# EPR Paradox

# EPR PARADOX

EPR paradox is a seeming paradox conceived as a thought experiment by Albert Einstein (1879–1955), Boris Podolsky, and Nathan Rosen in 1935 as a challenge to the Copenhagen Interpretation of quantum mechanics. The title of the published paper was "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" Einstein did not believe that objects only had properties when they were observed. He tried to conceive of a situation in which the observation of one thing would lead to a situation in which it was one hundred percent certain that another state would be completely certain irrespective of whether it had been observed.

Atomic states can be created so that their total spin is zero. If the state is allowed to decay into two spinning particles that move off rapidly in opposite direction close to the speed of light then the particles must have spin of +1 and -1 units respectively, so that they add to zero, as required by the conservation of spin. According to the Copenhagen Interpretation of quantum mechanics the spin of either of the two particles does not exist until it is measured. Before any spin is measured any one of the two decay particles has a fifty percent chance of having spin +1 or spin -1. But EPR argued that if you measure one particle and find its spin to be +1 then you know that the other must be -1 without a measurement taking place. Einstein argued that this undermined Niels Bohr's (1885–1962) interpretation of quantum measurement. The word *paradox* became associated with this set up because it appears mysterious how the second particle to have its spin measured can "know" what the outcome of the measurement of the spin of the first particle turned out to be. However, Bohr showed that this was not in fact the case. The two particles are entangled by quantum reality in such a way that there is no violation of the Copenhagen Interpretation of quantum reality. The measurement of the spin of the first particle brings into being the spin of the second particle. More recently, experiments of this sort have been performed by Alain Aspect and colleagues in 1982 in ways that allow the predictions of quantum mechanics to be tested to see if there is any conflict with observation. So far, the predictions of quantum mechanics agree with all observations to high precision.

*See also* Copenhagen Interpretation; Einstein, Albert; Paradox; Physics, Quantum

*Bibliography*

aspect, alain; dalibard, jean; and roger, gnard. "experimental test of bell's inequalities using time varying analyzers." physical review letters 49 (1982): 1804.

d'espagnet, bernard. in search of reality. new york: springer-verlag, 1983.

herbert, nick. quantum reality: beyond the new physics. london: rider, 1985.

einstein, albert; podolsky, boris; rosen, nathan. "can quantum-mechanical description of the universe be considered complete?" physical review 47 (1935): 777-780.

john d. barrow

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