadwick – often in collaboration with Rutherford, and amid his myriad supervisory and administrative duties at the Cavendish – had patiently performed one experiment after another. They involved searching for evidence of highly penetrative gamma radiation spontaneously produced by hydrogen, then by any number of rare gases, and then an assortment of rare elements.

  None of them had yielded worthwhile results. ‘I did quite a number of silly experiments,’ Chadwick later wrote, adding: ‘I must say, the silliest were done by Rutherford.’2

  In the course of this work, Chadwick had become intrigued by the results achieved when the alkaline earth metal beryllium was bombarded with radioactive particles. These, in turn, had ejected particles of high energy and penetration. ‘For a short but exciting time we thought we had found some evidence of the neutron,’ Chadwick later reported. ‘But somehow the evidence faded away. I was still groping in the dark.’3

  Then, in the latter half of 1931, Chadwick had been alerted to results published in Paris by Marie Curie’s scientist daughter, Irène, and her husband Frédéric Joliot-Curie. They reported observations of paraffin wax that emitted high-velocity protons when bombarded by gamma radiation.

  However, their conclusion that the highly penetrative gamma rays were causing protons to be fired from the paraffin at speed did not make sense to Chadwick. Gamma rays were known to be capable of knocking out electrons, but a proton was almost 2000 times heavier than an electron, and similarly difficult to budge. It was as if a table tennis ball fired at a bowling ball had been causing the latter to hurtle away. Chadwick suspected that the French experiment might, albeit unknowingly, have revealed the true energy source to be the neutral particle.

  In keeping with his quiet, unassuming character, Chadwick waited an hour or more for his daily 11am meeting with Rutherford before explaining to him the details of what he had read in the Comptes Rendus physics journal, and what he understood it to signify.

  ‘As I told him about the Curie-Joliot observation and their views on it,’ Chadwick recounted of that exchange with Rutherford, ‘I saw his growing amazement; and finally he burst out “I don’t believe it”. Such an impatient remark was utterly out of character, and in all my long association with him I recall no similar occasion.’4

  Using polonium – the element identified by Marie Curie, and named after her homeland – as his radiation source, Chadwick found that the alpha particles fired at beryllium produced rays that were then trained upon a paraffin target. It was then found these rays
could pass unimpeded through a sheet of lead up to two centimetres thick. This could only occur if those particles held a similar substantial mass to a proton but carried no charge, which would have seen them electrically repelled. It was proof that the neutral particle existed.

  Chadwick spent the next ten days, surviving on two or three hours’ sleep each night, substantiating his contention.

  The existence of a third essential constituent of matter, alongside the proton and electron, was formally announced to the world in a letter published in Nature on 27 February 1932, entitled ‘Possible Existence of a Neutron’.

  There was no equivocation such as he had shown to Rutherford when, fortified by dinner and drinks at Trinity Hall beforehand, Chadwick confirmed his stunning revelation to the Cavendish’s Kapitza Club. He then pronounced: ‘Now I want to be chloroformed and put to bed for a fortnight.’5

  Oliphant would never forget the exhilaration felt by the entire Cavendish team on that occasion.

  The intense excitement of all in the Cavendish, including Rutherford, was already remarkable, for we had heard rumours of Chadwick’s results. His account of the experiments was extremely lucid and convincing, and the ovation he received from his audience was spontaneous and warm. All enjoyed the story of a long quest, carried through with such persistence and vision, and they rejoiced in the success of a colleague.6

  On the strength of his discovery, Chadwick would receive the physics Nobel Prize in 1935, thus unleashing a stampede of experimentation that ultimately yielded an atomic bomb.

  In accepting that he and his wife had achieved the same results, yet failed to grasp their place in scientific history, Frédéric Joliot-Curie wrote:

  The word neutron had been used by the genius Rutherford . . . at a conference to denote a hypothetical neutral particle which, together with protons, made up the nucleus. This hypothesis had escaped the attention of most physicists, including ourselves. But it was still present at the Cavendish where Chadwick worked . . . Old laboratories with long tradition always have hidden riches.7

  While the identification of a new particle was of front-page importance to the world at large, its significance in the field of atomic research was, quite literally, earth shattering. In scale, the new particle was similar to a proton, but its absence of charge meant that the electrical barrier blocking intruders from entering an atom’s core – particularly in elements such as uranium with the greatest and most impenetrable atomic mass – was no obstacle despite the neutron’s size.

  ‘In fact,’ Nobel Prize winning physicist I.I. (Isidor) Rabi later declared, ‘the forces of attraction which hold nuclei together may well pull the neutron into the nucleus. When a neutron enters a nucleus, the effects are about as catastrophic as if the moon struck the earth.’8

  * * *

  While Chadwick’s triumph was toasted by the laboratory, the work of Cockcroft and Walton had attracted little notice. In their efforts to verify Rutherford’s other hypothesis, they were still attempting to generate sufficiently large electrical currents to enable protons to be accelerated to similarly high energies. This would allow them to be employed as projectiles to disintegrate atomic nuclei.

  If Rutherford had begun his investigation of sub-atomic structure by firing alpha particle bullets from a lead-lined box, particle accelerators were seen as the field gun, capable of smashing apart the most intractable forms of matter. But at the start of 1932, as Rutherford had suggested five years earlier, that point appeared dauntingly distant.

