The Physicists Read online

  Still, classical physics lost its dominance and there was a change of direction among physicists, somewhere near the turn of the century. The initiative didn’t come from abstract thought, but from some puzzling observations (much more as romantics expect scientific revolutions to start, though history often tells us otherwise). With the development of efficient air pumps, scientists could now investigate air – and other gases – at very low pressures. When an electric current was passed through such a gas, physicists were surprised to find ‘rays’ streaming off one of the electrodes (the cathode). A German physicist, Eugen Goldstein, christened them cathode rays – but what were they?

  In 1895, another German physicist, Wilhelm Roentgen, found that cathode rays produce another, even stranger, type of radiation when they hit a solid object. He called them X-rays: X for unknown, for these highly penetrating rays were unlike anything then known. The following year, a French physicist, Henri Becquerel, found that minerals containing uranium also produce radiation, quite spontaneously. Where did radiation come from? Clearly from components of the mineral itself – but how? It is not known whether any of the early observers guessed that the answer was individual atoms. Scientific papers are always written as though no one ever anticipated anything.

  Much of this activity was taking place in Paris. That itself was rather odd, for France at the time wasn’t as scientifically developed as England or Germany. Academic salaries were meagre, and laboratory equipment primitive beyond belief. (The latest TV film of Marie Curie and her husband Pierre at work in Paris minimizes, rather than exaggerates, the paucity of resources with which they had to do their work.) But people with the scientific obsession aren’t easily put off by poverty of that kind. Rutherford used to boom: ‘I could do research at the North Pole.’

  There was a lot of determination and ability in Paris: in addition to Becquerel there was the Polish woman, Marie Sklodovska, who had just become Madame Curie, and the young Paul Langevin who was eventually to invent – among other things – sonar. In a partly accidental, but scientifically meticulous, fashion, some of the first discoveries in modern physics were made. The Curies isolated a new element, and called it radium. Radium had various curious properties; for example, it had a finite lifespan, losing weight by degrees as it emitted several distinct kinds of radiation. The idea that atoms might not always be permanent, but could sometimes disintegrate of their own accord, hung vaguely in the air.

  Then J J Thomson proved in 1897 that cathode ‘rays’ were not waves of radiation at all – Thomson deflected them with both magnetic and electric fields, evidence that they were minute particles of matter each carrying an electric charge. His experiment also showed that these particles – electrons – weighed far less than hydrogen atoms. That is, particles of a different order from any atoms were proved to exist. It took some years for scientists to guess or realize that these particles could be emitted from atoms themselves, or to speculate that atoms were not simple but had constituents of their own. From the first, these results cut across the grain of most preconceptions. There were classical physicists who died unconvinced. But in fact this convulsion of scientific thought, like those which came later, was quickly domesticated. Scientific reason showed itself too strong for doubt.

  he existence of the electron was soon taken for granted. As a matter of history, there was something of a personal row. Who had really discovered the electron? Who had the priority? Scientists, as a great practitioner, Peter Medawar, has recently reminded us, are very much like other people. They come in all shapes, sizes, temperaments. Some are very clever, as the outside world judges cleverness. Some aren’t. Some are noble, and again, some aren’t. There are often disputes about priority, and they can be very bitter. Newton, it is sad to say, was venomously ungenerous in this respect. Charles Darwin was magnanimous, and so was Alfred Wallace, who arrived at Darwin’s conclusions about the evolution of the species at the same time.

  That may have happened over the electron. Philipp Lenard, a German physicist, certainly thought so, and said so with vehemence. He also said, with even more vehemence, that he had got in first. But he didn’t get the major credit, which went to the Cavendish Professor, J J Thomson. Most neutral opinion seems to have thought that there was no injustice done. As someone said, in a great discovery the scientist must satisfy two criteria. He must know what the discovery is, and he must know how important it is. Thomson satisfied both criteria. He had a much more lucid intellect than Lenard who, incidentally, as an old man was one of the only two eminent German scientists who became active spokesmen for the Nazi faith. (The other was Werner Heisenberg, a great theoretician not yet born at the time of Lenard’s dispute with Thomson.)

