by Marcus Chown
By the late 1920s, a new puzzle began to worry Pauli and his peers. It concerned radioactive ‘beta decay’. A beta particle was one of the three distinct types of radiation spat out by the nucleus of an unstable atom as it ‘decayed’, rearranging its constituents to attain a more stable state. In 1899, three years after the discovery of radioactivity by Frenchman Henri Becquerel, New Zealand physicist Ernest Rutherford had shown that beta particles were ‘electrons’ – not the common-or-garden variety that orbited the nucleus of an atom, but electrons from inside the nucleus.
In the world of the atomic nucleus, greater stability is synonymous with lower energy. Consequently, when a nucleus decays, it drops from a higher to a lower energy ‘state’. The excess energy is spat out as an alpha particle, beta particle or gamma ray. Experimenters observed that alpha particles and gamma rays were emitted at precise energies, which made perfect sense if those energies were equal to the difference in energy between the initial and final states of the nucleus. However, the English physicist James Chadwick discovered something peculiar about beta particles in 1914: unlike their cousins, they were emitted not with a precise energy but with a continuous range of energies.
Think of a gun, which uses a fixed amount of energy to fire a bullet. Every bullet exits the gun at the same speed. It is never the case that one leaves at moderate speed, the next at high speed and the one after that so slowly that it dribbles out the end of the gun barrel. But this is precisely what the tiny electron bullets spat out in beta decay do. Not surprisingly, physicists were shocked at what Chadwick’s experiment was telling them.
This behaviour of beta particles might, of course, have a perfectly mundane explanation. Perhaps before escaping they bounced around inside an atom like a ball bearing in a pinball machine, striking multiple electrons and losing a portion of their energy to each one. However, by 1927, this possibility had been ruled out by an experiment by Charles Ellis and William Wooster at Cambridge University.7 The beta particle puzzle remained, and was so serious that it caused Niels Bohr, one of the founding fathers of quantum theory and the greatest physicist of the twentieth century after Einstein, to question one of the foundation stones of physics – that energy can neither be created nor destroyed but only transformed from one type to another. Perhaps in the world of the atom, Bohr suggested, processes do not obey the ‘law of conservation of energy’.
Enter Pauli, a physicist at the Swiss Federal Institute of Technology in Zurich. To him, the conservation of energy was like a life raft in a violent, storm-tossed sea, and abandoning it was absolutely unthinkable. ‘Bohr is on entirely the wrong track,’ he said. But what, then, was the solution to the beta particle puzzle?
Pauli was having the worst year of his life. Two years earlier, in November 1927, his mother, having been abandoned by her husband, committed suicide. The event had such a profound effect on Pauli that he left the Catholic Church, no doubt feeling abandoned by God. Then, on 23 December 1929, he married Käthe Deppner, a twenty-three-year-old cabaret dancer from Berlin six years his junior. When she met Pauli she was seeing a chemist called Paul Goldfinger, and she continued the affair during their marriage. An anguished Pauli, who was not even living with his wife, told a friend that he was only ‘loosely married’.8
Losing his wife to another man hurt, but Pauli felt the humiliation even more keenly because it affected his pride. ‘Had she taken a bullfighter I would have understood,’ he complained to friends. ‘With such a man I could not compete – but a chemist – such an average chemist!’9
Pauli’s troubled marriage to Deppner resulted in him developing a drink problem and a smoking habit.10 ‘With women and me things don’t work out at all,’ he wrote despairingly. ‘This, I am afraid, I have to live with, but it is not always easy. I am somewhat afraid that, in getting older, I will feel increasingly lonely.’11, 12
In the darkest times, occupying his mind with the problems thrown up by quantum theory may have served as an escape from his troubles, but this may have further strained his relationship with Deppner. She reported that Pauli received many letters from physicists, especially quantum pioneer Werner Heisenberg, and would walk around in their apartment ‘like a caged lion … formulating his answers in the most biting and witty manner’.13 It was during the eleven anguished months that he was loosely married to Deppner that Pauli came up with the idea for solving the puzzle of beta decay.
