by Peter Watson
Chadwick was in physics by mistake.32 A shy man, with a gruff exterior that concealed his innate kindness, he had wanted to be a mathematician but turned to physics after he stood in the wrong queue at Manchester University and was impressed by the physicist who interviewed him. He had studied in Berlin under Hans Geiger but failed to leave early enough when war loomed and was interned in Germany for the duration. By the 1920s he was anxious to be on his way in his career.33 To begin with, the experimental search for the neutron went nowhere. Believing it to be a close union of proton and electron, Rutherford and Chadwick devised various ways of, as Richard Rhodes puts it, ‘torturing’ hydrogen. The next bit is complicated. First, between 1928 and 1930, a German physicist, Walter Bothe, studied the gamma radiation (an intense form of light) given off when light elements such as lithium and oxygen were bombarded by alpha particles. Curiously, he found intense radiation given off not only by boron, magnesium, and aluminum – as he had expected, because alpha particles disintegrated those elements (as Rutherford and Chadwick had shown) – but also by beryllium, which was not disintegrated by alpha particles.34 Bothe’s result was striking enough for Chadwick at Cambridge, and Irène Curie, daughter of Marie, and her husband Frédéric Joliot in Paris, to take up the German’s approach. Both labs soon found anomalies of their own. H. C. Webster, a student of Chadwick, discovered in spring 1931 that ‘the radiation [from beryllium] emitted in the same direction as the … alpha particles was harder [more penetrating] than the radiation emitted in a backward direction.’ This mattered because if the radiation was gamma rays – light – then it should spray equally in all directions, like the light that shines from a lightbulb. A particle, on the other hand, would behave differently. It might well be knocked forward in the direction of an incoming alpha.35 Chadwick thought, ‘Here’s the neutron.’36
In December 1931 Irène Joliot-Curie announced to the French Academy of Sciences that she had repeated Bothe’s experiments with beryllium radiation but had standardised the measurements. This enabled her to calculate that the energy of the radiation given off was three times the energy of the bombarding alphas. This order of magnitude clearly meant that the radiation wasn’t gamma; some other constituent must be involved. Unfortunately Irène Joliot-Curie had never read Rutherford’s Bakerian lecture, and she took it for granted that the beryllium radiation was caused by protons. Barely two weeks later, in mid-January 1932, the Joliot-Curies published another paper. This time they announced that paraffin wax, when bombarded by beryllium radiation, emitted high-velocity protons.37
When Chadwick read this account in the Comptes rendus, the French physics journal, in his morning mail in early February, he realised there was something very wrong with this description and interpretation. Any physicist worth his salt knew that a proton was 1,836 times heavier than an electron: it was all but impossible for a proton to be dislodged by an electron. While Chadwick was reading the report, a colleague named Feather, who had read the same article and was eager to draw his attention to it, entered his room. Later that morning, at their daily progress meeting, Chadwick discussed the paper with Rutherford. ‘As I told him about the Curie-Joliot observation and their views on it, 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. I mention it to emphasise the electrifying effect of the Curie-Joliot report. Of course, Rutherford agreed that one must believe the observations; the explanation was quite another matter.’38 Chadwick lost no time in repeating the experiment. The first thing to excite him was that he found the beryllium radiation would pass unimpeded through a block of lead three-quarters of an inch thick. Next, he found that bombardment by the beryllium radiation knocked the protons out of some elements by up to 40 centimetres, fully 16 inches. Whatever the radiation was, it was huge – and in terms of electrical charge, it was neutral. Finally, Chadwick took away the paraffin sheet that the Joliot-Curies had used so as to see what happened when elements were bombarded directly by beryllium radiation. Using an oscilloscope to measure the radiation, he found first that beryllium radiation displaced protons whatever the element, and crucially, that the energies of the displaced protons were just too huge to have been produced by gamma rays. Chadwick had learned a thing or two from Rutherford by now, including a habit of understatement. In the paper, entitled ‘Possible Existence of a Neutron,’ which he rushed to Nature, he wrote, ‘It is evident that we must either relinquish the application of the conservation of energy and momentum in these collisions or adopt another hypothesis about the nature of radiation.’ Adding that his experiment appeared to be the first evidence of a particle with ‘no net charge,’ he concluded, ‘We may suppose it to be the “neutron” discussed by Rutherford in his Bakerian lecture.’39 The process observed was 4He + 9Be→ 12C + n where n stands for neutron of mass number 1.40
