210 RELATIVITY, THE GENERAL THEORYthat Einstein should accept his post in Zurich cheerfully and in good spirits [LI].
Soon thereafter, Lorentz the father figure spoke again. On February 29, 1912,
Einstein wrote to Zangger, 'I was called to Leiden by Lorentz to be his successor.
It was good that I was already committed to Zurich, for, if not, I would have had
to go there' [E8]. The Leiden position went to Ehrenfest, who took over in the fall
of 1912. Some time in 1913 Einstein sent Ehrenfest a letter which must often have
given its recipient food for thought: 'When Lorentz called me at that time I expe-
rienced an undeniable shudder' [E9].12b. From scalar to tensor
In August 1912, Einstein and his family arrived back in Zurich. On the tenth of
that month they were officially registered as residents of an apartment at Hof-
strasse 116. Some time between August 10 and August 16, it became clear to
Einstein that Riemannian geometry is the correct mathematical tool for what we
now call general relativity theory. The impact of this abrupt realization was to
change his outlook on physics and physical theory for the rest of his life. The next
three years were the most strenuous period in his scientific career.
In order to appreciate what happened in August 1912, it is essential to know
that before his arrival in Zurich Einstein had already concluded that the descrip-
tion of gravitation in terms of the single scalar c-field of the Prague days had to
go and that a new geometry of physical space-time was needed. I am convinced
that he arrived in Zurich with the knowledge that not just one but ten gravitational
potentials were needed. This opinion is based on some remarks in Einstein's
papers; on a study of all the letters from the period March-August 16, 1912,
which are in the Einstein archives in Princeton; and on recollections by myself
and by Ernst Gabor Straus, Einstein's assistant from 1944 to 1948, of conversa-
tions with Einstein.
To begin with, let us recall that the second of the 1912 papers discussed in the
previous chapter [E10] was completed in March. Toward the end of that month,
Einstein wrote to Besso, 'Recently, I have been working furiously on the gravi-
tation problem. It has now reached a stage in which I am ready with the statics.
I know nothing as yet about the dynamic field, that must follow next.. .. Every
step is devilishly difficult' [Ell].* Yet his initial response to the finished part, the
static case, was strongly positive. From Prague he wrote to Ehrenfest, 'The inves-
tigations on the statics of gravitation are ready and satisfy me very much. I really
believe I have found a piece of truth. I am now thinking about the dynamic case,
one again going from the special to the general' [E12]. This undated letter was
certainly written in 1912 and most probably before the middle of May, since by
that time, Einstein had become less assured. On May 20 he wrote to Zangger,
'The investigations on gravitation have led to some satisfactory results, although
"This important letter is not contained in the EB volume of their correspondence.