Pauli set out his solution to the problem on 4 December 1930, in an open letter to fellow scientists at a meeting in Germany.14 ‘Dear Radioactive Ladies and Gentlemen,’ it began. ‘Unfortunately, I cannot appear personally in Tübingen, since I am indispensable here in Zurich because of a ball on the night of 6 to 7 December.’ The dance was at the ‘Baur au Lac’, the most distinguished hotel in the centre of Zurich, and it was a mere ten days since his divorce. Emotionally bruised though he was, Pauli intended to get straight back on the horse and find himself another woman.
The letter was read out aloud to attendees at the Tübingen meeting, including Lise Meitner, who would later play a crucial role in the discovery of ‘nuclear fission’. Pauli pointed out that even if a fixed amount of energy was available in beta decay, the fact that the electron emitted from the nucleus did not have a fixed amount could be explained if it shared it with a hitherto unknown particle.
Think of the gun again. If a bullet emerged from the barrel with a second projectile, the two would share the available energy. If the second projectile took very little of the energy and the bullet took the lion’s share, it would be expelled at high speed. If the second projectile took most of the energy and the bullet had very little energy, it might emerge at such a low speed that it dribbled out the end of the gun. Depending on how much of the available energy was used by the second projectile, the bullet could have any of a range of possible energies.
However, no second particle had been identified accompanying the electron emitted in beta decay. Pauli’s new particle must therefore interact very rarely with the atoms of normal matter, and he estimated that it would take a ten-centimetre-thick lead wall to stop it in its tracks.
On the hypothetical particle’s other properties, Pauli was also quite specific. In order for it to not noticeably affect the mass of a nucleus, it must weigh very little, if anything at all. He did not realise that it might not actually exist in the nucleus but instead be created at the moment of emission, just as a photon of light is created at the moment of emission and is in no sense taken from a pre-existing ‘bag of photons’ within an atom. Pauli was also specific about the electric charge of the hypothetical particle, which, like energy, cannot be created or destroyed. In beta decay, for instance, there is no net change in the total charge – though the nucleus increases its positive charge, this is compensated for by the negative charge carried by the emitted electron.* In order for the new particle not to upset this delicate balance, it must therefore carry no charge. In recognition of its electrical neutrality, Pauli christened it a ‘neutron’, a name that would later be changed to ‘neutrino’.
‘I don’t feel secure enough to publish anything about this idea,’ Pauli wrote in his letter to the Tübingen meeting. The neutrino was a ‘desperate remedy’. The reason was that, in 1930, only three subatomic building blocks of matter were known: the ‘proton’ in the nucleus of the atom; the electron, which orbited the nucleus; and the photon, the particle of light. By adding another particle, Pauli was increasing the number of nature’s fundamental building blocks by a third.
The first time Pauli announced the neutrino in public was on 16 June 1931 at the inaugural summer meeting of the American Physical Society in Pasadena, but it gained more traction among physicists four months later, at a meeting in Rome organised by Enrico Fermi. Fermi, who would turn out to be the greatest Italian scientist since Galileo, had, like Pauli, made key contributions to quantum theory. He was instantly captivated by the Austrian physicist’s idea, not simply because it solved the problem of the spread of energy of beta
particles, but because it also fixed another problem: that of spin.
Physicists had discovered that subatomic particles behave as if they are spinning, even though they are not. Like everything else in the submicroscopic quantum realm, spin comes in indivisible chunks, or ‘quanta’. Since a spinning charge acts like a tiny magnet, it is possible to deduce the spin of a particle from the way in which it is deflected by a magnetic field. The proton, neutron and electron all turn out to have a spin of ½. (For historic reasons, the smallest chunk is half of a particular value.)15 In recognition of the behaviour of particles with ‘half-integer spin’, an idea that was elucidated principally by Fermi, they are known as ‘fermions’.
