x l I N T R O D U C T I O N T O V O L U M E 1 4 Sitter’s solution, despite the fact that Einstein eventually conceded that his criticism had been unwarranted. Yet, even though Einstein eventually admitted that De Sit- ter’s solution was a dynamical and matter-free solution to the Einstein equations, he still considered it an unphysical model that did not represent a possible cosmol- ogy (see Vol. 8, the editorial note, “The Einstein–De Sitter–Weyl–Klein debate,” p. 356). Moreover, even though he regretted his “only partly correct” criticism of De Sitter’s work in a letter to Ehrenfest of December 1918 (Vol. 8, Doc. 664), he never published a correction. In the Friedmann case, while issuing a correction, Einstein nevertheless harbored reservations. De Sitter’s solution was suspicious both because it violated Mach’s principle, and because it was non-static Fried- mann’s solution, in general, does not violate Mach’s principle, but is still non-static. It remains unclear whether Einstein’s deletion of the sentence expressing these doubts reflects his actual belief in the physical significance of the solution, or whether he still disbelieved its potential physical significance but decided to be cautious in print. The latter interpretation seems more likely. II During Einstein’s voyage to the Far East, a significant discovery had been made in St. Louis. In the fall of 1922, largely independent of any quantum considerations, Arthur Holly Compton had convinced himself that the observed shift in the wave- length of molybdenum X-rays scattered at right angles off a graphite target could not be accounted for by any classical scattering theory and could only be explained on the grounds of Einstein’s light quantum hypothesis. As late as October 1922, Compton had presented his data without any mention of the quantum hypothesis, but by the first days of December, at the Chicago meeting of the American Physical Society, he was able to explain the data: assuming that X-ray quanta were scattered off the loosely bound electrons in the target material, they would transfer momen- tum to the recoil electron, and consequently experience a shift in wavelength cor- responding to their loss of momentum. Compton had discovered the effect now named after him: “Word of Compton’s discovery spread like a shock wave emanating from Chicago. Eventually it provid- ed precisely the kind of experimental proof for quanta that Einstein had so recently sought […] Compton’s discovery was beyond doubt one of the most fundamental and far-reaching discoveries of this century.”[3] A brief summary of Compton’s Chicago presentation was published in the February 1923 issue of the Physical Review.[4]