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The New Dynamics
15a. From 1915 to 1980
Einstein arrived at the special theory of relativity after thinking for ten years about
the properties of light. Electromagnetism was not the only area of physics that
attracted his attention during those years. In the intervening time, he also thought
hard about statistical mechanics and about the meaning of Planck's radiation law.
In addition, he tried his hand at experiments. The final steps leading to his June
1905 paper were made in an intense burst of activity that lasted for less than two
months.
Einstein arrived at the general theory of relativity after thinking for eight years
about gravitation. This was not the only area of physics which attracted his atten-
tion during those years. In the intervening time, he also thought hard about quan-
tum physics and about statistical mechanics. In addition, he tried his hand at
experiments. The final steps leading to his November 25,1915, paper were made
in an intense burst of activity that lasted for less than two months.
In every other respect, a comparison of the development of the special and the
general theory is a tale of disparities. In June 1905, Einstein at once gave special
relativity its ultimate form in the first paper he ever wrote on the subject. By
contrast, before November 25, 1915, he had written more than a dozen papers on
gravitation, often retracting in later ones some conclusions reached earlier. The
November 25 paper is a monumental contribution, of that there can be no doubt.
Yet this paper—again in contrast with the paper of June 1905—represents only
a first beach-head in new territory, the only sure beacon at its time of publication
(but what a beacon) being the one-week-old agreement between theory and exper-
iment in regard to the perihelion precession of Mercury. Both in 1905 and in
1915, Einstein presented new fundamental principles. As I have stressed repeat-
edly, the theory of 1905 was purely kinematic in character. Its new tenets had
already been digested to a large extent by the next generation of physicists. By
contrast, general relativity consists of an intricate web of new kinematics and new
dynamics. Its one kinematic novelty was perfectly transparent from the start:
Lorentz invariance is deprived of its global validity but continues to play a central
role as a local invariance. However, the new dynamics contained in the equations
of general relativity has not been fully fathomed either during Einstein's life or in
the quarter of a century following his death. It is true that since 1915 the under-
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