  When the electron had been identified, there was no doubt from that time on about the existence of sub-atomic particles. Electrons must be an integral part of every atom, and Thomson replaced the earlier, chemists’ concept of the atom as a structureless ‘billiard ball’ with a more sophisticated model. Thomson’s atom was a diffuse sphere of positive electric charge, with the negatively charged electrons embedded in it. In the cathode ray tube electrical forces ripped the electrons out of the residual gas atoms and sent them flying down the tube as ‘cathode rays’.

  At last physicists began to take the atom seriously, and began to think about its interior structure. Many of the best minds in physics became devoted to the structure of microcosmic matter. Within forty years they had consummate success.

  3: Founding Fathers

  THOSE forty years, that is from the end of the nineteenth century to the outbreak of the Second World War, were a wonderful time for physicists to be alive. Both experimental physicists and theoreticians had their phases of triumph, and it is instructive to notice how the balance swung. There was a long record of experimental discoveries beginning with elucidation of the radiations emitted from radioactive elements and evidence that most of an atom’s mass lay in a central nucleus. Rutherford and Chadwick’s disintegration of atoms by particles from radioactive sources led on to disintegration of atoms by controlled means by Cockcroft and Walton. Meanwhile there was identification of further sub-atomic particles – neutrons and positive electrons. And in 1938 the most fateful experiment of all, the splitting of some uranium atoms with an emission of particles which could lead to further splitting, with the possibility of a chain reaction and the release of huge amounts of energy.

  Through most of this period, the dominant figure was Ernest Rutherford who was born in 1871. As has been said, he ranks with Faraday among British experimental physicists. As a man, he was wildly different from Faraday – exuberant, outgoing, not noticeably modest or unassuming. He was taken at face value, a simple face value, by most of the people round him. Internally, he was not so simple. There were deep layers of diffidence concealed beneath that robust and noisy façade. He was a prey to nerves. He found it hard to manage an overweighted nature. Kapitsa, one of the most gifted of his pupils, an engineering-physicist of genius in the high Russian tradition, and in addition equipped with psychological observation and insight, seems to have been the only member of Rutherford’s scientific entourage who understood him well. Kapitsa’s letters to his mother in Leningrad, dating from the time he first came to England to work with Rutherford, have recently been published in the Soviet Union. Within a few days Kapitsa was writing, ‘The Professor is a deceptive character. They [the English] think he is a hearty colonial. Not so. He is a man of immense temperament. He is given to uncontrollable excitement. His moods fluctuate violently. It will need great vigilance if I am going to obtain, and keep, his high opinion.’

  About the only item true in the stereotype of Rutherford is that he was a colonial. His father was a Scottish immigrant to New Zealand, who managed after scraping a living, doing odd jobs, to become a small farmer and a kind of general utility man, employing one or two workmen and doing anything in the way of domestic repairs. He seems to have had a good deal of technical ingenuity.

  Ruther
ford knew nothing in the way of privilege. New Zealand was a remote province. He received a good education, however, rather on the Scottish model. He was top of his school in all subjects, being very far from the dumb-ox kind of scientist who occasionally turns up. But when he came to England on a scholarship he felt an outsider who didn’t know the rules. There were a good many chips on those heavy shoulders. He couldn’t get along with English intellectual chit-chat, and insisted on behaving like a country boy who had never read a book (actually he was very widely read) with people of about one-hundredth of his cultivation, not to say intelligence.

