Quantum Mechanics and Physical Reality in Nature 136 1935, p. 65

London: Harrison & Sons, 1935. 1st Edition. FIRST EDITION OF NIELS BOHR’S IMPORTANT FIRST RESPONSE TO A PAPER PUBLISHED EARLIER IN THE SAME YEAR BY ALBERT EINSTEIN, EINSTEIN’S “LAST AND GREATEST ATTEMPT TO UNDERMINE QUANTUM UNCERTAINTY” (Peacock, The Quantum Revolution, 88). Einstein’s paper was co-authored with two younger colleagues, Boris Podolsky and Nathan Rosen; it essentially argued that quantum mechanics was not a complete physical theory. “Generally referred to as ‘EPR’, the Einstein paper quickly became a centerpiece in the debate over the interpretation of the quantum theory, a debate that continues today" (Stanford Encyclopedia of Philosophy). Bohr’s letter here precedes a later response in The Physical Review.

EPR had “introduced a thought experiment to argue that quantum mechanics was not a complete physical theory. Known today as the “EPR paradox,” the thought experiment was meant to demonstrate the innate conceptual difficulties of quantum theory. It said that the result of a measurement on one particle of an entangled quantum system can have an instantaneous effect on another particle, regardless of the distance of the two parts” (APS Physics, 14, 10, November 2005).

"By the time of the EPR paper many of the early interpretive battles over the quantum theory had been settled, at least to the satisfaction of working physicists. Bohr had emerged as the "philosopher" of the new theory and the community of quantum theorists, busy with the development and extension of the theory, were content to follow Bohr’s leadership when it came to explaining and defending its conceptual underpinnings. Thus in 1935 the burden fell to Bohr to explain what was wrong with the EPR "paradox" (Stanford Encyclopedia of Philosophy).

The publication of EPR “caused a furor in Copenhagen” (Peacock, 90). "When the EPR paper reached Niels Bohr… he realized that he had once again been cast in the role, which he played so well at the Solvay Conferences, of defending quantum mechanics from yet another Einstein assault. "This onslaught came down on us as a bolt from the blue," a colleague [Leon Rosenfeld] of Bohr’s reported. "Its effect on Bohr was remarkable Everything else was abandoned. We had to clear up such a misunderstanding at once” (Stanford Encyclopedia of Philosophy).

As noted earlier, Bohr’s more expansive remarks would come later, but given the uproar in the physics community regarding Einstein again questioning quantum uncertainty, he knew he had to respond immediately. Later, as said, he would offer an extended rebuke, but wanting to quell the ‘furor,’ he published the letter offered here.

Bohr begins his letter by laying out the heart of the EPR paper: that its authors “question of the completeness of quantum mechanical description” in arguing that ‘If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity’” (Bohr, Quantum Mechanics, Nature 136, 1935, p. 65).

Vehemently disagreeing with Einstein, the essence of Bohr’s response is a strong defense of the far stricter Copenhagen interpretation of quantum mechanics” (APS). At its essence, Bohr argues that EPR “had an unreasonably stringent requirement for completeness; [that] in fact, quantum mechanics is as complete as it can be. There is no more information to be had than [what] quantum mechanics can give us”(Peacock, 91). Item #558

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