Portal:Physics/Selected article/Week 16, 2007

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In quantum mechanics, the EPR paradox is a thought experiment which challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. "EPR" stands for Einstein, Podolsky, and Rosen, who introduced the thought experiment in a 1935 paper to argue that quantum mechanics is not a complete physical theory. It is sometimes referred to as the EPRB paradox for David Bohm, who converted the original thought experiment into something closer to being experimentally testable.

The EPR thought experiment, performed with electrons. A source (center) sends electrons toward two observers, Alice (left) and Bob (right), who can perform spin measurements.
The EPR thought experiment, performed with electrons. A source (center) sends electrons toward two observers, Alice (left) and Bob (right), who can perform spin measurements.

The EPR experiment yields a dichotomy. Either

  1. The result of a measurement performed on one part A of a quantum system has a non-local effect on the physical reality of another distant part B, in the sense that quantum mechanics can predict outcomes of some measurements carried out at B or
  2. Quantum mechanics is incomplete in the sense that some element of physical reality corresponding to B cannot be accounted for by quantum mechanics (that is, some extra variable is needed to account for it.)

Although originally devised as a thought experiment that would demonstrate the incompleteness of quantum mechanics, actual experimental results refute the principle of locality, invalidating the EPR trio's original purpose. The "spooky action at a distance" that so disturbed the authors of EPR consistently occurs in numerous and widely replicated experiments. Einstein never accepted quantum mechanics as a "real" and complete theory, struggling to the end of his life for an interpretation that could comply with relativity without implying "God playing dice", as he condensed his dissatisfaction with quantum mechanic's intrinsic randomness and counter-intuitivity.