Spin, like electric charge and momentum, is one of those quantities that never changes, or is conserved.† However, if a neutron (spin ½) changes into a proton (spin ½) and an electron (spin ½), the final spins add up to either 1 – if the proton and electron spin the same way – or 0 if they spin in opposite directions and their spins cancel each other out. Neither of these is the spin ½ of the initial neutron. However, Pauli, in his letter to the Tübingen meeting, had not only proposed that the neutrino has no electric charge, very little mass and that it interacts with normal matter very rarely – he had postulated that it has a spin of ½. This made it possible for the spins of the proton, electron and neutrino (½ + ½ – ½ = ½) to equal the spin of the initial neutron (½).
Never before in the history of physics had anyone predicted the existence of a new entity that solved so many problems simultaneously and whose characteristics – spin, electric charge, mass and ability to penetrate matter – were so precisely pinned down by experimental observations. It caught Fermi’s imagination to such an extent that, after the October 1931 meeting in Rome, he was spurred to develop a revolutionary theory of beta decay.16
In the couple of years it took for Fermi to incubate his ideas, two new subatomic particles came to light, as mentioned earlier. In August 1932, Carl Anderson, studying ‘cosmic rays’ at the California Institute of Technology, found the first particle of ‘antimatter’ – a positively charged twin of the electron, which he christened the ‘positron’.‡ And in January 1932, James Chadwick at Cambridge University discovered a second constituent of the nucleus, identical in mass to the positively charged proton but with no electric charge. It was the discovery of the ‘neutron’ that caused Fermi to suggest a new name for Pauli’s hypothetical particle, neutrino being Italian for ‘little neutral one’.
Fermi’s theory of beta decay, when it was published in 1934, was a triumph. It required the existence of a third fundamental force of nature, in addition to the well-known gravitational and electromagnetic forces. The new ‘interaction’, which Fermi christened ‘the weak force’, operated only over a very short range within the atomic nucleus, which was why nobody had noticed it before. It acted to change a neutron in a nucleus into a proton and simultaneously create an electron and an antineutrino.
Fermi’s theory also permitted the reverse process, in which a proton captured a neutrino, causing it to change into a neutron and emit a positron. (In fact, this is the process that creates a neutrino; beta decay creates an antineutrino, which was what Pauli was actually describing.) The physicists Hans Bethe and Rudolf Peierls immediately pointed out that such ‘inverse beta decay’ would, in theory, permit a neutrino flying through space to be stopped by matter and to therefore be detected, though this would happen extremely rarely.
Fermi did not call the new interaction the weak force for nothing. It was about ten trillion times weaker than the electromagnetic force that holds together the atoms in our bodies. It was so weak, in fact, that the chance of a neutrino being stopped by a proton in an atomic nucleus was calculated to be close to zero.17 Whereas Pauli had thought a neutrino might be halted by a piece of lead about ten centimetres thick, according to Fermi’s theory it would require a layer of lead many light years thick.§¶18 As the American novelist Michael Chabon would later observe, ‘Eight solid light years of lead … is the thickness of that metal in which you would need to encase yourself if you wanted to keep from being touched by neutrinos. I guess the little fuckers are everywhere.’19
Despite Fermi’s theory of beta decay bolstering the case for the neutrino, many remained sceptical of its existence. And who could honestly blame them? As Nobel Prize-winning American physicist Leon Lederman would one day observe, ‘Neutrinos … win the minimalist contest: zero charge, zero radius, and very possibly zero mass.’20
One of the sceptics was the English astronomer Arthur Eddington. ‘Just now nuclear physicists are writing a great deal about hypothetical particles called neutrinos supposed to account for certain peculiar facts observed in beta-ray disintegration,’ he said. ‘We can perhaps best describe the neutrinos as little bits of spin-energy that have got detached. I am not much impressed by the neutrino theory.’
Eddington stopped short of saying that he did not believe in neutrinos. ‘I have to reflect that a physicist may be an artist, and you never know where you are with artists.’ If neutrinos did exist, Eddington recognised the problem of proving it, but even here he was cautious. ‘Dare I say that experimental physicists will not have sufficient ingenuity to make neutrinos? Whatever I may think, I am not going to be lured into a wager against the skill of experimenters,’ he said. ‘If they succeed in making neutrinos, perhaps even in developing industrial applications of them, I suppose I shall have to believe – though I may feel that they have not been playing quite fair.’21
The undetectability of neutrinos was a major concern even to those who believed in their existence. The irony is that Pauli, a man who so feared loneliness, had postulated the existence of the loneliest entity in creation – a particle so mind-bogglingly antisocial that it interacts with hardly anything in the universe. ‘I have done a terrible thing,’ he said. ‘I have postulated a particle that cannot be detected.’ Leading physicists were in agreement that finding the neutrino would be impossible, and Pauli himself bet a case of champagne that nobody would ever catch one.
