“Nicht Sein Kann was Nicht Sein Darf,” or the Prehistory of EPR, 1909–1935: Einstein’s Early Worries about the Quantum Mechanics of Composite Systems

Abstract

The story of Einstein’s misgivings about quantum mechanics and about his debate with Bohr has been told many times—by the participants themselves,¹ by their colleagues and contemporaries,² and by historians and philosophers of science of later generations.³ So the question arises: Why tell the story yet again? The answer is that there is more to be said. I will argue that the standard histories have overlooked what was from early on the principal reason for Einstein’s reservations about quantum mechanics, namely, the non-separability of the quantum mechanical account of interactions, something ultimately unacceptable to Einstein because it could not be reconciled with the field-theoretic manner of describing interactions.⁴ Showing the significance of this issue for Einstein is important not only for the sake of setting right the historical record, but also because it makes Einstein’s critique of quantum mechanics far more interesting—from the point of view of the physics involved—than if we see it resting merely on a stubborn old man’s nostalgic attachment to classical determinism.

The quote used in the title is taken from a letter of Wolfgang Pauli to Werner Heisenberg, 15 June 1935 (Pauli 1985, p. 402), in which Pauli takes issue with the EPR argument. Pauli himself took the quote from a poem by Christian Morgenstern, “Die unmögliche Tatsache,” reprinted in the collection, “Alle Galgenlieder” (Berlin, 1932), p. 163.