The Joliot-Curies were much embarrassed by their failure to spot what was, for Rutherford and Chadwick, the obvious (though the French would make their own discoveries later). Chadwick, who had worked day and night for ten days to make sure he was first, actually announced his results initially to a meeting of the Kapitza Club at Cambridge, which had been inaugurated by Peter Kapitza, a young Russian physicist at the Cavendish. Appalled by the formal, hierarchical structure of Cambridge, Kapitza had started the club as a discussion forum where rank didn’t matter. The club met on Wednesdays, and on the night when Chadwick, exhausted, announced that he had discovered the third basic constituent of matter, he delivered his address – very short – and then remarked tartly, ‘Now I want to be chloroformed and put to bed for a fortnight.’41 Chadwick was awarded the Nobel Prize for his discovery, the result of dogged detective work. The neutral electrical charge of the new particle would allow the nucleus to be probed in a far more intimate way. Other physicists were, in fact, already looking beyond his discovery – and in some cases they didn’t like what they saw.
Physics was becoming the queen of sciences, a fundamental way to approach nature, with both practical and deeply philosophical implications. The trans-mutability of nature apart, its most philosophical aspect was its overlap with astronomy.
At this point we need to return – briefly – to Einstein. At the time he produced his theory of relativity, most scientists took it for granted that the universe was static. The nineteenth century had produced much new information about the stars, including ways to measure their temperatures and distances, but astronomers had not yet observed that heavenly bodies are clustered into galaxies, or that they were moving away from one another.42 But relativity had a surprise for astronomers: Einstein’s equations predicted that the universe must either be expanding or contracting. This was a wholly unexpected consequence, and so weird did it appear, even to Einstein himself, that he tinkered with his calculations to make his theoretical universe stand still. This correction he later called the biggest blunder of his career.43
Curiously, however, a number of scientists, while they accepted Einstein’s theory of relativity and the calculations on which it was based, never accepted the cosmological constant, and the correction on which it was based. Alexander Friedmann, a young Russian scientist, was the first man to cause Einstein to think again (‘cosmological constant’ was actually his term). Friedmann’s background was brutish. His mother had deserted his father – a cruel, arrogant man – taking the boy with her. Convicted of ‘breaking conjugal fidelity,’ she was sentenced by the imperial court to celibacy and forced to give up Alexander. He didn’t see his mother again for nearly twenty years. Friedmann taught himself relativity, during which time he realised Einstein had made a mistake and that, cosmological constant or no, the universe must be either expanding or contracting.44 He found this such an exciting idea that he dared to improve on Einstein’s work, developing a mathematical model to underline his conviction, and sent it to the German. By the early 1920s, however, Arthur Eddington had confirmed some
of Einstein’s predictions, and the great man had become famous and was snowed under with letters: Friedmann’s ideas were lost in the avalanche.45 Undaunted, Friedmann tried to see Einstein in person, but that move also failed. It was only when Friedmann was given an introduction by a mutual colleague that Einstein finally got to grips with the Russian’s ideas. As a result, Einstein began to have second thoughts about his cosmological constant – and its implications. But it wasn’t Einstein who pushed Friedmann’s ideas forward. A Belgian cosmologist, Georges Lemaître, and a number of others built on his ideas so that as the 1920s advanced, a fully realised geometric description of a homogeneous and expanding universe was fleshed out.46
A theory was one thing. But planets and stars and galaxies are not exactly small entities; they occupy vast spaces. Surely, if the universe really was expanding, it could be observed? One way to do this was by observation of what were then called ‘spiral nebulae.’ Nowadays we know that nebulae are distant galaxies, but then, with the telescopes of the time, they were simply indistinct smudges in the sky, beyond the solar system. No one knew whether they were gas or solid matter; and no one knew what size they were, or how far away. It was then discovered that the light emanating from spiral nebulae is shifted toward the red end of the spectrum. One way of illustrating the significance of this redshift is by analogy to the Doppler effect, after Christian Doppler, the Austrian physicist who first explained the observation in 1842. When a train or a motorbike comes toward us, its noise changes, and then, as it goes past and away, the noise changes a second time. The explanation is simple: as the train or bike approaches, the sound waves reach the observer closer and closer together – the intervals get shorter. As the train or bike recedes, the opposite effect occurs; the source of the noise is receding at all times, and so the interval between the sound waves gets longer and longer. Much the same happens with light: where the source of light is approaching, the light is shifted toward the blue end of the spectrum, while light where the source is receding is shifted toward the red end.