Los Alamos, New Mexico, November 1955
Frederick Reines had been doing the impossible for more than a decade. When he joined the Manhattan Project in 1944, it had seemed impossible that they would be able to create a runaway nuclear chain reaction, releasing a million times more energy, pound for pound, than dynamite. But they achieved that feat at Alamagordo on 16 July 1945. ‘I have become Death, the destroyer of worlds,’ Robert Oppenheimer, director of the Manhattan Project, had quoted from the Bhagavadgita, as they watched a mushroom cloud rise into the dawn sky above the New Mexico desert.
Later, it had seemed impossible that they could create the ‘super’, a device that used an atomic bomb as a trigger and unleashed the energy of the Sun itself. But they achieved that feat too, with the detonation of the hydrogen bomb in Enewetak Atoll on 1 November 1952.
They had always been faced with impossible challenges, but they had met them head-on, and triumphed. For the test of a boosted atomic bomb in 1951, for instance, they had known that their electronics would be fried when the intense flash of gamma rays from the explosion generated a huge surge of electricity in the signal cables running from the bomb tower to the instrumentation bunker. The only thing providing shielding on the scale they required was the island on which they were testing the bomb, so they simply dug up one side of the island and piled it on top of the other.22
The impossible challenges of the bomb tests had instilled in all of them a ‘can-do’ spirit and a tendency to ‘think big’. It was exactly this mindset that had led Reines to seriously consider the impossible challenge of detecting the neutrinos from the explosion of a nuclear bomb.
In 1951, he had returned to the US from a series of successful bomb tests on Enewetak Atoll. Tired and jaded after six gruelling years with the weapons programme, he was in desperate need of a break. He asked the leader of the theoretical division at Los Alamos for time off from his duties to think about fundamental
physics, and Carson Mark, who was an enlightened man, granted him his request. Reines was given a bare office, where he sat staring at a blank pad of paper for several months. He asked himself what he wanted to do with his life, and for a long time he did not know. But then he thought of the neutrino.
At Los Alamos, Reines had served on the ‘Bomb-Test Steering and Liaison Group’. On occasion, it had tossed around the wild idea of piggy-backing physics experiments on nuclear tests and using the intense burst of heat radiation, gamma rays and neutrons to study fundamental phenomena. Reines knew that a nuclear fireball generated one additional type of radiation. When a nucleus of uranium or plutonium ‘fissions’, it creates two unstable ‘daughter’ nuclei. Each nucleus, in its desperate quest for stability, undergoes on average six beta decays, every time spitting out an antineutrino. As a result, a nuclear explosion creates an intense burst of antineutrinos.
The chance of detecting one antineutrino was impossibly low, but if there were vast numbers of them, Reines reasoned, the odds of ensnaring one would be hugely improved.
One day, in the summer of 1951, Reines heard that Enrico Fermi himself was visiting Los Alamos and was installed in an office just down the corridor. Since creating the theory of beta decay in Rome in the early 1930s, Fermi had won the 1938 Nobel Prize in Physics and fled Mussolini’s fascist dictatorship for America. On 2 December 1942, he had changed the course of history: in a crude ‘pile’ of uranium and graphite on a squash court under the West Stands of the University of Chicago’s Stagg Field, he had unleashed the stupendous energy of the atomic nucleus in the world’s first sustained nuclear chain reaction.
Reines knocked nervously on Fermi’s door. When he told him of his plan to detect neutrinos in the blast of a nuclear explosion, Fermi, to his surprise, did not dismiss the idea out of hand and agreed that a nuclear explosion offered the best chance of detecting the elusive particles.