Such confidence in Rutherford’s revolutionary ideas had not always been so evident. In the late 1890s Rutherford had developed the ideas of the French physicist Henri Becquerel. In turn, Becquerel had built on Wilhelm Conrad Röntgen’s discovery of X rays, which we encountered in chapter three. Intrigued by these mysterious rays that were given off from fluorescing glass, Becquerel, who, like his father and grandfather, was professor of physics at the Musée d’Histoire Naturelle in Paris, decided to investigate other substances that ‘fluoresced.’ Becquerel’s classic experiment occurred by accident, when he sprinkled some uranyl potassium sulphate on a sheet of photographic paper and left it locked in a drawer for a few days. When he looked, he found the image of the salt on the paper. There had been no naturally occurring light to activate the paper, so the change must have been wrought by the uranium salt. Becquerel had discovered naturally occurring radioactivity.2
It was this result that attracted the attention of Ernest Rutherford. Raised in New Zealand, Rutherford was a stocky character with a weatherbeaten face who loved to bellow the words to hymns whenever he got the chance, a cigarette hanging from his lips. ‘Onward Christian Soldiers’ was a particular favourite. After he arrived in Cambridge in October 1895, he quickly began work on a series of experiments designed to elaborate Becquerel’s results.3 There were three naturally radioactive substances – uranium, radium, and thorium – and Rutherford and his assistant Frederick Soddy pinned their attentions on thorium, which gave off a radioactive gas. When they analysed the gas, however, Rutherford and Soddy were shocked to discover that it was completely inert – in other words, it wasn’t thorium. How could that be? Soddy later described the excitement of those times in a memoir. He and Rutherford gradually realised that their results ‘conveyed the tremendous and inevitable conclusion that the element thorium was spontaneously transmuting itself into [the chemically inert] argon gas!’ This was the first of Rutherford’s many important experiments: what he and Soddy had discovered was the spontaneous decomposition of the radioactive elements, a modern form of alchemy. The implications were momentous.4
This wasn’t all. Rutherford also observed that when uranium or thorium decayed, they gave off two types of radiation. The weaker of the two he called ‘alpha’ radiation, later experiments showing that ‘alpha particles’ were in fact helium atoms and therefore positively charged. The stronger ‘beta radiation’, on the other hand, consisted of electrons with a negative charge. The electrons, Rutherford said, were ‘similar in all respects to cathode rays.’ So exciting were these results that in 1908 Rutherford was awarded the Nobel Prize at age thirty seven, by which time he had moved from Cambridge, first to Canada and then back to Britain, to Manchester, as professor of physics.5 By now he was devoting all his energies to the alpha particle. He reasoned that because it was so much larger than the beta electron (the electron had almost no mass), it was far more likely to interact with matter, and that interaction would obviously be crucial to further understanding. If only he could think up the right experiments, the alpha might even tell him something about the structure of the atom. ‘I was brought up to look at the atom as a nice hard fellow, red or grey in colour, according to taste,’ he said.6 That view had begun to change while he was in Canada, where he had shown that alpha particles sprayed through a narrow slit and projected in a beam could be deflected by a magnetic field. All these experiments were carried out with very basic equipment – that was the beauty of Rutherford’s approach. But it was a refinement of this equipment that produced the next major breakthrough. In one of the many experiments he tried, he covered the slit with a very thin sheet of mica, a mineral that splits fairly naturally into slivers. The piece Rutherford placed over the slit in his experiment was so thin – about three-thousandths of an inch – that in theory at least alpha particles should have passed through it. They did, but not in quite the way Rutherford had expected. When the results of the spraying were ‘collected’ on photographic paper, the edges of the image appeared fuzzy. Rutherford could think of only one explanation for that: some of the particles were being deflected. That much was clear, but it was the size of the deflection that excited Rutherford. From his experiments with magnetic fields, he knew that powerful forces were needed to induce even small deflections. Yet his photographic paper showed that some alpha particles were being knocked off course by as much as two degrees. Only one thing could explain that. As Rutherford himself was to put it, ‘the atoms of matter must be the seat of very intense electrical forces.’7
Science is not always quite the straight line it likes to think it is, and this result of Rutherford’s, though surprising, did not automatically lead to further insights. Instead, for a time Rutherford and his new assistant, Ernest Marsden, went doggedly on, studying the behaviour of alpha particles, spraying them on to foils of different material – gold, silver, or aluminium.8 Nothing notable was observed. But then Rutherford had an idea. He arrived at the laboratory one morning and ‘wondered aloud’ to Marsden whether (with the deflection result still in his mind) it might be an idea to bombard the metal foils with particles sprayed at an angle. The most obvious angle to start with was 45 degrees, which is what Marsden did, using foil made of gold. This simple experiment ‘shook physics to its foundations.’ It was ‘a new view of nature … the discovery of a new layer of reality, a new dimension of the universe.’9 Sprayed at an angle of 45 degrees, the alpha particles did not pass through the gold foil – instead they were bounced back by 90 degrees onto the zinc sulphide screen. ‘I remember well reporting the result to Rutherford,’ Marsden wrote in a memoir, ‘when I met him on the steps leading to his private room, and the joy with which I told him.’10 Rutherford was quick to grasp what Marsden had already worked out: for such a deflection to occur, a massive amount of energy must be locked up somewhere in the equipment used in their simple experiment.
But for a while Rutherford remained mystified. ‘It was quite the most incredible event that has ever happened to me in my life,’ he wrote in his autobiography. ‘It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration I realised that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greatest part of the mass of the atom was concentrated in a minute nucleus.’11 In fact, he brooded for months before feeling confident he was right. One reason was because he was slowly coming to terms with the fact that the idea of the atom he had grown up with – J. J. Thomson’s notion that it was a miniature plum pudding, with electrons dotted about like raisins – would no longer do.12 Gradually he became convinced that another model entirely was far more likely. He made an analogy with the heavens: the nucleus of the atom was orbited by electrons just as planets went round the stars.