l x x i v I N T R O D U C T I O N T O V O L U M E 1 3 neither collaboration with colleagues nor experimental apparatus. From the above letter to Bohr we know that he took some of Bohr’s writings along on the trip. Nev- ertheless, no thoughts about quantum-theoretical questions left any trace in his di- ary or in any other papers. What he did think about, as documented in this volume, was the problem of find- ing a unified theoretical framework for a field-theoretic understanding of gravita- tion and electromagnetism, and the possibility of understanding the essence of matter in such a framework as well. In particular, Einstein seems to have been ar- guing in his mind with two of the foremost theoreticians of the time who were also interested in foundational questions of general relativity and a unified field theory. The authors whose work he was most deeply engaged with were Hermann Weyl and Arthur S. Eddington, although we have no direct evidence as to which, if any, of their writings he took along.[49] In any case, in his diary we find two notes documenting what he was thinking about scientifically on his outbound trip. Already on 9 October 1922, his third day at sea, he recorded having thought about the “gravitation-electricity problem.” He expressed agreement with Weyl “that a field or an invariant ds detached from anything electric has no reality.” It appears that his speculations were far-reaching and centered on matters of principle. He wanted to go beyond Weyl’s generaliza- tion of Riemannian geometry, and he weighed the possibility that a field theory had to be based on something other than differential equations: “Nevertheless, it seems to me still possible that one will be able to hold on to the field theory whether one could do the same with the expression of natural laws in terms of differential equa- tions seems doubtful” (Doc. 379, [p. 2v]). A second entry from his travel diary some two weeks later indicates that he had carried out some calculations in the framework of Weyl’s theory, but that he appar- ently obtained a “useless result” (Doc. 379, [p. 6]). Unfortunately, we have no fur- ther evidence of his scientific musings during his outbound trip. That situation changes on his way back. On the day of his departure from Japan on 29 December 1922, he found the ship “spacious and comfortable” and apparent- ly settled into some serious work immediately. On that very day, he noted that he had found the electromagnetic energy tensor, a problem he had discussed earlier with Ishiwara, and wrote a letter to Ishiwara. On the next day, he entered into his diary: “Idea for elaboration of Weyl-Eddington-Theory” (Doc. 379, [p. 28v]). Stopovers in Shanghai and Hong Kong during the next week may have distracted him again from his calculations, although he did continue his musings while at sea. After leaving Hong Kong on 6 January 1923, he immediately got back to work and noted: “New idea for the electromagnetic problem of the general theory of relativ- ity” (Doc. 379, [p. 30]). He continued to work, and three days later penned a paper on gravitation and electricity. gμν