EinsteinPodolskyRosen

The Einstein-Podolsky-Rosen paradox, commonly abbreviated EPR, refers to a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen that challenges the completeness of quantum mechanics. Published in Physical Review, the work presents a thought experiment involving two particles prepared in a correlated quantum state and then separated by a large distance.

The central claim rests on two ideas: locality and elements of reality. Locality is the assumption that a measurement performed on one part of a system cannot instantaneously affect a distant part. Elements of reality are criteria proposed by EPR: if, without disturbing a system, one can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to that quantity. In their scenario, by choosing to measure either position or momentum of one particle, an experimenter could predict with certainty the corresponding property of the distant partner. If quantum mechanics is complete, these distant properties should have definite values only when described by the wavefunction; otherwise, the theory would be incomplete.

From these considerations, EPR argued that quantum mechanics cannot provide a complete description of physical reality and that a more complete theory with hidden variables might exist to preserve locality. The paper sparked decades of debate about the nature of reality, measurement, and nonlocal correlations in quantum systems.

Impact and legacy: the EPR argument helped inaugurate the discussion of quantum entanglement and nonlocality. It influenced subsequent theoretical work, including Bell’s theorem in 1964, which showed how local hidden-variable theories could be experimentally tested. Experiments probing Bell inequalities have largely supported quantum predictions, though loopholes remain in some tests. The EPR paradox remains a foundational reference in discussions of the interpretation of quantum mechanics and the role of entanglement in quantum information science.