by Peter Watson
For Mark Oliphant, one of Rutherford’s protégés in the 1920s, the main hallway of the Cavendish, where the director’s office was, consisted of ‘uncarpeted floor boards, dingy varnished pine doors and stained, plastered walls, indifferently lit by a skylight with dirty glass.1 For C. P. Snow, however, who also trained there and described the lab in his first novel, The Search, the paint and the varnish and the dirty glass went unremarked. ‘I shall not easily forget those Wednesday meetings in the Cavendish. For me they were the essence of all the personal excitement in science; they were romantic, if you like, and not on the plane of the highest experience I was soon to know [of scientific discovery]; but week after week I went away through the raw nights, with east winds howling from the fens down the old streets, full of a glow that I had seen and heard and been close to the leaders of the greatest movement in the world.’ Rutherford, who followed Maxwell as director of the Cavendish in 1919, evidently agreed. At a meeting of the British Association in 1923 he startled colleagues by suddenly shouting out, ‘We are living in the heroic age of physics!’2
In some ways, Rutherford himself – now a rather florid man, with a moustache and a pipe that was always going out – embodied in his own person that heroic age. During World War I, particle physics had been on hold, more or less. Officially, Rutherford was working for the Admiralty, researching submarine detection. But he carried on research when his duties allowed. And in the last year of war, in April 1919, just as Arthur Eddington was preparing his trip to West Africa to test Einstein’s predictions, Rutherford sent off a paper that, had he done nothing else, would earn him a place in history. Not that you would have known it from the paper’s title: ‘An Anomalous Effect in Nitrogen.’ As was usual in Rutherford’s experiments, the apparatus was simple to the point of being crude: a small glass tube inside a sealed brass box fitted at one end with a zinc-sulphide scintillation screen. The brass box was filled with nitrogen and then through the glass tube was passed a source of alpha particles – helium nuclei – given off by radon, the radioactive gas of radium. The excitement came when Rutherford inspected the activity on the zinc-sulphide screen: the scintillations were indistinguishable from those obtained from hydrogen. How could that be, since there was no hydrogen in the system? This led to the famously downbeat sentence in the fourth part of Rutherford’s paper: ‘From the results so far obtained it is difficult to avoid the conclusion that the long-range atoms arising from collision of [alpha] particles with nitrogen are not nitrogen atoms but probably atoms of hydrogen…. If this be the case, we must conclude that the nitrogen atom is disintegrated.’ The newspapers were not so cautious. Sir Ernest Rutherford, they shouted, had split the atom.3 He himself realised the importance of his work. His experiments had drawn him away, temporarily, from antisubmarine research. He defended himself to the overseers’ committee: ‘If, as I have reason to believe, I have disintegrated the nucleus of the atom, this is of greater significance than the war.’4
In a sense, Rutherford had finally achieved what the old alchemists had been aiming for, transmuting one element into another, nitrogen into oxygen and hydrogen. The mechanism whereby this artificial transmutation (the first ever) was achieved was clear: an alpha particle, a helium nucleus, has an atomic weight of 4. When it was bombarded on to a nitrogen atom, with an atomic weight of 14, it displaced a hydrogen nucleus (to which Rutherford soon gave the name proton). The arithmetic therefore became: 4+14-1=17, the oxygen isotope, O17.5
The significance of the discovery, apart from the philosophical one of the transmutability of nature, lay in the new way it enabled the nucleus to be studied. Rutherford and Chadwick immediately began to probe other light atoms to see if they behaved in the same way. It turned out that they did – boron, fluorine, sodium, aluminum, phosphorus, all had nuclei that could be probed: they were not just solid matter but had a structure. All this work on light elements took five years, but then there was a problem. The heavier elements were, by definition, characterised by outer shells of many electrons that constituted a much stronger electrical barrier and would need a stronger source of alpha particles if they were to be penetrated. For James Chadwick and his young colleagues at the Cavendish, the way ahead was clear – they needed to explore means of accelerating particles to higher velocities. Rutherford wasn’t convinced, preferring simple experimental tools. But elsewhere, especially in America, physicists realised that one way ahead lay with particle accelerators.