  He was a great man, and a good one. He didn’t like being outfaced, though, by people who had learned tricks denied him. He wasn’t comfortable in the company of well-trained theoreticians. Of course he could have mastered theoretical physics, or anything else in science, but some of those shoulder chips got in the way. When he was Cavendish professor, Cambridge became the world centre of experimental physics, but it didn’t rank with Copenhagen and Göttingen in theory – except for the accidental occurrence of a young theoretical physicist of great genius, Paul Dirac. His appearance had nothing to do with Rutherford; that the divide in Cambridge between theoretical and experimental physics was sharp, did have something to do with Rutherford.

  As a physicist, he had extraordinary intuition. He seems scarcely ever to have tried a problem which wouldn’t go. If any scientist had a nose for, to use Medawar’s phrase, ‘the solution of the possible’, Rutherford had. His attack was simple and direct, or rather he saw his way, through the hedges of complication, to a method which was the simplest and most direct.

  An example is the most dramatic event of his career, the experiments by which he proved the existence of the atomic nucleus. The Curies had shown that radium emits various kinds of ‘radiation’, and one of these was now known to consist of a stream of electrically charged particles. These ‘alpha particles’ were identical to helium atoms with their electrons removed; but they originated not from helium gas but sprang spontaneously from the radium atoms as they disintegrated.

  Even though atomic disintegration was still little understood, Rutherford saw these high-speed alpha particles as useful projectiles. He intercepted them with a thin sheet of gold foil, to see what happened as they passed through. If atoms were diffuse spheres of electrical charge, as Thomson had imagined, then most of the alpha particles should have gone straight through; a few should be deflected slightly. But some of the alpha particles bounced straight back again. It was like firing artillery shells at a piece of tissue paper, and getting some of them returning in the direction of the gun.

  Rutherford could only explain this by postulating that these alpha particles were hitting small, massive concentrations within the atoms. He thus concluded that most of an atom’s mass resided in a minute, positively charged nucleus at the centre, while the electrons went around the outside – very much like the planets orbiting the massive sun. Most of the atom was just empty space. If an atom were expanded to the size of the dome of St Paul’s Cathedral, virtually all its mass would lie within a central nucleus no larger than an orange. The large majority of alpha particles passed the atoms’ emptiness and carried on through the foil; but just occasionally one would hit a nucleus head-on, and rebound along the way it had come.

  Positive, like all Rutherford’s physics. He said that he knew it was convincing, and maintained that he was completely surprised. One wonders if he hadn’t had a secret inkling. He was superlatively good at making predictions about nature.

  It is hard to think of a prediction of his which didn’t come off. He predicted the existence of an electrically neutral particle within the atomic nucleus, which was duly proved when Chadwick discovered the neutron in 1932. He predicted the splitting of atoms by accelerated protons, duly achieved by Cockcroft that same year. He made just one negative prediction: as late as 1933, he announced that the energies in the atom were unlikely ever to be used. That apart, he was almost always right. His Cavendish ‘boys’ as he called them – men as gifted as Chadwick, Kapitsa, Cockcroft, Blackett (all Nobel prize winners), Oliphant, Dee, half a dozen others – tended to think that, though he might be overpowering or deafeningly noisy, he was next door to infallible.

  That was his kind of paternal leadership. His own greatest individual work wasn’t done at Cambridge at all. With singular folly, Cambridge didn’t try to keep the young Rutherford – possibly because the place wasn’t big enough to hold both him and his seniors. He went off to a professorship at Montreal. American universities bid for him, better talent-spotters than Cambridge, but at the time America wasn’t a major force in the scientific world, and Rutherford returned to England, after a touching and deliberate resignation by the head of the physics department at Manchester, Arthur Schuster, who thought that Rutherford must at any cost be preserved for this country. It was at Manchester that Rutherford proved the nuclear structure of the atom.

