1935. “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”: The EPR Experiment and Complementarity

Abstract

This chapter offers an analysis of Bohr’s exchanges with Einstein concerning the completeness and locality of quantum mechanics, most especially Einstein, Podolsky, and Rosen’s article and Bohr’s reply, both published under the same title—“Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”—in 1935. EPR’s article introduced a thought experiment, the EPR experiment, and offered a particular argument concerning it, EPR’s argument (to be distinguished from the EPR experiment), which led EPR to conclude that quantum mechanics is incomplete, or else nonlocal, in the sense of entailing instantaneous physical connections between events, connections forbidden by relativity. These conclusions were questioned by Bohr in his reply, which offers a different analysis of the EPR experiment and derives different conclusions concerning its meaning and implications. While important for Bohr and Einstein (and a few others at the time), the EPR experiment and the Bohr–Einstein exchange concerning it had been somewhat marginal to the debate concerning quantum theory, until Bell’s and related theorems discovered in the 1960s and then the experimental findings associated with these theorems. These developments have brought the EPR experiment to center stage of this debate, indeed largely defined now by the key questions posed by the experiment, especially those concerning the locality of quantum phenomena and quantum mechanics.