Modern Mind: An Intellectual History of the 20th Century

by Peter Watson

  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.

  As a theory, the planetary model was elegant, much more so than the ‘plum pudding’ version. But was it correct? To test his theory, Rutherford suspended a large magnet from the ceiling of his laboratory. Directly underneath, on a table, he fixed another magnet. When the pendulum magnet was swung over the table at a 45-degree angle and when the magnets were matched in polarity, the swinging magnet bounced through 90 degrees just as the alpha particles did when they hit the gold foil. His theory had passed the first test, and atomic physics had now become nuclear physics.13

  For many people, particle physics has been the greatest intellectual adventure of the century. But in some respects there have been two sides to it. One side is exemplified by Rutherford, who was brilliantly adept at thinking up often very simple experiments to prove or disprove the latest advance in theory. The other project has been theoretical physics, which involved the imaginative use of already existing information to be reorganised so as to advance knowledge. Of course, experimental physics and theoretical physics are intimately related; sooner or later, theories have to be tested. Nonetheless, within the dis
cipline of physics overall, theoretical physics is recognised as an activity in its own right, and for many perfectly respectable physicists theoretical work is all they do. Often the experimental verification of theories in physics cannot be tested for years, because the technology to do so doesn’t exist.

  The most famous theoretical physicist in history, indeed one of the most famous figures of the century, was developing his theories at more or less the same time that Rutherford was conducting his experiments. Albert Einstein arrived on the intellectual stage with a bang. Of all the scientific journals in the world, the single most sought-after collector’s item by far is the Annalen der Physik, volume XVII, for 1905, for in that year Einstein published not one but three papers in the journal, causing 1905 to be dubbed the annus mirabilis of science. These three papers were: the first experimental verification of Max Planck’s quantum theory; Einstein’s examination of Brownian motion, which proved the existence of molecules; and the special theory of relativity with its famous equation, E=mc2.

  Einstein was born in Ulm, between Stuttgart and Munich, on 14 March 1879, in the valley of the Danube near the slopes that lead to the Swabian Alps. Hermann, his father, was an electrical engineer. Though the birth was straightforward, Einstein’s mother Pauline received a shock when she first saw her son: his head was large and so oddly shaped, she was convinced he was deformed.14 In fact there was nothing wrong with the infant, though he did have an unusually large head. According to family legend, Einstein was not especially happy at elementary school, nor was he particularly clever.15 He later said that he was slow in learning to talk because he was ‘waiting’ until he could deliver fully formed sentences. In fact, the family legend was exaggerated. Research into Einstein’s early life shows that at school he always came top, or next to top, in both mathematics and Latin. But he did find enjoyment in his own company and developed a particular fascination with his building blocks. When he was five, his father gave him a compass. This so excited him, he said, that he ‘trembled and grew cold.’16

  Though Einstein was not an only child, he was fairly solitary by nature and independent, a trait that was encouraged by his parents’ habit of encouraging self-reliance in their children at a very early age. Albert, for instance, was only three or four when he was given the responsibility of running errands, alone in the busy streets of Munich.17 The Einsteins encouraged their children to develop their own reading, and while studying math at school, Albert was discovering Kant and Darwin for himself at home – very advanced for a child.18 This did, however, help transform him from being a quiet child into a much more ‘difficult’ and rebellious adolescent. His character was only part of the problem here. He hated the autocratic approach used in his school, as he hated the autocratic side of Germany in general. This showed itself politically, in Germany as in Vienna, in a crude nationalism and a vicious anti-Semitism. Uncomfortable in such a psychological climate, Einstein argued incessantly with his fellow pupils and teachers, to the point where he was expelled, though he was thinking of leaving anyway. Aged sixteen he moved with his parents to Milan, attended university in Zurich at nineteen, though later he found a job as a patent officer in Bern. And so, half educated and half-in and half-out of academic life, he began in 1901 to publish scientific papers. His first, on the nature of liquid surfaces, was, in the words of one expert, ‘just plain wrong.’ More papers followed in 1903 and 1904. They were interesting but still lacked something – Einstein did not, after all, have access to the latest scientific literature and either repeated or misunderstood other people’s work. However, one of his specialities was statistical techniques, which stood him in good stead later on. More important, the fact that he was out of the mainstream of science may have helped his originality, which flourished unexpectedly in 1905. One says unexpectedly, so far as Einstein was concerned, but in fact, at the end of the nineteenth century many other mathematicians and physicists – Ludwig Boltzmann, Ernst Mach, and Jules-Henri Poincaré among them – were inclining towards something similar. Relativity, when it came, both was and was not a total surprise.19

