206 RELATIVITY, THE GENERAL THEORYLet us summarize the Prague papers.* By the spring of 1912, Einstein knew
of the red shift and the deflection of light. He had realized that the Lorentz trans-
formations are not generally applicable, that a larger invariance group was
needed, and that the laws of physics would have to be correspondingly more com-
plicated. From the study of a primitive scalar model field theory, his attention had
been drawn to the generality of the variational principle d$ds = 0 for mechanical
systems. He understood that the sources of the gravitational field were not just
ponderable matter but also field energy. He realized that gravitational field energy
is to be included as a source and that the gravitational field equations were there-
fore bound to be nonlinear. He saw that the equivalence principle apparently held
only locally. As yet, he had no theory of gravitation. But he had learned a lot of
physics.
References
El. A. Einstein, letter to Pauline Einstein, April 4, 1910.
Ela. , letter to M. Besso, March 6, 1952; EB, p. 464.
E2. , letter to J. Laub, summer 1910, undated.
E3. , letter to J. Laub, October 11, 1910.
E4. , letter to J. Laub, December 28, 1910.
E5. —, letter to M. Grossmann, March 1911.
E6. —, letter to H. Zangger, August 24, 1911.
£7. , letter to M. Besso, May 13, 1911; EB, p. 19.
E7a. , letter to A. Stern, March 17, 1912.
E8. ,AdP 35, 898(1911).
E9. , Jahrb. Rad. Elektr. 4, 411 (1907).
E10. , AdP 17, 891 (1905), Sec. 8.
Ell. , letter to J. Laub, August 10, 1911.
E12. —, letter to H. Zangger, November 16, 1911.
E13. , letter to M. Besso, September 11, 1911; EB, p. 26.
E14. , letter to M. Besso, September 21, 1911; EB, p. 32.
E15. , letter to M. Besso, December 26, 1911; EB, p. 40.
E16. , AdP 38, 355(1912).
E17. , AdP 38, 443(1912).
E18. , letter to P. Ehrenfest, June 7, 1912.
E19. , Viertelj. Schrift Ger. Medizin, 44, 37 (1912).
Fl. P. Frank, Albert Einstein, Sein Leben und Seine Zeit, p. 137, Vieweg, Braun-
schweig, 1979.
F2. ,[Fl],p. 169.
HI. J. Havranek, Acta Univ. Carolmae 17, 105 (1977).
Jl. R. Jost, Viertelj. Schrift. Naturf. Ges. Zurich 124, 7 (1979).
J2. S. Jaki, Found. Phys. 8, 927 (1978).
*A short third paper on the c-field theory, also written in Prague [E19], will be discussed later.