l x 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 5 on 13 March 1926 (Schrödinger 1926c), which Einstein must have seen for the first time during the second week of April. In a letter to Zangger early that month he had complained: “I like De Broglie’s idea, but so far nothing can be done with it” (Doc. 243). But nine days later, he praised Schrödinger in a letter to Ehrenfest: “The Born-Heisenberg thing is probably not correct, after all. It does not seem pos- sible to make the correspondence between a matrix function and a normal one unique... In contrast, Schrödinger created a very different and highly clever theory of quantum states by allowing De Broglie’s waves to act in phase space... Not such an infernal machine but rather a clear idea and ‘unavoidable’ in its application” (Doc. 253). Similarly, he expressed excitement in his letter of the same day to Lorentz, recommending Schrödinger as a speaker at the upcoming 1927 Solvay conference instead of himself: Schrödinger “has a theory of quantum states in press, a truly brilliant implementation of De Broglie’s idea” (Doc. 254). Lorentz responded by clarifying that he envisaged two quantum lectures at the Solvay con- ference, one of which would address fundamental quantum dynamics, to be given by Heisenberg or Schrödinger, and a second on quantum statistics. Lorentz hoped that Einstein would take it upon himself to speak on the latter (Doc. 269). Einstein agreed and again praised Schrödinger, whose “version of the quantum rule impresses me much this seems to me to be a piece of the truth, even though the meaning of waves in an n-dimensional q-space remains so much in the dark.” For the first time Einstein pointed out what he considered to be the crucial problem with Schrödinger’s wave function, namely, that it is not defined on spacetime, but on configuration space, where n is the number of particles in the system described by the wave function (Doc. 272). He also raised this issue in letters to Besso, Epstein, Sommerfeld, and Ehrenfest, and complained to the latter about the unin- telligibility of Paul Dirac’s recent work as well (Docs. 271, 304, 353, 356, 362). Einstein began the correspondence with Schrödinger on wave mechanics shortly afterward. On 16 April 1926 he wrote: “Mr. Planck has shown me your theory with justified enthusiasm, which I then also studied with the greatest interest.” He immediately started to tackle the details of the theory, but in the margin he added: “The idea of your work shows real brilliance” (Doc. 256). Schrödinger replied warmly: “Approval by you and Planck is more valuable to me than approval by half the world... Besides, this whole matter would not have been created now or ever (I mean, by me) if your second gas degeneracy paper had not shoved the impor- tance of De Broglie’s ideas in front of my nose” (Doc. 264). Five days later, on 28 April 1926, Heisenberg delivered a lecture in the Berlin Physics Colloquium. Afterward, Einstein invited him to his home. In Heisenberg’s much later recollections (Heisenberg 1969), he credited Einstein with having laid the foundation, during their conversation, to his own discovery of the uncertainty relations in February 1927, almost a year later.