I N T R O D U C T I O N T O V O L U M E 1 5 l x x x i IX. The New Quantum Mechanics Throughout his scientific life, Einstein was engaged in studying and deepening the understanding of the quantum nature of heat, light, and matter. Over a period of twenty-five years he had contributed numerous papers to research on quantum theory. In 1925, he quickly recognized the great importance of the new quantum theories and he was equally quick in realizing the conceptual difficulties involved. As this volume shows, from the beginning Einstein preferred wave mechanics over matrix mechanics, and he emphasized the important fact that Schrödinger’s wave function is defined on configuration space rather than on spacetime (Docs. 304, 307, 310, 353, 362).[54] On 15 July 1925, Einstein received a letter in which Max Born announced: “Heisenberg’s new work, soon to be published, looks very mysterious, but is cer- tainly true and profound” (Doc. 23). Werner Heisenberg’s paper was published on 18 September 1925. It was followed ten days later by a paper coauthored by Born and Pascual Jordan, and then by a third, joint paper by Born, Heisenberg, and Jordan in February 1926 (see Illustrations 22 and 24). In his first paper, Heisenberg 1925, Heisenberg reinterpreted the classical equa- tions of motion for position and momentum as expressing relations between arrays of numbers (hence the term “Umdeutung” in the title of the paper) Jordan and Born recognized these arrays as matrices with infinitely many rows and columns subject to a noncommutative multiplication law in Jordan and Born 1925 and in their joint Born, Heisenberg, and Jordan 1926 paper, the authors extended the pro- cedure to systems with arbitrarily many degrees of freedom. These three papers laid the foundations of the new matrix mechanics. Einstein appreciated the ingenuity of the new approach but was dissatisfied. Two days after the publication of Heisenberg’s first paper he wrote to Ehrenfest: “Heisenberg laid a big quantum egg. In Göttingen they believe in it (I do not)” (Doc. 114). He began a lively correspondence with Heisenberg and Jordan. Unfor- tunately, Einstein’s letters to both of them are not extant, with the exception of one postcard to Jordan (Doc. 212). Nevertheless, we can reconstruct Einstein’s comments and criticism from the long letters that both Heisenberg and Jordan wrote back to him, from which it becomes clear that Einstein’s letters, too, must have been extensive. The main part of the correspondence with Einstein took place while Born, Jordan, and Heisenberg were working on the definitive form of their new quantum mechanics. Indeed, in his first letter to Einstein on 27 October 1925 (Doc. 98), Jordan sent draft notes of what would eventually become the landmark paper Born, Heisenberg, and Jordan 1926.