In the same year Einstein advanced a revolutionary theory of the photo-electric effect, which has exercised a decisive influence on the modern quantum theory of light. The essence of this effect is that the speed with which electrons are liberated from a metal surface illuminated by ultraviolet light depends only on the colour of the light and not on its brightness. Einstein suggested that the light (from which the escaping electrons must derive their energy) is not continuously distributed in space, but is like a gas with a discrete molecular structure – the ‘molecules’ being photons or units of radiant energy of amounts proportional to the frequency of the light. This assumption gave a concrete physical mechanism for the quantum theory of white light advanced by Planck in 1900, and it provided satisfactory estimates of the speed of photo-electrons. But the importance of Einstein’s theory of photons far transcended the occasion of its suggestion. Its real significance is that it accustomed physicists to accept the dual character of light, which sometimes behaves like a continuous train of waves, and sometimes a hail of bullets, and that in 1924 it suggested to de Broglie that matter itself had a similar ‘dual personality’ and could behave either as a wave or a corpuscle. These conceptions have dominated all subsequent speculations about the ultimate elements of matter and light.
Special Theory of Relativity
Although Einstein’s researches in the quantum theory were of vital significance and, in one direction, seemed to show a clearer grasp of its implications than was possessed by its originator, it is with the theories of relativity that his name will always be associated. The ‘Special Theory of Relativity’ was published in the same extraordinary year. It expressed in a simple and systematic form the effects produced on the basic instruments of physics – the ‘rigid’ scale and the perfect clock – by relative motion, and thus codified the earlier mathematical investigations of Voigt, the physical speculations of Larmor and the pioneer work of Lorentz. For the first time the optics of moving media received a satisfactory formulation, and Newtonian dynamics itself was generalized so as to express the effect of motion on apparent mass. In particular, Einstein’s deduction that mass and energy are proportional became the basic law of atomic transformation. Apart from its spectacular demonstration in atomic energy, it is supported also by a host of experiments in nuclear physics, in which it is used daily as a tool with which nuclear physicists work. Equally, the design of large engineering machines, such as ‘synchrotrons’, in which nuclear particles are accelerated to high energies, depends directly on its use.
In this group of varied and important publications he showed at once qualities of imagination and insight which were even more vital to his work than mathematical ability, which indeed was a necessary qualification but was not (by the highest standards) exceptional. It was also well for his immediate career that he had more than one contribution to offer.
As soon as the remarkable researches published by Einstein in 1905 became known many attempts were made to secure for him a professorial post. As a result of these efforts he became a Privatdozent at Berne in 1908 and Professor extraordinarius at Zürich in 1909. In 1911 he became Professor of Theoretical Physics at Prague, but returned to Zürich to the corresponding post in 1912. During 1913 Planck and Nernst persuaded Einstein to go to Berlin as director of the projected research institute for physics, as a member of the Royal Prussian Academy of Science and as a professor in the University of Berlin – with no duties or obligations. He occupied this post until 1933.
General Theory of Relativity
The ‘General Theory of Relativity’, published in 1916, was the fruit of many years of speculation by Einstein on the questions: ‘Can we distinguish the effects of gravitation and of acceleration?’ and ‘Are light rays bent by gravity?’ To answer these questions he was led to build a great and complex theory, which needs for its systematic expression a new mathematical discipline invented by Ricci and Levi-Civita. The divergences between the predictions of the planetary theory based on Einstein’s theory and those based on the classical theory of Newton are all extremely small, but in one case (the slow changes in the orbit of Mercury) Einstein’s theory provides an explanation which had never been found on Newtonian principles. Moreover, it successfully predicted the deflection of light from distant stars as it grazed the sun’s disc – an effect subsequently verified by British astronomical expeditions in 1919 – and also the reddening of light from very massive stars – which was much later confirmed by observations on the dark companion of Sirius. The success with which ‘general relativity’ gave quantitative predictions of the new phenomena has created a presumption in its favour which has substantially survived.
The application of general relativity to cosmology was implicit in Einstein’s original theory, but became explicit through a modification which he introduced into it in 1917. His contribution in this field was an attempt to provide an answer to an old and ‘insoluble’ problem: ‘How can the universe of stars be uniform in density, fill all space and yet be of finite total mass?’ The subsequent relation of observational evidence of ‘the expanding universe’ to the possible forms of theory that might be developed was done mainly by others, including Lemaître, de Sitter, and Eddington, to whom Einstein served as a stimulus.
During the 1914–18 war two other notable events occurred in his life – he refused to sign the ‘Manifesto of Ninety-two German Intellectuals’ which identified German culture and German militarism, and he contracted a second marriage, with his cousin Elsa. In 1921 he appeared publicly as a supporter of Zionism and he actively collaborated with Weizmann in the establishment of the University of Jerusalem. During the post-war years he travelled and lectured in Holland, Czechoslovakia, Austria, the United States (where he not only lectured on relativity but took part in Weizmann’s campaign for the Jewish National Fund), and England (where he lectured at King’s College, London, and calmed the fears of the Archbishop of Canterbury that relativity was a threat to theology). In 1922 he lectured in Paris, Shanghai, and Kobe, returning home via Palestine and Spain.
The Nobel Prize
In the same year he was awarded a Nobel prize, strangely enough, for his work in quantum theory, as the committee were not sure whether his theory of relativity was technically a ‘discovery’! He was awarded a Copley Medal by the Royal Society in 1925. He visited South America in 1925 and lectured at Pasadena (California) during the winters of 1930–31, 1931–32, and 1932–33. In the summer of 1932 he lectured at Oxford, and was made an honorary Doctor of Science. The great purge of Jewish scientists began under Hitler in 1933 and Einstein decided not to return to Germany, where scientific freedom had ceased to exist. He lived for some months at La Cocque in Belgium and resigned from the Prussian Academy. In the winter of 1933, at the invitation of Flexner, he emigrated to America and became a Professor at the Institute for Advanced Study at Princeton, a post which he held until 1945. His second wife, Elsa, had died in 1936.
Indeterminacy Opposed
In his later years he was venerated – and loved – but became somewhat isolated in his work from the main stream of modern physics. Remembering his early contribution to the quantum theory, it might have been supposed that he would have accepted readily the principle of indeterminacy, which came to play so large a part in it, and that, in his quest for a further unification of the laws of Nature, he would have tried to weld together the discontinuous and indeterminate picture given by the quantum theory with the continuous and determinate picture of relativity. But for Einstein physics was firmly rooted in causality; God did not play at dice, and he would not admit the ultimate validity of any theory based on chance or indeterminacy. The quantum theory remained, therefore, for him as a passing phase, however important to working physicists. Instead, he attempted further generalizations of relativity, which should incorporate both gravitation and electromagnetism, together with the nuclear fields of force. This work, however, has received no better reception than have all other ‘unified field theories’.
When we consider the basic character of the problems he attacked, the vast cosmical scale on which he worked, and his immense influence on physical cosmology as well as physics, we can only compare Einstein with Newton. If Newton’s central achievement was to establish the reign of gravitation in its full simplicity and universality, the essence of Einstein’s work was to reveal gravitation as a phenomenon expressible in terms of world