Despite the spectacular success of quantum mechanics (QM) over the last 80 years in explaining phenomena observed at the atomic and subatomic level, the conceptual status of the theory is still a topic of lively controversy. Most of the discussion centers around two famous paradoxes (or, as some would have it, pseudoparadoxes) associated, respectively, with the names of Einstein, Podolsky, and Rosen (EPR) and with Schrodinger's cat. In this Viewpoint, I will concentrate on the paradox of Schrodinger's cat or, as it is often known (to my mind somewhat misleadingly), the quantum measurement paradox.
We review both the Einstein, Podolsky, Rosen (EPR) paper about the completeness of quantum theory, and Schrodinger's responses to the EPR paper. We find that both the EPR paper and Schrodinger's responses, including the cat paradox, are not consistent with the current understanding of quantum theory and thermodynamics. Because both the EPR paper and Schrodinger's responses play a leading role in discussions of the fascinating and promising fields of quantum computation and quantum information, we hope our review will be helpful to researchers in these fields.
The ‘EPR paradox’ is directly related to the 1935 paper of Einstein, Podolsky, and Rosen, even though Einstein did not include the word ‘paradox’ in it. All Einstein and his co authors did was to construct an argument that concluded that quantum mechanics was ‘not complete.’ This EPR conclusion is examined in detail here and found to be based on the erroneous assumption of an ‘all values’ spread in x that leads to an unphysical exact measurement of the momentum p . The word paradox, in relation the second part of the EPR paper, was introduced by Schrödinger in 1936 and popularized by Bohm in the 1950s. Hence, we should not attach responsibility to EPR for the ‘paradox’ situation. It was Schrödinger and Bohm who attached and propagated the term paradox in relation to the EPR paper. And the literature made it famous. This is particularly lamentable since the physics of quantum entanglement is utterly free of paradoxes .
The Einstein-Podolsky-Rosen (EPR) paradox was presented as an argument that quantum mechanics is an incomplete description of physical reality. However, the premises on which the argument is based are falsifiable by Bell experiments. In this paper, we examine the EPR paradox from the perspective of Schrodinger's reply to EPR. Schrodinger pointed out that the correlated states of the paradox enable the simultaneous measurement of
x^ and
p^, one by direct, the other by indirect measurement. Schrodinger's analysis takes on a timely importance because a recent experiment realizes these correlations for macroscopic atomic systems. Different to the original argument, Schrodinger's analysis applies to the experiment at the time when the measurement settings have been fixed. In this context, a subset of local realistic assumptions (not negated by Bell's theorem) implies that
x and
p are simultaneously precisely defined. Hence, an alternative EPR argument can be presented that quantum mechanics is incomplete, based on a set of (arguably) nonfalsifiable premises. As systems are amplified, macroscopic realism can be invoked, and the premises are referred to as weak macroscopic realism (wMR). In this paper, we propose a realization of Schrodinger's gedanken experiment where field quadrature phase amplitudes
X^ and
P^ replace position and momentum. Assuming wMR, we derive a criterion for the incompleteness of quantum mechanics, showing that the criterion is feasible for current experiments. Questions raised by Schrodinger are resolved. By performing simulations based on an objective-field (
Q-based) model for quantum mechanics, we illustrate the emergence on amplification of simultaneous predetermined values for
X^ and
P^. The values can be regarded as weak elements of reality, along the lines of Bell's macroscopic beables.
The famously controversial 1935 paper by Einstein, Podolsky, and Rosen (EPR) took aim at the heart of the flourishing field of quantum mechanics. The paper provoked responses from the leading theoretical physicists of the day, and brought entanglement and nonlocality to the forefront of discussion. This book looks back at the seminal year in which the EPR paper was published and explores the intense debate it unleashed. These conversations in print and in private correspondence offer significant insight into the minds of pioneering quantum physicists including Niels Bohr, Erwin Schrödinger and Albert Einstein himself. Offering the most complete collection of sources to date – many published or translated here for the first time – this text brings a rich new context to this pivotal moment in physics history. Both researchers and students in the history and philosophy of science, and enthusiasts alike, will find this book illuminating.