In 1949 Pais had organized a Festschrift in honor of Einstein's seventieth birthday for the Reviews of Modern Physics, Volume 21, Issue 3. He did not himself write one of the thirty-seven articles, but it is striking that so little is about Einstein and quantum theory, primarily the Louis de Broglie's article and Phillip Frank's retrospective.

To be sure, thirty years later Pais published a long article "Einstein and the Quantum Theory" in 1979 in Rev Mod Phys, Volume 51, Issue 4. This appeared virtually unchanged as chapters 18-26 of Subtle is the Lord seven years later. Pais began with an outline of Einstein's contributions to quantum theory.

the physics community at large had received the light-quantum hypothesis with disbelief and with skepticism bordering on derision. As one of the architects of the pre-1925 quantum theory, the “old” quantum theory, Einstein had quickly found both enthusiastic and powerful support for one of his two major contributions to this field: the quantum theory of specific heat. (There is no reason to believe that such support satisfied any particular need in him.) By sharp contrast, from 1905 to 1923, he was a man apart in being the only one, or almost the only one, to take the light-quantum seriously.

The critical reaction to Einstein’s light-quantum hypothesis of 1905 is of great importance for an understanding of the early developments in quantum physics. It was also a reaction without parallel in Einstein’s scientific career. Deservedly, his papers before 1905 had not attracted much attention. But his work on Brownian motion drew immediate and favorable response. The same is true for relativity. Planck became an advocate of the special theory only months after its publication; the younger generation took note as well. Lorentz, Hilbert, F. Klein, and others had followed the evolution of his ideas on general relativity; after 1915 they and others immediately started to work out its consequences. Attitudes to his work on unified field theory were largely critical. Many regarded these efforts as untimely, but few rejected the underlying idea out of hand. In regard to the quantum theory, however, Einstein almost constantly stood apart, from 1905 until his death. Those years cover two disparate periods, the first of which (1905-1923) I have just mentioned. During the second period, from 1926 until the end of his life, he was the only one, or again nearly the only one, to maintain a profoundly skeptical attitude toward quantum mechanics. I shall discuss Einstein’s position on quantum mechanics in Chapter 25, but cannot refrain from stating at once that Einstein’s skepticism should not be equated with a purely negative attitude. It is true that he was forever critical of quantum mechanics, but at the same time he had his own alternative program for a synthetic theory in which particles, fields, and quantum phenomena all would find their place. Einstein pursued this program from about 1920 (before the discovery of quantum mechanics!) until the end of his life. Numerous discussions with him in his later years have helped me gain a better understanding of his views.

But let me first return to the days of the old quantum theory. Einstein’s contributions to it can be grouped under the following headings.

(a) The Light-Quantum. In 1900 Planck discovered the blackbody radiation law without using light-quanta. In 1905 Einstein discovered light-quanta without using Planck’s law. Chapter 19 is devoted to the light-quantum hypothesis. The interplay between the ideas of Planck and Einstein is discussed. A brief history of the photoelectric effect from 1887 to 1915 is given. This Chapter ends with a detailed account of the reasons why the light-quantum paper drew such a negative response.

(b) Specific Heats. Toward the end of the nineteenth century, there existed grave conflicts between the data on specific heats and their interpretation in terms of the equipartition theorem of classical statistical mechanics. In 1906 Einstein completed the first paper on quantum effects in the solid state. This paper showed the way out of these paradoxes and also played an important role in the final formulation of the third law of thermodynamics. These topics are discussed in Chapter 20.

(c) The Photon. The light-quantum as originally defined was a parcel of energy. The concept of the photon as a particle with definite energy and momentum emerged only gradually. Einstein himself did not discuss photon momentum until 1917. Relativistic energy momentum conservation relations involving photons were not written down till 1923. Einstein’s role in these developments is discussed in Chapter 21, which begins with Einstein’s formulation in 1909 of the particle-wave duality for the case of electromagnetic radiation and also contains an account of his discovery of the A and B coefficients and of his earliest concern with the breakdown of classical causality. The Chapter concludes with remarks on the role of the Compton effect. The reader may wonder why the man who discovered the relation E = hv for light in 1905 and who propounded the special theory of relativity in that same year would not have stated sooner the relation p = hv/c. I shall comment on this question in Section 25d.

(d) Einstein’s work on quantum statistics is treated in Chapter 23, which also includes a discussion of Bose’s contribution.

(e) Einstein’s role as a key transitional figure in the discovery of wave mechanics will be discussed in Chapter 24.

Subtle is the Lord, p.357-359

Inward Bound: Of Matter and Forces in the Physical World. Oxford University Press, USA. (1986)

Niels Bohr's Times, In Physics, Philosophy, and Polity. Oxford University Press, USA. (1991)