The first crucial tests were made in 1922, by Vesto Slipher at the Lowell Observatory in Flagstaff, Arizona.47 The Lowell had originally been built in 1893 to investigate the ‘canals’ on Mars. In this case, Slipher anticipated finding redshifts on one side of the nebulae spirals (the part swirling away from the observer) and blueshifts on the other side (because the spiral was swirling toward earth). Instead, he found that all but four of the forty nebulae he examined produced only redshifts. Why was that? Almost certainly, the confusion arose because Slipher could not really be certain of exactly how far away the nebulae were. This made his correlation of redshift and distance problematic. But the results were nonetheless highly suggestive.48
Three years elapsed before the situation was finally clarified. Then, in 1929, Edwin Hubble, using the largest telescope of the day, the 100-inch reflector scope at Mount Wilson, near Los Angeles, managed to identify individual stars in the spiral arms of a number of nebulae, thereby confirming the suspicions of many astronomers that ‘nebulae’ were in fact entire galaxies. Hubble also located a number of ‘Cepheid variable’ stars. Cepheid variables – stars that vary in brightness in a regular way (periods that range from 1—50 days) – had been known since the late eighteenth century, but it was only in 1908 that Henrietta Leavitt, at Harvard, showed that there is a mathematical relationship between the average brightness of a star, its size, and its distance from earth.49 Using the Cepheid variables that he could now see, Hubble was able to calculate how far away a score of nebulae were.50 His next step was to correlate those distances with their corresponding redshifts. Altogether, Hubble collected information on twenty-four different galaxies, and the results of his observations and calculations were simple and sensational: he discovered a straightforward linear relationship. The farther away a galaxy was, the more its light was redshifted.51 This became known as Hubble’s law, and although his original observations were made on twenty-four galaxies, since 1929 the law has been proven to apply to thousands more.52
Once more then, one of Einstein’s predictions had proved correct. His calculations, and Friedmann’s, and Lemaître’s, had been borne out by experiment: the universe was indeed expanding. For many people this took some getting used to. It involved implications about the origins of the universe, its character, the very meaning of time. The immediate impact of the idea of an expanding universe made Hubble, for a time, almost as famous as Einstein. Honours flowed in, including an honorary doctorate from Oxford, Time put him on its cover, and the observatory became a stopping-off place for famous visitors to Los Angeles: Aldous Huxley, Andrew Carnegie, and Anita Loos were among those given privileged tours. The Hubbies were taken up by Hollywood: the letters of Grace Hubble, Edwin’s wife, written in the early thirties, talk of dinners with Helen Hayes, Ethel Barrymore, Douglas Fairbanks, Walter Lippmann, Igor Stravinsky, Frieda von Richthofen (D. H. Lawrence’s widow), Harpo Marx and Charlie Chaplin.53 Jealous colleagues pointed out that, far from being a Galileo or Copernicus of his day, Hubble was not all that astute an observer, and that since his findings had been anticipated by others, his contribution was limited. But Hubble did arduous spadework and produced enough accurate data so that sceptical colleagues could no longer scoff at the theory of an expanding universe. It was one of the most astonishing ideas of the century, and it was Hubble who put it beyond doubt.
At the same time that physics was helping explain massive phenomena like the universe, it was still making advances in other areas of the minuscule world, in particular the world of molecules, helping us to a better understanding of chemistry. The nineteenth century had seen the first golden age of chemistry, industrial chemistry in particular. Chemistry had largely been responsible for the rise of Germany, whose nineteenth-century strength Hitler was so concerned to recover. For example, in the years before World War I, Germany’s production of sulphuric acid had gone from half that of Britain to 50 percent more; its production of chlorine by the modern electrolytic method was three times that of Britain; and its share of the world’s dyestuffs market was an incredible 90 percent.
