Einstein, Albert

a letter concerning the general theory of relativity written in the year of its experimental confirmation

News that the predicted effect of gravity on light had been observed reached the media in November 1919, making Einstein a world figure.

During the summer, Einstein's stepdaughter Ilse Einstein, his secretary at the time, was away on holiday. The correspondence was handled by a temporary assistant, Lotte, whom Einstein mentions in a letter to his stepdaughters on August 17 (see The Collected Papers of Albert Einstein, Vol. 9, Doc. 90, pp. 132-134.) Writing to a colleague at the Technische Hochschule Berlin-Charlottenburg (now the Technische Universitaet Berlin), while he was teaching at the University of Berlin, Einstein tries to unpack a confusion which arose due to varying assumptions in the Special Theory and in the General Theory [in translation]: "In fact, one needs to distinguish between the special theory of relativity laid down in 1905 and the one which resulted from its extension into the general theory of relativity. According to the first, velocities over 300,000 km/sec. are excluded; the application of a rotating coordinate system is not permitted in this case. Therefore, according to this theory, the rotation still has an absolute character. The general theory of relativity however allows arbitrarily rotating coordinate systems, but knows no upper limit of velocity relative to the coordinate system. You can find a more detailed treatment of this, for example, in my little book "On the Special and General Theory of Relativity" published by Vieweg."

Our thanks to Diana K. Buchwald, General Editor of the Einstein Papers Project for her help with the translation.