Category: physics
Associated thinker(s): Albert Einstein; Boris Podolsky; Nathan Rosen
Original publication or approximate period: “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” (1935)
Difficulty: Advanced
Type: thought experiment
Themes: entanglement; locality; completeness
Related experiments: schrodingers-cat; wigners-friend
Prepare two systems with strongly correlated positions and momenta, then separate them. In the idealized original argument, a position measurement on one lets you predict the other’s position; choosing momentum instead lets you predict its momentum. EPR propose that a quantity predictable with certainty without disturbing the distant system corresponds to an element of physical reality.
If the distant system is unaffected by the local measurement choice, the argument seeks simultaneous elements of reality for both quantities, although the ordinary wave function does not assign them both sharp values. The alternatives are counterfactual measurement choices, not a claim that one run measures both quantities sharply.
Is quantum mechanics complete if local measurement choices allow certain predictions about distant systems?
EPR clarified a tension between a picture of separately existing distant systems and the structure of quantum states. Its legacy is not just a debate about interpretation: later Bell arguments yield constraints on broad classes of locally causal accounts that can be confronted with experiments. The original 1935 reasoning and those later experimental tests must still be distinguished.
EPR’s proposed conclusion is that the wave function is incomplete given their reality and locality reasoning. Bohr’s response challenges how the conditions for predictions and measurement contexts enter that inference. Later Bell analysis shows that a large class of local hidden-variable explanations cannot reproduce quantum correlations. Modern experimental violations constrain that class, rather than proving that controllable signals can travel faster than light.
Entanglement does not let an observer select a distant measurement outcome. The original scenario uses position and momentum; the familiar pair of spins is a later Bohm-style version. “Realism has been disproved” is an overbroad description of results that depend on precise assumptions.
Bohm’s spin formulation makes the structure easier to describe with discrete outcomes. Bell inequalities replace a purely philosophical exchange with conditional mathematical bounds and empirical tests.
The scholarly historical entry supports original authorship, date, argument, and later variations. No claim is made that EPR performed the proposed experiment or that one experiment has selected every aspect of a quantum interpretation.
Verified on 2026-10-30. Source coverage: [1] 1935 authorship, original position/momentum argument, Bohm variation and Bell developments. Reflection questions and illustrative numerical examples are original editorial material.