Between 1924 and 1932, when Chadwick finally isolated the neutron, there were no breakthroughs in nuclear physics. Quantum physics, on the other hand, was an entirely different matter. Niels Bohr’s Institute of Theoretical Physics opened in Copenhagen on 18 January 1921. The land had been given by the city, appropriately enough next to some soccer fields (Niels and his brother, Harald, were both excellent players).6 The large house, on four floors, shaped like an ‘L,’ contained a lecture hall, library, and laboratories (strange for an institute of theoretical physics), as well as a table-tennis table, where Bohr also shone. ‘His reactions were very fast and accurate,’ says Otto Frisch, ‘and he had tremendous will power and stamina. In a way those qualities characterised his scientific work as well.’7 Bohr became a Danish hero a year later when he won the Nobel Prize. Even the king wanted to meet him. But in fact the year was dominated by something even more noteworthy – Bohr’s final irrevocable linking of chemistry and physics. In 1922 Bohr showed how atomic structure was linked to the periodic table of elements drawn up by Dmitri Ivanovich Mendeléev, the nineteenth-century Russian chemist. In his first breakthrough, just before World War I, Bohr had explained how electrons orbit the nucleus only in certain formations, and how this helped explain the characteristic spectra of light emitted by the crystals of different substances. This idea of natural orbits also married atomic structure to Max Planck’s notion of quanta. Bohr now went on to argue that successive orbital shells of electrons could contain only a precise number of electrons. He introduced the idea that elements that behave in a similar way chemically do so because they have a similar arrangement of electrons in their outer shells, which are the ones most used in chemical reactions. For example, he compared barium and radium, which are both alkaline earths but have very different atomic weights and occupy, respectively, the fifty-sixth and eighty-eighth place in the periodic table. Bohr explained this by showing that barium, atomic weight 137.34, has electron shells filled successively by 2, 8,18, 18, 8, and 2 (=56) electrons. Radium, atomic weight 226, has on the other hand electron shells filled successively by 2, 8, 18, 32, 18, 8, and 2 (=88) electrons.8 Besides explaining their position on the periodic table, the fact that the outer shell of each element has two electrons means barium and radium are chemically similar despite their considerable other differences. As Einstein said, ‘This is the highest form of musicality in the sphere of thought.’9
During the 1920s the centre of gravity of physics – certainly of quantum physics – shifted to Copenhagen, largely because of Bohr. A big man in every sense, he was intent on expressing himself accurately, if painfully slowly, and forcing others to do so too. He was generous, avuncular, completely devoid of those instincts for rivalry that can so easily sour relations. But the success of Copenhagen also had to do with the fact that Denmark was a small country, neutral, where national rivalries of the Americans, British, French, Germans, Russians, and Italians could be forgotten. Among the sixty-three physicists of renown who studied at Copenhagen in the 1920s were Paul Dirac (British), Werner Heisenberg (German), and Lev Landau (Russian).10
There was also the Swiss-Austrian, Wolfgang Pauli. In 1924 Pauli was a pudgy twenty-three-year-old, prone to depression when scientific problems defeated him. One problem in particular had set him prowling the streets of the Danish capital. It was something that vexed Bohr too, and it arose from the fact that no one, just then, understood why all the electrons in orbit around the nucleus didn’t just crowd in on the inner shell. This is what should have happened, with the electrons emitting en
ergy in the form of light. What was known by now, however, was that each shell of electrons was arranged so that the inner shell always contains just one orbit, whereas the next shell out contains four. Pauli’s contribution was to show that no orbit could contain more than two electrons. Once it had two, an orbit was ‘full,’ and other electrons were excluded, forced to the next orbit out.11 This meant that the inner shell (one orbit) could not contain more than two electrons, and that the next shell out (four orbits) could not contain more than eight. This became known as Pauli’s exclusion principle, and part of its beauty lay in the way it expanded Bohr’s explanation of chemical behaviour.12 Hydrogen, for example, with one electron in the first orbit, is chemically active. Helium, however, with two electrons in the first orbit (i.e., that orbit is ‘full’ or ‘complete’), is virtually inert. To underline the point further, lithium, the third element, has two electrons in the inner shell and one in the next, and is chemically very active. Neon, however, which has ten electrons, two in the inner shell (filling it) and eight in the four outer orbits of the second shell (again filling those orbits), is also inert.13 So together Bohr and Pauli had shown how the chemical properties of elements are determined not only by the number of electrons the atom possesses but also by the dispersal of those electrons through the orbital shells.