  Nor was the quest to achieve higher voltages in the name of science confined to the Cavendish. Years before Cockcroft and Walton began assembling a rectifying circuit from items they sourced and scrounged, three members of the University of Berlin’s Physics Institute had taken their ambitious project to the Italian Alps. They had installed an antenna between two mountains to await the regular summer thunderstorms, and when they came, saw sparks jumping almost twenty metres between metal spheres strung on lengths of steel cable. The resulting charge was estimated to deliver an electrical potential of around 15 million volts. It also cost the life of one of the researchers, who was blown off the mountainside by the unforgiving force they sought to tame.

  Attempts to artificially replicate massive voltages at Cambridge also brought a very real risk. Oliphant would ruefully recall the day he went to make an adjustment to electrical apparatus that was, in the interests of safety, located behind one of the lab’s internal brick walls. However, on this occasion, his fellow experimenter Bernard Kinsey had failed to switch off the 20,000-volt source beforehand. So when Oliphant – standing on the basement’s stone floor, which was forever slightly damp like the jute sacks of his boyhood pranks – reached for the equipment, he was sent flying by a shock that rendered him unconscious.

  He woke to find smoke wafting from the molten soles of his shoes, and Kinsey bent over him. ‘My God, my God, what have I done, have I killed him?’ Kinsey fretted.

  At which point Rutherford appeared, railing at Kinsey, whose dread had eased when Oliphant came to.

  Kinsey then launched his defence: ‘But, God’s bladders Professor, how was I to know he’d touch the bloody thing?’9

  Four decades later, Oliphant would receive a letter from another former Cavendish colleague who recalled the burned rubber singed into the ‘garage’ floor, noting: ‘I always thought that those heel marks should have been preserved under a glass plate to show the toughness of the real experimental physicist.’10

  The perilous pursuit of ever-higher energy sources was also challenging Cockcroft and Walton. They had learned through similarly fraught experience that results gained from a linear accelerator – where electrical fields were used to increase the speed at which charged particles travelled – were limited by the levels of voltage that could be applied to them. Once the electrical energy being imparted reached too great a level, the basic apparatus underwent electrical breakdown, in which the materials employed as insulators began operating as conductors.

  As so often manifests along discovery’s uncharted path, a solution arose from timely coincidence. In the midst of their so far unsuccessful attempts to accelerate particles, the pair were required to vacate their experimental workspace in the Cavendish basement, so it could be taken over by physical chemists. As a consequence, they built their equipment anew in a high-roofed, reclaimed lecture theatre in the nearby Balfour Building. This allowed Cockcroft to integrate, from the outset, an ingenious voltage-multiplying circuit he had yet to test with higher loads. Just as influential was his successful request for Rutherford to invest £1000 (around £65,000 today) in a 300-kilovolt transformer, a machine capable of delivering similar voltage to an average lightning strike.11

  It had taken several years for Cockcroft and Walton to piece together the prototype electrostatic accelerator that finally took shape next door to the Cavendish. In its earlier iterations, it had been variously constructed from glass cylinders recycled from petrol pumps, steel bicycle tubing, lead batteries, tungsten wire filaments and sheets of galvanised iron – all made airtight by a specially devised, low-pressure plasticine compound. Once installed in its new premises, the newly improved system could be exhaustively tested with steadily increased voltages. Air leaks caused by softening of the plasticine, or by errant sparks that pierced holes in the glass tubes, were constantly being mended by the two researchers, who often teetered on wooden ladders to reach the extremities of the sprawling device.

  Rutherford’s patience had begun to fray when he learned that the voltage issues had been resolved but problems remained with the velocity and direction of the proton beam that would carry the atom-smashing bullets. He urged the pair to ‘get on with it’; while anxious to see the final effects, he was little interested in the technical specifications required to reach that point.

  By mid-spring of 1932, the apparatus underwent its final conditioning, and the positively charged protons, directed by magnetic force, were unleashed upon a lithium target. Then, on
14 April, Walton recorded the telltale incandescence of alpha particles striking a zinc sulfide screen and immediately summoned Cockcroft. Upon verifying the authenticity of his colleague’s observations, Cockcroft phoned Rutherford, who arrived shortly afterwards.

  Scottish science journalist Ritchie Calder was coincidentally visiting the Cavendish at that epochal moment.

  Ernest, Lord Rutherford of Nelson, pushed aside a Geiger counter, a soldering iron and a cluttering of bits and pieces, and hoisted his six-foot frame and its matching bulk on to the laboratory bench. With his hat tipped to the back of his head and his feet dangling, he might have passed as a farmer at a cattle roup [auction] in his ancestral Perthshire or at a flax-sale in his native South Island, New Zealand. ‘Take over Cockcroft,’ he said, ‘it’s your show.’

  In the darkened hall, switches were thrown. The generators warmed, with the hum of a gathering storm. There was the throb of the pumps as they sucked the air out of the vacuum tubes. Lightning crackled and flashed as the high-tension spheres sparked. A tall glass pillar glowed with a luminous blue haze. Presently, there was a clicking sound, and a counter, like a mileage recorder in a motor-car, began to clock in the fragments of the splitting atoms.12

  Rutherford somehow folded his ample frame into the observation hut, which was not much more than a tea chest fitted with a blackout curtain, to witness the scintillations happening above his head, where the accelerating tube met the screen. After being helped out of the tiny box, he straightened himself upon a nearby stool and proclaimed: ‘Those scintillations look mighty like alpha-particle ones. I should know an alpha-particle scintillation when I see one, for I was in at the birth of the alpha particle and I have been observing them ever since.’13