THE EINSTEIN-GROSSMANN COLLABORATION 211
until now I have been unable to penetrate beyond the statics of gravitation' [El 3].
Soon thereafter, there are hints of difficulties: 'The further development of the
theory of gravitation meets with great obstacles' (... stosst auf grosse Hinder-
nisse) [El4]. This undated letter to Zangger also contains a reference to von
Laue's discovery of X-ray diffraction. Since Einstein wrote congratulations to von
Laue in June [El5], it is most probable that the letter to Zangger was written in
that same month. Another letter, certainly written in June, contains a similar com-
ment: 'The generalization [of the static case] appears to be very difficult' [E16].
These repeated references to his difficulties are never accompanied by expressions
of doubt about his conclusions concerning the red shift and the bending of light.
He never wavered in his opinion that these phenomena were to be part of the
future physics. For example, he wrote in June, 'What do the colleagues say about
giving up the principle of the constancy of the velocity of light? Wien tries to help
himself by questioning the gravitational [action of] energy. That, however, is
untenable ostrich polities' [El6]. It is my understanding that Einstein was sure he
was moving in the right direction but that he gradually came to the conviction that
some essential theoretical tools were lacking.
There is a brief and cryptic statement in the last paper Einstein wrote in
Prague, in July, which indicates that he was onto something new. This paper (a
polemic against Abraham to which I shall return later) contains the following
phrase: 'The simple physical interpretation of the space-time coordinates will
have to be forfeited, and it cannot yet be grasped what form the general space-
time transformation equations could have [my italics]. I would ask all colleagues
to apply themselves to this important problem!' [El7]. Observe the exclamation
mark at the end of this sentence. I do not know how often such a symbol is found
in Einstein's writings, but I do know that it occurs only rarely.
On August 10, as said, Einstein registered as a Zurich resident. On August 16,
he writes a letter to Hopf. Gone are the remarks about devilish difficulties and
great obstacles. Instead, he writes, 'Mit der Gravitation geht es glanzend. Wenn
nicht alles trugt habe ich nun die allgemeinsten Gleichungen gefunden' [E18].*
What happened in July and early August 1912?
Two statements by Einstein tell the story. In his Kyoto address (December
1922), he said, 'If all [accelerated] systems are equivalent, then Euclidean geom-
etry cannot hold in all of them. To throw out geometry and keep [physical] laws
is equivalent to describing thoughts without words. We must search for words
before we can express thoughts. What must we search for at this point? This
problem remained insoluble to me until 1912, when I suddenly realized that
Gauss's theory of surfaces holds the key for unlocking this mystery. I realized that
Gauss's surface coordinates had a profound significance. However, I did not know
*'It is going splendidly with gravitation. If it is not all deception, then I have found the most general
equations.' One Einstein biographer wrote general for most general [S4], a nontrivial modification
of this crucial phrase.