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The '''EPR paradox''' is the argument published in 1935 by [[Albert Einstein]], Boris Podolsky and Nathan Rosen under the title "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" Their conclusion was that it cannot: that quantum mechanics, though correct as far as it goes, must be an incomplete description of an underlying reality.

==The argument==

EPR set out a '''criterion of reality''': 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 that quantity.

They then considered two particles that have interacted and separated, prepared so that their total momentum and their relative position are both definite. Measuring the momentum of particle 1 fixes the momentum of particle 2 with certainty. Measuring instead the position of particle 1 fixes the position of particle 2 with certainty. Particle 2 is far away and, on the assumption that no influence travels faster than light, is not disturbed by what is done to particle 1. So both its position and its momentum must be elements of reality — yet quantum mechanics assigns no simultaneous values to both. Hence the description is incomplete.

The argument is valid. Its premises are the criterion of reality and locality, and given those the conclusion follows. Bohr replied within months in the same journal, in a paper whose meaning is still argued over, denying that the criterion of reality could be applied without reference to the whole experimental arrangement. Schrödinger responded in the same year with the papers that introduced the term ''Verschränkung'' — [[Quantum Entanglement|entanglement]] — and the cat.

David Bohm reformulated the argument in 1951 in terms of two spin-½ particles in a singlet state, replacing continuous variables with dichotomic ones. This is the version everyone now uses, and it is the version [[John Stewart Bell]] took up.

==What became of it==

For thirty years the EPR argument was regarded as a matter of interpretation, not of fact. Bell's 1964 paper changed that. Bell accepted EPR's reasoning and asked what a local completion of quantum mechanics — exactly the theory EPR were arguing must exist — would predict. The answer was that it would obey an inequality that quantum mechanics violates. So the question of whether EPR's conclusion could be realised became experimental, and the experiments have gone against it: see [[Bell's Theorem]] for the results, including the loophole-free tests of 2015.

The historically accurate summary is therefore neither "Einstein was refuted" nor "Einstein was right". EPR correctly identified that quantum mechanics either is incomplete or is nonlocal. Bell showed those are not the only options they thought they were, and experiment then excluded the local option. What Einstein wanted — a local, complete, deterministic account — is not available. That is a real loss for the classical picture, and it should be stated as such rather than argued around.

==On this wiki==

The EPR argument is a natural point of attachment for a collection that is largely committed to realist and local physics, and this wiki holds a body of work on it.

* [[Vesselin C Noninski]], "[[EPR Paradox and the Physical Meaning of an Experiment in Quantum Mechanics]]" — on what an EPR measurement physically consists of.
* [[Dan Brasoveanu]], "[[Significant Facts Revealed by the EPR Paradox and Bell's Theorem]]" — treats the two together, which is the right way round.
* [[Wladimir Guglinski]], "[[Demystifying the EPR Paradox]]".
* [[Al F Kracklauer]], "[[A Local-Real Model of EPR Correlations]]" — an attempt at exactly the local completion EPR called for.
* [[Thomas Smid]], "[[Classical Interpretation of EPR- Bell Test Photon Correlation Experiments]]".
* [[Caroline H Thompson]], "[[Timing, 'Accidentals' and Other Artifacts in EPR Experiments]]" — a methodological critique of how coincidences are counted, aimed at the experiments rather than at the argument.
* "[[On Causality and EPR Configurations]]" and [[Augusto Garuccio]]'s "[[Entangled States and the Compatibility Between Quantum Mechanics and Relativity]]" take up the compatibility question — whether EPR correlations sit comfortably with [[Special relativity|relativity]] — which is where the genuine tension lies, since the correlations do not permit signalling but do resist any Lorentz-invariant story about what happens first.
* [[Carroll O Alley]] and collaborators report laboratory work in "[[Experiments with Entangled Two-Photon States From Type-H Parametric Down Conversion: Evidence for Wave-Particle Duality]]".

Readers should note the chronology: most of this material predates the loophole-free experiments, and arguments resting on detector inefficiency or coincidence-window artefacts have been overtaken. Arguments resting on the ''interpretation'' of the correlations have not been, and remain live.

==See also==

* [[Bell's Theorem]]
* [[Quantum Entanglement]]
* [[Albert Einstein]]
* [[John Stewart Bell]]
* [[David Bohm]]
* [[Quantum mechanics]]

[[Category:Quantum Theory]]
[[Category:Philosophy]]
[[Category:Theory & Models]]

EPR Paradox - Revision history

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