THE HAPPIEST THOUGHT OF MY LIFE' 179I mentioned in Chapter 7 the contributions Einstein made to special relativity
after the completion of his September 1905 paper on that subject. Some of these
sequels appeared in 1906 and early 1907. In that period he also added to his 1905
work on Brownian motion (Chapter 5). However, his main activities during that
time concerned the quantum theory. In 1906 he gave his own interpretation of
Planck's 1900 work on the quantum theory and completed the fundamental paper
on the quantum theory of the specific heats of solids (Chapters 19 and 20).
His first important paper on relativity theory after 1905 is the 1907 review.
This article was written at the request of Stark, the editor of the Jahrbuch. On
September 25, 1907, Einstein had accepted this invitation [E4]. On November 1,
Einstein further wrote to Stark: 'I am now ready with the first part of the work
for your Jahrbuch; I am working zealously on the second [part] in my unfortu-
nately scarce spare time' [E5]. Since this second part contains the remarks on
gravitation, it seems most probable that Einstein's 'happiest thought' came to him
sometime in November 1907. We certainly know where he was when he had this
idea. In his Kyoto lecture he told the story:
I was sitting in a chair in the patent office at Bern when all of a sudden a
thought occurred to me: 'If a person falls freely he will not feel his own weight.'
I was startled. This simple thought made a deep impression on me. It impelled
me toward a theory of gravitation. [II]
Was Einstein first drawn to gravitation because he wanted to include it in spe-
cial relativity or because he saw that he could extend special relativity with its
help? The way I read the quoted lines from the Morgan manuscript, the answer
would seem to be that, by asking for the inclusion, he at once or almost at once
came upon the extension. That is also Einstein's own recollection, again found in
the Kyoto lecture: 'In 1907, while I was writing a review of the consequences of
special relativity ... I realized that all the natural phenomena could be discussed
in terms of special relativity except for the law of gravitation. I felt a deep desire
to understand the reason behind this. ... It was most unsatisfactory to me that,
although the relation between inertia and energy is so beautifully derived [in spe-
cial relativity], there is no relation between inertia and weight. I suspected that
this relationship was inexplicable by means of special relativity' [II]. The absence
of the equation for the static Newtonian gravitational potential $:
(where p is the matter density and G the Newtonian gravitational constant) in the
1907 review indicates that the generalization of this equation to special relativity
was not his ultimate purpose. Equation 9.1 does not appear in his papers until
February 1912 [E6], but by then he already knew that this equation is not gen-
erally true even in the static case, as we shall see in Chapter 11.
Three main issues are raised in Section V of the Jahrbuch article.
- The Equivalence Principle. 'Is it conceivable that the principle of relativity
also holds for systems which are accelerated relative to each other?' That is Ein-