  It is instructive to remember how little money was spent on these great scientific researches. Faraday’s apparatus (some still preserved in the Royal Institution) was humble, knocked up in the laboratory. Things hadn’t changed much by Rutherford’s time. His experiments were built with the help of one laboratory technician, or if he were feeling well-financed, perhaps two. There was no engineering. All was home made. The old phrase was ‘string and sealing wax’, and it is not far from the truth. The Cavendish was a great experimental laboratory, but it would look like a badly equipped high school compared with the big physics institutions of today. It was not until trained engineers such as Kapitsa and Cockcroft became active that the Cavendish knew any approach to big physics. Rutherford marvelled and cheered them on, but sometimes thought that it might be overdone.

  Until the Second World War, there was little industrial support for physicists. Chemists had been looked after by the chemical industry for many years: other industries had been peculiarly obtuse in not seeing any conceivable use for physicists. Young men in the l930s, with doctorates and good research to their credit, considered themselves lucky to get decent jobs in schools. A few years later, in the war, they were being snatched up as the rarest and most valuable of all human commodities.

  It seems strange now that the Cavendish at its peak should have stayed so remote from industry. With the harsh wind of approaching war, however, Cockcroft, the Cavendish all-purpose functionary, was set to indoctrinate selected young men in the latest military prospect – what was later called by the American name of radar, and was the most successful British scientific weapon in the Hitler war. Few unobtrusive steps have paid off better. By the by, that happened in the same university which contemporary opinion seems to believe was devoted entirely to espionage.

  Rutherford and his colleagues had little to do with money. It seems to have bored Rutherford himself about as much as academic philosophy. He was a remarkably unmercenary man. He could have earned large fees as a consultant. He would have thought that a ludicrous waste of time. As a professor at Montreal he was paid £500 a year. At Manchester and Cambridge he got about £1600, a good academic salary for the period, but he never earned more than that. When he died, he left almost exactly the amount of his Nobel prize, which at that time was something like £7,000.

  In that brilliant period, an even rarer character was leaving his mark on world physics. This was Albert Einstein. He was born in 1879, and thus was eight years younger than Rutherford. When he was in his twenties, people were talking of him as the new Copernicus. A little later, it wasn’t unusual or extravagant to say that here, for the first and only time, was someone in the class of Newton.

  Einstein’s greatest work touched only remotely on the new particle physics. His Theory of Relativity – actually in two parts, Special Relativity and General Relativity – made him rightly world-famous. But it dealt not with the small-scale structure of matter, but the largest scales of space, time and speed. Ironically enough, relativity was at the time so controversial that Einstein was not aw
arded his Nobel prize for the greatest of his theories, but for some early work on the effect of light on metal surfaces. His explanation of this photo-electric effect, however, did form an important basis on which later particle physicists could build when they came to describe atoms in terms of quantum theory – of which more later.

  Einstein was the most independent of all great scientists, relying with absolute confidence on his own solitary thoughts. He had set out to understand all aspects of the natural world: space, time, the unity and harmony of the entire world picture. He had set out to find the most universal of universal laws. He did so.

  This makes him sound portentous. In his own personality, he wasn’t, not in the least. He was cheerful, unaffected and amiable to everyone, and extremely witty. He was the best company of all the great physicists. In his serious moments, and there were many as the political scene darkened, he did speak from a depth of moral experience. He was as certain of his moral insight as of his physical insight. He wanted to do his best for his fellow humans, but he was the least sentimental of men. He recognized no collective loyalties except to the human race. He had renounced his German nationality at the age of sixteen, one of the most astonishing – and revealing – acts that any boy has ever done. But he wasn’t really a boy – he was full of animal spirits and vigour, but his nature was formed at a very early age.

  He had no use for the minute differences which divide men from one another. It is true that, though he didn’t believe in Judaism any more than he believed in any other religion, he preserved a special feeling for his own Jewish people. Conversely, while he was given all honours in Germany and the conditions for his major achievement, he does seem to have had a special negative feeling for Germans. In later life he did not admit to having made a real German friend – outside German Jews. That was odd, in one so benevolent and so removed from ordinary human rancour. But, though he was as much above pettiness of spirit as anyone, he was human after all.