The greatest breakthrough in theoretical chemistry in the twentieth century was achieved by one man, Linus Pauling, whose idea about the nature of the chemical bond was as fundamental as the gene and the quantum because it showed how physics governed molecular structure and how that structure was related to the properties, and even the appearance, of the chemical elements. Pauling explained the logic of why some substances were yellow liquids, others white powders, still others red solids. The physicist Max Perutz’s verdict was that Pauling’s work transformed chemistry into ‘something to be understood and not just memorised.’54
Born the son of a pharmacist, near Portland, Oregon, in 1901, Pauling was blessed with a healthy dose of self-confidence, which clearly helped his career. As a young graduate he spurned an offer from Harvard, preferring instead an institution that had started life as Throop Polytechnic but in 1922 was renamed the California Institute of Technology, or Caltech.55 Partly because of Pauling, Caltech developed into a major centre of science, but when he arrived there were only three buildings, surrounded by thirty acres of weedy fields, scrub oak, and an old orange grove. Pauling initially wanted to work in a new technique that could show the relationship between the distinctively shaped crystals into which chemicals formed and the actual architecture of the molecules that made up the crystals. It had been found that if a beam of X rays was sprayed at a crystal, the beam would disperse in a particular way. Suddenly, a way of examining chemical structure was possible. X-ray crystallography, as it was called, was barely out of its infancy when Pauling got his Ph.D., but even so he quickly realised that neither his math nor his physics were anywhere near good enough to make the most of the new techniques. He decided to go to Europe in order to meet the great scientists of the day: Niels Bohr, Erwin Schrödinger, Werner Heisenberg, among others. As he wrote later, ‘I had something of a shock when I went to Europe in 1926 and di
scovered that there were a good number of people around that I thought to be smarter than me.’56
So far as his own interest was concerned, the nature of the chemical bond, his visit to Zurich was the most profitable. There he came across two less famous Germans, Walter Heitler and Fritz London, who had developed an idea about how electrons and wave functions applied to chemical reactions.57 At its simplest, imagine the following: Two hydrogen atoms are approaching one another. Each is comprised of one nucleus (a proton) and one electron. As the two atoms get closer and closer to each other, ‘the electron of one would find itself drawn to the nucleus of the other, and vice versa. At a certain point, the electron of one would jump to the new atom, and the same would happen with the electron of the other atom.’ They called this an ‘electron exchange,’ adding that this exchange would take place billions of times a second.58 In a sense, the electrons would be ‘homeless,’ the exchange forming the ‘cement’ that held the two atoms together, ‘setting up a chemical bond with a definite length.’ Their theory put together the work of Pauli, Schrödinger, and Heisenberg; they also found that the ‘exchange’ determined the architecture of the molecule.59 It was a very neat piece of work, but from Pauling’s point of view there was one drawback about this idea: it wasn’t his. If he were to make his name, he needed to push the idea forward. By the time Pauling returned to America from Europe, Caltech had made considerable progress. Negotiations were under way to build the world’s biggest telescope at Mount Wilson, where Hubble would work. A jet propulsion lab was planned, and T. H. Morgan was about to arrive, to initiate a biology lab.60 Pauling was determined to outshine them ad. Throughout the early 1930s he released report after report, all part of the same project, and all having to do with the chemical bond. He succeeded magnificently in building on Heitler and London’s work. His early experiments on carbon, the basic constituent of life, and then on silicates showed that the elements could be systematically grouped according to their electronic relationships. These became known as Pauling’s rules. He showed that some bonds were weaker than others and that this helped explain chemical properties. Mica, for example, is a silicate that, as all chemists know, splits into thin, transparent sheets. Pauling was able to show that mica’s crystals have strong bonds in two directions and a weak one in a third direction, exactly corresponding to observation. In a second instance, another silicate we all know as talc is characterised by weak bonds all around, so that it crumbles instead of splitting, and forms a powder.61
