Einstein–Podolsky–Rosen paradox

A 1935 argument that quantum mechanics is incomplete, based on entangled particles whose distant measurements appear to challenge locality.

Quantum entanglement: A quantum correlation between systems that cannot be explained as independent states of each part. The paradox uses entangled particles to make distant measurement outcomes tightly correlated.
Einstein–Podolsky–Rosen paper: A 1935 Physical Review article arguing that quantum mechanics does not provide a complete description of reality. It is the original publication that gives the paradox its name and structure.
Bohr–Einstein debates: A series of exchanges in which Niels Bohr and Albert Einstein disputed the interpretation and completeness of quantum mechanics. The EPR paper became a defining episode in their extended disagreement.
Local hidden-variable theory: A theory in which physical outcomes are determined by hidden variables and distant events do not exert instantaneous influence. It represents the kind of local completion EPR considered possible before Bell’s theorem constrained such theories.
EPR state: An idealized continuous-variable entangled state with perfectly correlated positions and anticorrelated momenta. This state supplies the simultaneous position and momentum predictions at the argument’s core.
Albert Einstein: A theoretical physicist whose work transformed modern physics and who challenged quantum mechanics’ completeness. Einstein helped formulate the argument as a challenge to distant action and the quantum state.
Bell's theorem: A theorem showing that no theory satisfying specified locality conditions can reproduce all quantum predictions. Bell converted the EPR tension into inequalities that distinguish local theories from quantum mechanics.
Copenhagen interpretation: A family of interpretations treating quantum predictions as complete while limiting claims about unmeasured properties. It accepts quantum mechanics’ predictive framework rather than adding EPR-style definite values for unmeasured quantities.
Quantum measurement: A physical interaction that yields a definite outcome associated with a quantum observable. Choosing which observable to measure determines which distant property can be predicted.
Boris Podolsky: A theoretical physicist who coauthored the 1935 EPR paper with Einstein and Nathan Rosen. Podolsky wrote the paper’s formal argument, which Einstein later described as not expressing his own approach well.

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