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Predefinição:No footnotes Predefinição:Quantum In quantum mechanics, the EPR paradox (or Einstein–Podolsky–Rosen 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.[1][2]
According to its authors the EPR experiment yields a dichotomy. Either
- 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...
- 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.)
As it was shown later by Bell one cannot introduce the notion of "elements of reality" without affecting the predictions of the theory. That is, one cannot complete quantum mechanics with these "elements", because this automatically leads to some logical contradictions.
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 complying with the Heisenberg Uncertainty Principle. As he once said: "God does not play dice", skeptically referring to the Copenhagen Interpretation of quantum mechanics which says there exists no objective physical reality other than that which is revealed through measurement and observation.
The EPR paradox is a paradox in the following sense: if one adds to quantum mechanics some seemingly reasonable (but actually wrong, or questionable as a whole) conditions (referred to as locality) — realism (not to be confused with philosophical realism), counterfactual definiteness, and completeness (see Bell inequality and Bell test experiments) — then one obtains a contradiction. However, quantum mechanics by itself does not appear to be internally inconsistent, nor — as it turns out — does it contradict relativity. As a result of further theoretical and experimental developments since the original EPR paper, most physicists today regard the EPR paradox as an illustration of how quantum mechanics violates classical intuitions.