This paper traces the interpretation of entangled states from their discovery in 1926 to the culmination of this first round of interpretation in Einstein, Podolsky, and Rosen’s thought experiment of 1935. While some adopted a holist interpretation of entangled states, others believed that entangled states were really disguised mixtures of product states, a view this paper denotes entanglement exceptionalism. This paper argues that entanglement exceptionalism might have been the predominant view pre‑1935. This, in turn, offers a new framing of EPR as arguing that entanglement exceptionalism is an unstable view that, under reasonable assumptions, must result in the conclusion that quantum mechanics is incomplete. Furthermore, this paper speculates that it was well known that one could not adopt EPR’s assumptions and reject entanglement exceptionalism. Consequently, only entanglement exceptionalists would have been inclined to adopt EPR’s assumptions, and they may therefore have been EPR’s main audience. The paper ends by recounting how Wendell Furry, in the aftermath of the EPR paper, showed that entanglement exceptionalism does not reproduce the empirical predictions of quantum mechanics, so that, independently of EPR’s assumptions, entanglement exceptionalism had to be abandoned. The paper argues that Furry, John von Neumann, and Niels Bohr, likely among many others, took these properties of entangled states as yet another reason to adopt a form of anti‑realism with respect to quantum mechanics. This, finally, provides a corrective to the story that entanglement was ignored between 1935 and the 1960s. Entanglement was well known in this period, but Bohr and those following him were convinced that it was unproblematic for their various versions of anti‑realist interpretations.
Abstract The 1935 Einstein–Podolsky–Rosen (EPR) paper, “Can Quantum–Mechanical Description of Physical Reality Be Considered Complete?”, stands as a landmark in the foundational debates over quantum mechanics. While the paper has since become one of Einstein’s most cited works, it initially received little attention, and the feature of non-locality, which was central to the argument in the paper, was largely neglected for decades. In this paper, I offer two reasons for this early neglect of non-locality in the immediate aftermath of EPR. First, early responses to the EPR paper defended the completeness of quantum mechanics by reformulating the thought experiment the authors had presented, thereby obscuring the feature of quantum mechanics the authors had wished to highlight. Bohr’s influential reply, for instance, substituted the specific thought experiment presented by EPR with the two-slit experiment, thus suppressing the entanglement between spatially separated systems. A second reason for the marginalization of non-locality was the tendency to see Einstein’s critique of quantum mechanics as proceeding from a commitment to determinism, though this in fact played no role in the EPR paper or in his later writings and correspondence. This paper argues that the failure to recognize non-locality was partly a result of the rhetorical manoeuvring by EPR’s critics, but also to a broader philosophical climate dominated by concerns over determinism, which effectively eclipsed all other conceptual issues, Only with Bell’s theorem in the 1960s did non-locality return as a serious focus of foundational inquiry.
The EPR paper and Bohr's reply to it brought to an end the public debate between Einstein and Bohr. In the aftermath, Bohr's construal of quantum mechanics established itself as the basis of the orthodox interpretation – the ‘Copenhagen interpretation’ as it came to be called. The vast majority of physicists regarded Einstein's criticism as reactionary, and dismissed it with disdain. One of the main causes of this disdainful attitude was the general opinion that Einstein's criticism committed him to the existence of hidden states, and the existence of these, it was thought, had been conclusively ruled out by the work of von Neumann and others. What light does the subsequent work on the question of hidden states, and in particular the work of Bell, throw on the Bohr–Einstein dispute? Completeness and hidden states There are three distinct questions concerning hidden states. First, might there be such states, unbeknown to quantum mechanics? Second, can quantum mechanics, as it stands, take account of such states? Third, could some theory containing descriptions of such states reduplicate the predictions of quantum mechanics? Concerning the first of these questions, Einstein's view was that it is very likely that such states exist, and since quantum mechanics takes no account of them, it is incomplete and best regarded as a statistical theory analogous in some respects to classical statistical mechanics.