Albert Einstein to Arnold Sommerfeld 1
DOI: 10.1063/1.1897512
Translated and annotated by Bertram Schwarzschild
Einstein was still a patent officer in the Swiss capital, two years after his annus mirabilis, when he wrote this letter to Arnold Sommerfeld, the eminent professor of theoretical physics at the University of Munich. At the time, the idea of a maximum signal-propagation velocity was still in dispute. The letter also discusses Einstein’s equivalence principle, which would become the foundation of his general theory of relativity, completed eight years later. Einstein remained at the patent office until October 1909, when he finally got his first academic position—a professorship at ETH, the Swiss Federal Institute of Technology in Zürich.
Bern, 5 January 1908
Highly honored Professor,
Many thanks for sending me your latest papers. Of greatest interest to me, of course, is the one on the speed of signal propagation. Last summer I had a lively exchange of letters on this subject with Professor [Wilhelm] Wien. I didn’t succeed in convincing him of my view. 2 From [Emil] Wiechert’s result—that the Maxwell–Lorentz equations can be replaced by action at a distance propagating at the speed of light (c)—I concluded that a signal whose propagation is due solely to electromagnetic interaction between point particles cannot possibly propagate faster than c.
Recently I’ve been working on the question of whether the relativity principle can be generalized to uniformly accelerated coordinate systems. After all, the fact that all bodies are equally accelerated by the gravitational field strongly invites us to view an accelerated coordinate system and an unaccelerated coordinate system with a homogeneous gravitational field as completely identical things. On the basis of that assumption, one arrives at entirely plausible conclusions. 3 As soon as I receive reprints of my paper on this subject, I’ll send you one.
Yours with great respect,
References
1. The Collected Papers of Albert Einstein, vol. 5, M. J. Klein, A. J. Fox, R. Schulmann, eds., Princeton U. Press, Princeton, NJ (1993), p. 85.
2. In the printed version of a lecture in 1907, Sommerfeld had written that the phase velocity of a light pulse, but not the signal velocity, can exceed c in a dispersive medium. Wien had expressed doubt about Sommerfeld’s conclusion.
3. A. Einstein, Jahrbuch d. Radioaktivität u. Elektronik 4, 411 (1907), available inThe Collected Papers of Albert Einstein, vol. 2 English translation, A. Beck, trans., Princeton U. Press, Princeton, NJ (1989), p. 252. This early paper already discusses the consequences of the equivalence principle for clocks and light in a gravitational field. Enthusiastically telling Sommerfeld of his bold conjecture, Einstein could hardly have known that, in a December 1907 letter to Hendrik Lorentz, Sommerfeld had characterized Einstein’s work as containing an unhealthy dogmatism in the “abstract-conceptual manner of the Semite” [die abstrakt-begriffliche Art des Semiten].
More about the Authors
Albert Einstein. ETH, the Swiss Federal Institute of Technology, Zürich.