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 energy 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 poden. 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.