358 THE QUANTUM THEORYThe 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 Brown-
ian 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 men-
tioned. 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 con-
tributions 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 momen-
tum emerged only gradually. Einstein himself did not discuss photon momentum