The Einstein-Podolsky-Rosen (EPR) paradox was enunciated in 1935 and since then it has made a lot of ink flow. Being a subtle result, it has also been largely misunderstood. Indeed, if questioned about its solution, many physicists will still affirm today that the paradox has been solved by the Bell-test experimental results, which have shown that entangled states are real. However, this remains a wrong view, as the validity of the EPR ex-absurdum reasoning is independent from the Bell-test experiments, and the possible structural shortcomings it evidenced cannot be eliminated. These were correctly identified by the Belgian physicist Diederik Aerts, in the eighties of last century, and are about the inability of the quantum formalism to describe separate physical systems. The purpose of the present article is to bring Aerts' overlooked result to the attention again of the physics' community, explaining its content and implications.
The Einstein-Podolsky-Rosen (EPR) paradox was enunciated in 1935 and since then it has made a lot of ink flow. Being a subtle result, it has also been largely misunderstood. Indeed, if questioned about its solution, many physicists will still affirm today that the paradox has been solved by the Bell-test experimental results, which have shown that entangled states are real. However, this remains a wrong view, as the validity of the EPR ex-absurdum reasoning is independent from the Bell-test experiments, and the possible structural shortcomings it evidenced cannot be eliminated. These were correctly identified by the Belgian physicist Diederik Aerts, in the eighties of last century, and are about the inability of the quantum formalism to describe separate physical systems. The purpose of the present article is to bring Aerts' overlooked result to the attention again of the physics' community, explaining its content and implications.
In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) published a thought experiment that is entirely correct, has been demonstrated in real experiments, and is now the most famous in quantum physics. Their pioneering work described, for the first time, quantum correlations and can be regarded as a very early glimpse into today's 'deep' quantum technologies, by which I mean those that enhance functionality by making use of quantum correlations. However, their work also contains a paradox that Erwin Schroedinger had already recognised as such in 1935 and which has since been cemented by the so-called Bell experiments. Here, I am now able to pinpoint the origin of the paradox within the chain of reasoning, which ultimately resolves the paradox.
In 1935, Albert Einstein, Boris Podolsky, and Nathan Rosen ('EPR') reported on a thought experiment that they believed showed that quantum theory provided an incomplete description of reality. Today we know that quantum theory is a complete and correct description of Nature (in flat space-time). The EPR thought experiment is predicted by quantum theory and has since been experimentally confirmed. However, EPR experiments have no physical pictorial explanation, and are considered part of a "paradox". Here I resolve the "paradox". I show independently of any interpretation of quantum theory that EPR (thought) experiments are a direct and compelling consequence of the existence of true randomness and the conservation of energy. It becomes obvious why EPR (thought) experiments allow for the precise prediction of truly random measurement values. Local hidden variables are not motivated.
In 1935 Einstein, Podolsky, and Rosen (EPR) published an important paper in which they claimed that the whole formalism of quantum mechanics together with what they called "Reality Criterion" imply that quantum mechanics cannot be complete. That is, there must exist some elements of reality that are not described by quantum mechanics. There must be, they concluded, a more complete description of physical reality behind quantum mechanics. There must be a state, a hidden variable, characterizing the state of affairs in the world in more details than the quantum mechanical state, something that also reflects the missing elements of reality. Under some further but quite plausible assumptions, this conclusion implies that in some spin-correlation experiments the measured quantum mechanical probabilities should satisfy particular inequalities (Bell-type inequalities). The paradox consists in the fact that quantum probabilities do not satisfy these inequalities. And this paradoxical fact has been confirmed by several laboratory experiments in the last three decades. The problem is still open and hotly debated among both physicists and philosophers. It has motivated a wide range of research from the most fundamental quantum mechanical experiments through foundations of probability theory to the theory of stochastic causality as well as the metaphysics of free will.
Roman Schnabel's article argues that the Einstein-Podolsky-Rosen (EPR) paradox can be resolved by identifying a flaw in what the author calls the "EPR implication" and by using radioactive alpha decay as an example showing that predictability does not exclude genuine randomness. The paper is clearly written and addresses an important foundational question. In our view, however, its main conclusion does not follow. The article narrows the original EPR argument, attributes too much to Bell-inequality violations, and replaces the central EPR structure - which involves incompatible observables and locality-based reasoning - with a simpler case of correlated random events. The result is an interesting interpretive remark, but not, we think, a satisfactory scientific resolution of the EPR problem.
More than 70 years after its publication, the article by Einstein, Podolsky, and Rosen (EPR) [88] is still cited hundreds of times every year in the literature; this is a very exceptional case of longevity for a scientific article! There is some irony in this situation since, for a long time, the majority of physicists did not pay much attention to the EPR reasoning. They considered it as historically interesting, but with no precise relevance to modern quantum mechanics; the argument was even sometimes completely misinterpreted. Astriking example is given in the Einstein-Born correspondence [89] where Born, even in comments that he wrote after Einstein's death, clearly shows that he never completely understood the nature of the objections raised by EPR. Born went on thinking that the point of Einstein was a stubborn rejection of indeterminism (“look, Albert, indeterminism is not so bad!”), while actually the major concern of EPR was locality and/or separability (we come back later to these terms, which are related to the notion of space-time). If giants like Born could be misled in this way, no surprise that, later on, many others made similar mistakes! This is why, in what follows, we will take an approach that may look elementary, but at least has the advantage of putting the emphasis on the logical structure of the arguments and their generality. Doing so, we will closely follow neither the historical development of the ideas nor the formulation of the original article, but rather will emphasize the generality of the EPR reasoning.
The famous EPR paradox shows that if we describe quantum particles in the usual way -- by their wave functions -- then we get the following seeming contradiction. If we entangle the states of the two particles, then move them far away from each other, and measure the state of the first particle, then the state of the second particle immediately changes -- which contradicts to special relativity, according to which such immediate-action-at-a-distance is not possible. It is known that, from the physical viewpoint, this is not a real paradox: if we measure any property of the second particle, the results will not change whether we perform the measurement on the first particle or not. What the above argument shows is that the usual wave function description of a quantum state does not always adequately describe the corresponding physics. In this paper, we propose a new, more physically adequate description of a quantum state, a description in which there is no EPR paradox: measurements performed at the first particle does not change the state of the remote second one.