The next year, 1925, was the high point of the golden age, and the centre of activity moved for a time to Göttingen. Before World War I, British and American students regularly went to Germany to complete their studies, and Göttingen was a frequent stopping-off place. Moreover, it had held on to its prestige and status better than most in the Weimar years. Bohr gave a lecture there in 1922 and was taken to task by a young student who corrected a point in his argument. Bohr, being Bohr, hadn’t minded. ‘At the end of the discussion he came over to me and asked me to join him that afternoon on a walk over the Hain Mountain,’ Werner Heisenberg wrote later. ‘My real scientific career only began that afternoon.’14 In fact it was more than a stroll, for Bohr invited the young Bavarian to Copenhagen. Heisenberg didn’t feel ready to go for two years, but Bohr was just as welcoming after the delay, and they immediately set about tackling yet another problem of quantum theory, what Bohr called ‘correspondence.’15 This stemmed from the observation that, at low frequencies, quantum physics and classical physics came together. But how could that be? According to quantum theory, energy – like light – was emitted in tiny packets; according to classical physics, it was emitted continuously. Heisenberg returned to Göttingen enthused but also confused. And Heisenberg hated confusion as much as Pauli did. And so when, toward the end of May 1925, he suffered one of his many attacks of hay fever, he took two weeks’ holiday in Heligoland, a narrow island off the German coast in the North Sea, where there was next to no pollen. An excellent pianist who could also recite huge tracts of Goethe, Heisenberg was very fit (he liked climbing), and he cleared his head with long walks and bracing dips in the sea.16 The idea that came to Heisenberg in that cold, fresh environment was the first example of what came to be called quantum weirdness. Heisenberg took the view that we should stop trying to visualise what goes on inside an atom, as it is impossible to observe directly something so small.17 All we can do is measure its properties. And so, if something is measured as continuous at one point, and discrete at another, that is the way of reality. If the two measurements exist, it makes no sense to say that they disagree: they are just measurements.
This was Heisenberg’s central insight, but in a hectic three weeks he went further, developing a method of mathematics, known as matrix math, originating from an idea by David Hilbert, in which the measurements obtained are grouped in a two-dimensional table of numbers where two matrices can be multiplied together to give another matrix.18 In Heisenberg’s scheme, each atom would be represented by one matrix, each ‘rule’ by another. If one multiplied the ‘sodium matrix’ by the ‘spectral line matrix,’ the result should give the matrix of wavelengths of sodium’s spectral lines. To Heisenberg’s, and Bohr’s, great satisfaction, it did; ‘For the first time, atomic structure had a genuine, though very surprising, mathematical base.’19 Heisenberg called his creation/discovery quantum mechanics.
The acceptance of Heisenberg’s idea was made easier by a new theory of Louis de Broglie in Paris, also published in 1925. Both Planck and Einstein had argued that light, hitherto regarded as a wave, could sometimes behave as a particle. De Broglie reversed this idea, arguing that particles could sometimes behave like waves. No sooner had de Broglie broached this theory than experimentation proved him right.20 The wave-particle duality of matter was the second weird notion of physics, but it caught on quickly. One reason was the work of yet another genius, the Austrian Erwin Schrödinger, who was disturbed by Heisenberg’s idea and fascinated by de Broglie’s. Schrödinger, who at thirty-nine was quite ‘old’ for a physicist, added the notion that the electron, in its orbit around the nucleus, is not like a planet but like a wave.21 Moreover, this wave pattern determines the size of the orbit, because to form a complete circle the wave must conform to a whole number, not fractions (otherwise the wave would descend into chaos). In turn this determined the distance of the orbit from the nucleus. Schrödinger’s work, set out in four long papers in Annalen der Physik in spring and summer 1926, was elegant and explained the position of Bohr’s orbits. The mathematics that underlay his theory also proved to be much the same as Heisenberg’s matrices, only simpler. Again knowledge was coming together.22
