Abstract
This chapter offers a detailed reading of Bohr’s reply to EPR, which is one of the most important, and one of the most difficult and controversial of Bohr’s works. The present reading is arguably the first ever at this level of detail; and, in addition to supplementing the analysis given in Chapter 8, it aims to clarify some of the difficulties, acknowledged by Bohr himself, that the article customarily poses for its readers.
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Notes
- 1.
It is true that, as noted in Chapter 6, Bohr does not appear to readily acknowledge such mistakes in the case of complementarity, as against renouncing his reservations concerning the idea of the photon or abandoning the BKS proposal. In these cases, however, Bohr gave up his views because the experimental evidence against them became available. In the case of complementarity we deal with interpretation of the same evidence and the same theory, which is a more subtle matter, and Bohr might have seen his revisions as refinements rather than changes or corrections, perhaps not always justifiably. In any event, Bohr appears to have firmly believed (in this case justifiably, in my view) in the essential correctness of his counterargument to EPR, whose argument he continued to view critically throughout his life (cf. Bohr 1962). By a strange coincidence, the article also had “suffered” a rather peculiar history of reprinting. Thus, the version reproduced in (Wheeler and Zurek 1983) has the order of pages mixed (page 149 should preceed page 148), and the version reproduced in PWNB 4 contains significant errors, which seriously inhibit Bohr’s meaning. At one point, it substitutes “causal” for Bohr’s “rational,” which literally destroys Bohr’s whole point (PWNB 4, p. 75). There are other errors as well. For these reasons, as earlier in this study, I cite here from the original version, which is, however, also directly and, for a change, correctly reproduced in Bohr (1972–1996, vol. 7, pp. [292]–[298]).
- 2.
Bohr adds a note on the essential equivalence of his thought experiment to actual experiments, such as those related to the Compton effect (Bohr 1935b, p. 698n.).
- 3.
Among these commentators are B. d’Espagnat (d’Espagnat 1989, pp. 94–95, 255), J. Faye (Faye 1991, pp. 181–182), H. Folse (Folse 1987), J. Honner (Honner 1987, pp. 125–141), D. Murdoch (Murdoch 1987, p. 194), and H. Stapp (Stapp 1989, p. 162). Folse, however, appears to depart from this view in his subsequent commentaries (e.g., Folse 2002).
- 4.
If one wants to describe this situation in terms of phenomena in Bohr’s sense, it would appear as follows. All phenomena that occur in this setup (and the same is true for the complementary setup designed for momentum measurements) are associated with S 1, and any phenomenon associated with S 2 would necessarily involve a separate local measuring arrangement and require an actual measurement to be performed upon it. S 2—especially once it passes through the slit, which is when we are specifically concerned with it in the EPR context—is never part of any phenomenon in Bohr’s sense, unless some local measurement directly associated with it is performed. Some phenomena involved pertain to the apparatus itself, including the measurement (entirely classical) of the distance between the slits, and these may be seen as the preconditions of the final (EPR) measurement, performed at the next stage on the first object. It is the latter measurement that makes the prediction concerning the final position of S 2 possible, which is strictly in accordance with Bohr’s definition of phenomenon as relating to a single registered measurement. It is also clear that temporal considerations are important here and further indicate that it would be inaccurate to include the final (predicted) position of S 2, measured or not, in the same phenomenon with the final positional measurement on the first object, thus defined strictly locally in physical terms. Hence, it is difficult to agree with those commentators cited above (Note 3), who argue that Bohr sees both particles of a given EPR pair as linked to a single phenomenon (in Bohr’s sense) and sees the overall situation as nonlocal.
- 5.
Murdoch (Murdoch 1987, pp. 170–171) is an exception. However, his reading of Bohr’s reply is essentially different from the one offered here and, in my view, does not offer an adequate analysis of Bohr’s argument. In particular, Murdoch maintains, close to the lines of Einstein’s reading of Bohr’s reply, that Bohr “held no strong view on the question of locality,” and hence, would in principle allow for nonlocality (Murdoch 1987, p. 185). Murdoch also reads Bohr’s concept of phenomena as applied to the EPR experiment along these lines (Murdoch 1987, p. 194).
- 6.
There are well-known difficulties concerning the time variable in quantum mechanics, since it is not defined as a Hilbert-space operator and cannot be related to the energy operator H in the way the position and momentum operators are related, PQ – QP = ih/2π. Indeed it is not possible to construct an operator T such that HT – TH = ih/2π for a physically meaningful H. These difficulties do not affect Bohr’s argument, discussed in Chapter 8, concerning Einstein’s photon-box experiments, to which he refers in the note (Bohr 1935b, pp. 701–702n.). Cf. also (Peres 1993, pp. 413–415).
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Plotnitsky, A. (2010). Essential Ambiguity and Essential Influence: Reading Bohr’s Reply to EPR. In: Epistemology and Probability. Fundamental Theories of Physics, vol 161. Springer, New York, NY. https://doi.org/10.1007/978-0-387-85334-5_9
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