'THE HAPPIEST THOUGHT OF MY LIFE' 183this article at least as much as the perfect relativity paper of 1905, not so much
for its details as for its courage.
Einstein's treatment of simultaneity in 1905 was the result of many years of
thinking that had led him to a new physical interpretation of global Lorentz
invariance. Only two years later, he realized that the extension of the principle of
special relativity demanded a reevaluation of the validity of this most precious tool.
In 1907, he already clearly knew that there was something amiss with this invar-
iance if his equivalence principle was to hold up in all generality. He did not
know then that Lorentz invariance was to return in a new, local version. Others
might have shied away from the equivalence principle in order to retain the global
invariance. Not so Einstein. With a total lack of fear he starts on the new road.
For the next eight years he has no choice. He has to go on. From then on also his
style changes. If the work of 1905 has the quality of Mozart, then the work of
1907-15 is reminiscent of Beethoven. The quotation at the head of this chapter is
the motto of the last movement of Beethoven's opus 135: Must it be? It must be.
References
El. A. Einstein, Nature 106, 782 (1921).
E2. , letter to R. W. Lawson, January 22, 1920.
E3. , Jahrb. Rad. Elektr. 4, 411 (1907).
E4. , letter to J. Stark, September 25, 1907, quoted in [HI].
E5. , letter to J. Stark, November 1, 1907, quoted in [HI].
E6. , AdP38, 355 (1912).
E7. —, [E6], p. 365.
E8. , letter to K. Habicht, December 24, 1907.
HI. A. Hermann, Sudhoff's Archiv. 50, 267 (1966).
II. J. Ishiwara, Einstein Koen-Roku. Tokyo-Tosho, Tokyo, 1977.
LI. R. W. Lawson, Nature 106, 781 (1921).
L2. , letter to A. Einstein, November 26, 1919.