Modern Mind: An Intellectual History of the 20th Century (52 page)

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.
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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.
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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.
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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.’
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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.
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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.’
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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.’
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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.
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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 prediction. 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.
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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.
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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 ad, he was Rutherford’s right-hand man now. At the time, however, he did not ready 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.
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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.
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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.

Chadwick was in physics by mistake.
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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.
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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.
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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.
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Chadwick thought, ‘Here’s the neutron.’
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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.
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When Chadwick read this account in the
Comptes rendus,
the French physics journal, in his morning mad 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.’
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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.’
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The process observed was
⁴
He +
⁹
Be→
¹²
C +
n
where
n
stands for neutron of mass number 1.
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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.’
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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.
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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.
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