The final layer of weirdness came in 1927, again from Heisenberg. It was late February, and Bohr had gone off to Norway to ski. Heisenberg paced the streets of Copenhagen on his own. Late one evening, in his room high up in Bohr’s institute, a remark of Einstein’s stirred something deep in Heisenberg’s brain: ‘It is the theory which decides what we can observe.’23 It was well after midnight, but he decided he needed some air, so he went out and trudged across the muddy soccer fields. As he walked, an idea began to germinate in his brain. Unlike the immensity of the heavens above, the world the quantum physicists dealt with was unimaginably small. Could it be, Heisenberg asked himself, that at the level of the atom there was a limit to what could be known? To identify the position of a particle, it must impact on a zinc-sulphide screen. But this alters its velocity, which means that it cannot be measured at the crucial moment. Conversely, when the velocity of a particle is measured by scattering gamma rays from it, say, it is knocked into a different path, and its exact position at the point of measurement is changed. Heisenberg’s uncertainty principle, as it came to be called, posited that the exact position and precise velocity of an electron could not be determined at the same time.24 This was disturbing both practically and philosophically, because it implied that in the subatomic world cause and effect could never be measured. The only way to understand electron behaviour was statistical, using the rules of probability. ‘Even in principle,’ Heisenberg said, ‘we cannot know the present in all detail. For that reason everything observed is a selection from a plenitude of possibilities and a limitation on what is possible in the future.’25
Einstein, no less, was never very happy with the basic notion of quantum theory, that the subatomic world could only be understood statistically. It remained a bone of contention between him and Bohr until the end of his life. In 1926 he wrote a famous letter to the physicist Max Born in Göttingen. ‘Quantum mechanics demands serious attention,’ he wrote. ‘But an inner voice tells me that this is not the true Jacob. The theory accomplishes a lot, but it does not bring us closer to the secrets of the Old One. In any case, I am convinced that He does not play dice.’26
For close on a decade, quantum mechanics had been making news. At the height of the golden age, German preeminence was shown by the fact that more papers on the subject were published in that language than in all others put together.27 During that time, experimental particle physics had been stalled. It is difficult at this distance to say why, for in 1920 Ernest Rutherford had made an extraordinary pr
ediction. Delivering the Bakerian lecture before the Royal Society of London, Rutherford gave an insider’s account of his nitrogen experiment of the year before; but he also went on to speculate about future work.28 He broached the possibility of a third major constituent of atoms in addition to electrons and protons. He even described some of the properties of this constituent, which, he said, would have ‘zero nucleus charge.’ ‘Such an atom,’ he argued, ‘would have very novel properties. Its external [electrical] field would be practically zero, except very close to the nucleus, and in consequence it should be able to move freely through matter.’ Though difficult to discover, he said, it would be well worth finding: ‘it should readily enter the structure of atoms, and may either unite with the nucleus or be disintegrated by its intense field.’ If this constituent did indeed exist, he said, he proposed calling it the neutron.29
Just as James Chadwick had been present in 1911, in Manchester, when Rutherford had revealed the structure of the atom, so he was in the audience for the Bakerian lecture. After all, he was Rutherford’s right-hand man now. At the time, however, he did not really share his boss’s enthusiasm for the neutron. The symmetry of the electron and the proton, negative and positive, seemed perfect, complete. Other physicists may never have read the Bakerian lecture – it was a stuffy affair – and so never have had their minds stimulated. Throughout the late 1920s, however, anomalies built up. One of the more intriguing was the relationship between atomic weight and atomic number. The atomic number was derived from the nucleus’s electrical charge and a count of the protons. Thus helium’s atomic number was 2, but its atomic weight was 4. For silver the equivalent numbers were 47 and 107, for uranium 92 and 235 or 238.30 One popular theory was that there were additional protons in the nucleus, linked with electrons that neutralised them. But this only created another, theoretical anomaly: particles as small and as light as electrons could only be kept within the nucleus by enormous quantities of energy. That energy should show itself when the nucleus was bombarded and had its structure changed – and that never happened.31 Much of the early 1920s was taken up by repeating the nitrogen transmutation experiment with other light elements, so Chadwick scarcely had time on his hands. However, when the anomalies showed no sign of being satisfactorily resolved, he came round to Rutherford’s view. Something like a neutron must exist.