  Einstein’s three great papers of that marvellous year were published in March, on quantum theory, in May, on Brownian motion, and in June, on the special theory of relativity. Quantum physics, as we have seen, was itself new, the brainchild of the German physicist Max Planck. Planck argued that light is a form of electromagnetic radiation, made up of small packets or bundles – what he called quanta. Though his original paper caused little stir when it was read to the Berlin Physics Society in December 1900, other scientists soon realised that Planck must be right: his idea explained so much, including the observation that the chemical world is made up of discrete units – the elements. Discrete elements implied fundamental units of matter that were themselves discrete. Einstein paid Planck the compliment of thinking through other implications of his theory, and came to agree that light really does exist in discrete units – photons. One of the reasons why scientists other than Einstein had difficulty accepting this idea of quanta was that for years experiments had shown that light possesses the qualities of a wave. In the first of his papers Einstein, showing early the openness of mind for which physics would become celebrated as the decades passed, therefore made the hitherto unthinkable suggestion that light was both, a wave at some times and a particle at others. This idea took some time to be accepted, or even understood, except among physicists, who realised that Einstein’s insight fitted the available facts. In time the wave-particle duality, as it became known, formed the basis of quantum mechanics in the 1920s. (If you are confused by this, and have difficulty visualising something that is both a particle and a wave, you are in good company. We are dealing here with qualities that are essentially mathematical, and all visual analogies will be inadequate. Niels Bohr, arguably one of the century’s top two physicists, said that anyone who wasn’t made ‘dizzy’ by the very idea of what later physicists called ‘quantum weirdness’ had lost the plot.)

  Two months after his paper on quantum theory, Einstein published his second great work, on Brownian motion.20 Most people are familiar with this phenomenon from their school days: when suspended in water and inspected under the microscope, small grains of pollen, no more than a hundredth of a millimetre in size, jerk or zigzag backward and forward. Einstein’s idea was that this ‘dance’ was due to the pollen being bombarded by molecules of water hitting them at random. If he was right, Einstein said, and molecules were bombarding the pollen at random, then some of the grains should not remain stationary, their movement cancelled out by being bombarded from all sides, but should move at a certain pace through the water. Here his knowledge of statistics paid off, for his complex calculations were borne out by experiment. This was generally regarded as the first proof that molecules exist.

  But it was Einstein’s third paper that year, the one on the special theory of relativity, published in June, that would make him famous. It was this theory which led to his conclusion that E=mc2. It is not easy to explain the special theory of relativity (the general theory came later) because it deals with extreme – but fundamental – circumstances in the universe, where common sense breaks down. However, a thought experiment might help.21 Imagine you are standing at a railway station when a train hurtles through from left to right. At the precise moment that someone else on the train passes you, a light on the train, in the middle of a carriage, is switched on. Now, assuming the train is transparent, so you can see inside, you, as the observer on the platform, will see that by the time the light beam reaches the back of the carriage, the carriage will have moved forward. In other words, that light beam has travelled slightly less than half the length of the carriage. However, the person inside the train will see the light beam hitting the back of the carriage at the same time as it hits the front of the carriage, because to that person it has travelled exactly half the length of the carriage. Thus the time the light beam takes to reach the back of the carriage is different for the two observers. But it is the s
ame light beam in each case, travelling at the same speed. The discrepancy, Einstein said, can only be explained by assuming that the perception is relative to the observer and that, because the speed of light is constant, time must change according to circumstance.

  The idea that time can slow down or speed up is very strange, but that is exactly what Einstein was suggesting. A second thought experiment, suggested by Michael White and John Gribbin, Einstein’s biographers, may help. Imagine a pencil with a light upon it, casting a shallow on a tabletop. The pencil, which exists in three dimensions, casts a shallow, which exists in two, on the tabletop. As the pencil is twisted in the light, or if the light is moved around the pencil, the shallow grows or shrinks. Einstein said in effect that objects essentially have four dimensions in addition to the three we are all familiar with – they occupy space-time, as it is now called, in that the same object lasts over time.22 And so if you play with a four-dimensional object the way we played with the pencil, then you can shrink and extend time, the way the pencil’s shallow was shortened and extended. When we say ‘play’ here, we are talking about some hefty tinkering; in Einstein’s theory, objects are required to move at or near the speed of light before his effects are shown. But when they do, Einstein said, time really does change. His most famous prediction was that clocks would move more slowly when travelling at high speeds. This anti-commonsense notion was actually borne out by experiment many years later. Although there might be no immediate practical benefit from his ideas, physics was transformed.23