WHEN did modern physics begin? That isn’t a question with much meaning, since the process is continuous. For our present purposes, we can make a crude statement about physicists, practically and intellectually. Modern physics began with the discovery of the particles of which atoms are made: first electrons, then protons and neutrons. These discoveries began to be made in the last years of the nineteenth century.
Through most of the nineteenth century, classical physics was advancing fast. Scientists were studying the large-scale laws of matter and energy: Newton’s law of gravitation explained how the planets and stars move; the laws of thermodynamics laid bare the properties of energy and heat, with practical results in the steam engine; and electricity and magnetism were being swiftly unravelled. Some people, even eminent scientists, believed that scientific effort was getting near to its end, and that there remained only mopping-up operations – they sensed the day of total victory in man’s understanding of the physical universe. The same feeling, that scientists have reached final statements, has occurred in other domains of science since: it has always been an intuition gone wrong. They were not greatly concerned with the structure of matter on the smallest scale. The general run of scientists assumed that matter was made of atoms, indestructible, eternal, and that these presumably differed from one element to another, as chemical experiments indicated.
Chemists, far more than physicists, were concerned with atoms, for it was now clear that chemical reactions were simply the rearrangement of atoms into larger groupings called molecules. Chemists knew the relative weights of the atoms of the different elements. The Russian chemist Mendeleev had found that when he arranged the elements according to their atomic weight, curious patterns emerged – elements with similar chemical properties recurred at regular intervals. Although physicists thought vaguely there must be something in Mendeleev’s law, they usually brushed the topic aside. Atoms were a convenient concept – especially for chemists – but the major nineteenth-century physicists had plenty to keep them busy without speculating about atoms.
The physicists were settling the great laws, the macrocosmic laws, of electromagnetism and thermodynamics, as difficult to penetrate as the microcosmic laws of their successors, and obviously of immense applied significance. Faraday was the greatest of experimental physicists (the only competitor being Rutherford in the next century) and he applied his gifts to probing the properties of electricity and magnetism, and the relation between them. When Faraday started his researches, electricity and magnetism were nothing but playthings. Before he died, the laws of the electromagnetic field were being worked out, and big electrical industries were already set up, though not in his own country.
Faraday was one of the saints of science, gentle, unassuming, generous, preserving the virtues of the Sandemanian sect (a relaxed derivative of Calvinism) in which he was brought up. He was one of the very few of the great scientists to be born among the very poor. Somehow he was spotted as a bright and dexterous lad, and he became a laboratory assistant to Sir Humphry Davy, who treated him with some condescension (as from parvenu bourgeois to proletarian) but gave him a kind of scientific opening. Faraday didn’t repine. Quite rapidly, he became one of the Victorian glories, and his lectures at the Royal Institution one of London’s treats. Dickens offered to help write the lectures so as to make them accessible to a wider audience. Victorians were remarkably good at recognizing and celebrating their own great men.
Meanwhile another man of supreme gifts was at work turning Faraday’s results into mathematical form – one of the great theoretical feats of the nineteenth century. Clerk Maxwell was, like Faraday, a man of unusual sweetness and light. Unlike Faraday, he was comfortably off, a Scottish landowner, and when his health failed (he died in his forties) he retired from the Cavendish Chair of Physics to his own estate. The Chair had just been created at Cambridge, thus initiating the only research school in England at a time when American universities such as Michigan had already had well-organized research for thirty years past. Maxwell left a pleasant legend in Cambridge. He was high-spirited and entertaining. His only vice was the writing of indifferent light verse with an obsessive facetiousness that has since been emulated by other scientists.
There was another mind, at least as powerful as Maxwell’s, operating in hermit solitude on the other side of the Atlantic. Willard Gibbs was, single-handed, establishing the conceptual laws of thermodynamics, and thus the whole of classical physical chemistry. Originally, thermodynamics was the science of how heat and energy are related, and the impetus of studying it came from the practical importance of the steam engine. Gibbs’ theoretical insight discovered that the same laws of thermodynamics control the chemical reactions between atoms. It was said that you had only to read Gibbs’ great works to understand everything about chemical thermodynamics – but since his exposition was in a notation known only to himself, it would probably be easier to work the subject out for oneself. Gibbs was a shy eccentric, something like Kant, with habits so regular that people could set their watches by him. He lived with his sister in New Haven, and was impossible to stir. He was, along with the analytical philosopher C S Pierce, the most original abstract thinker born in America so far. It is uncommon to meet an American student who has heard either of those two great names.
Theirs were the heights of classical physics before the modern age (more correctly the particle age) began. Of course, classical physics didn’t end in the 1890s, when the electron was discovered. Essential work is being done today. Most of the problems of hydrodynamics and aerodynamics are solved by applying the laws of classical physics. G I Taylor, one of this country’s most gifted theoreticians, devoted his life to them, except when he was called on like a fire-engine for one of the jobs that required his superlative technical mastery – as when he computed the properties of the blast-wave from a nuclear explosion. The principles of space travel are classical, and Tsiolkovsky, the early twentieth-century Russian scientist-engineer of genius who predicted much of what has occurred, would have no difficulty in making his way round a modern space centre were he still alive – he would no doubt wish that he could have laid his hands on our metallurgy and propellants.
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.