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The task ahead If one tries to view the problem situation faced by Eddington from his point of view, it breaks into three parts. The expectation was that these three would prove connected in some way when the problem was solved. The first part of the problem is to make some connexion between 136 as a number of algebraic elements and 1/136 as related to (though by 1936 Eddington was aware that it was not equal to) e2/ħc. As I said in the last chapter, the fine-structure constant occurs in many different contexts. Ideally, if it is to be related to algebraic structures, it should be possible to use any of these contexts with the principle of identification to establish the relation. Eddington chose, naturally enough, the occurrence of α as a coefficient in Dirac's equation of the hydrogen atom. Hitherto I have been speaking only of Dirac's equation for the free electron; that is the simplest case. But, as will have been clear from Dirac's explanation quoted in Chapter 6, his work was inspired – like so much work in both the old and new quantum theories – by difficulties in explaining the hydrogen spectrum fully. The way in which the equation for the free electron was modified to take account of the electrostatic force between proton and electron was well known, if not at all understood, in the quantum wave mechanics of Schrödinger. Dirac took it over without question and Eddington followed Dirac. No-one has since questioned this method of introducing an electric force, so I shall do no more now than to remark that it is a little mysterious. [ABSTRACT FROM AUTHOR]

This chapter discusses the historical impact of the advent of general relativity, in 1916, on British physicists and astronomers, and, at a personal level, its effect on Eddington's life. In this way I prepare for the next chapter in which I shall show that the intellectual effect of the general theory on Eddington was to accentuate certain philosophical ideas which he already had. These ideas take us some way along the road to understand the mystery described in Chapter 1. The genesis of general relativity We left Eddington reinstalled in Cambridge in 1914, clearing up his earlier astronomical work and just about to begin the investigation directed towards the understanding of the mechanism of the Cepheid variables. By that time the dust was beginning to settle on Einstein's 1905 paper in which the concept of time had been so strikingly changed. The Cambridge aether-theorists were working their way towards reconciliation but, doubtless, were more concerned in 1914 with the international disaster that was to sweep away the comforts of the long Edwardian summer. Then in 1915–16 there appeared a new paper by Einstein (Einstein 1916) which followed up his earlier one by changing completely the concept of space. This is usually described as Einstein's successful attempt to extend the idea of relativity to include gravitation. The problem of gravitation was considered particularly important because special relativity seemed to give a privileged position to the other well-known field theory, electromagnetism. The Lorentz transformations left Maxwell's equations unchanged. Yet in some ways gravitation was the more fundamental field because of its universal action. [ABSTRACT FROM AUTHOR]