l i i I N T R O D U C T I O N T O V O L U M E 1 4 In the end, Einstein conceded that he had been unable to make any progress in the theory. To Ehrenfest he wrote in late November 1924: “The thing is still totally nebulous and confused” (Doc. 384, see also Docs. 371, 444). But he remained hopeful that his exchanges with Bauer would yield results: “My ideas in that direc- tion are so far quite undetermined,” he wrote to Robert A. Millikan (Doc. 400). By then, Einstein had exchanged much correspondence with Millikan and others regarding a possible visit to Caltech. Ehrenfest, who visited Caltech in 1924, had informed Einstein that an invitation might be forthcoming for the next winter term, and although Einstein demurred (Docs. 255, 259), Millikan sent an invitation to him to spend three months during the upcoming winter quarter in Pasadena. Ein- stein declined, citing his teaching responsibilities in Berlin and his planned trip to South America, yet offered to visit Caltech sometime after January 1926. He later confirmed a visit for the winter of 1926 or, possibly, for 1927. In April 1925, Mil- likan again inquired about a visit in early 1926 and, if he came, whether Einstein would agree to deliver some public lectures at UC Berkeley as well (Docs. 329, 360, 400, 468). Eventually, Einstein’s first trip to Pasadena would take place at the end of 1930. VI In the period covered by this volume the German economy was further devastated by the effects of galloping hyperinflation, the demands of Allied reparations, and the occupation of the Rheinland and the Ruhr region. Repeated efforts by the Ger- man government to halt the runaway inflation and stabilize the mark finally bore fruit after the establishment of the Rentenbank and the introduction of the Renten- mark in October 1923. Further measures to improve the state of the German econ- omy, reorganize reparation payments, and end the occupation of the Ruhr were agreed upon in the Dawes Plan of August 1924.[21] Even though Einstein’s per- sonal financial situation had improved significantly in the early 1920s, money mat- ters continued to cause disagreements and crises in his relationship with his first wife Mileva MariF and his eldest son Hans Albert. The divorce decree of February 1919 stipulated that, should Einstein receive a Nobel Prize, the prize money minus 40,000 marks would be deposited in a Swiss bank as the property of his first wife, Mileva Einstein-Maric. The interest on this money would be entirely at her disposal, but she would be able to draw on the prin- cipal only with Einstein’s consent (Vol. 9, Doc. 6). After having learned that he