What is the EPR thought experiment

EPR paradox

EPR paradox, thought experiment named after A. Einstein, B. Podolski and N. Rosen (1939) on the question of the incompleteness of quantum theory, which was to be demonstrated on the basis of the behavior of coherent two-particle states when measuring one of the particles.

The experiment is presented here in the new formulation by D. Bohm:

Two Spin 1/2 particle in the singlet state


are emitted from a source and move away from each other. If the distance between the two particles is so great that they can no longer communicate with each other, the following correlations can be found in the above system when measuring the single-particle spin states: If the z-Component S.z particle 1 has an upward spin, particle 2 has a downward spin and vice versa. If you measure that x-Component S.x of particle 1 and finds + / 2, then has S.x particle 2 has the value - / 2 and vice versa. In this system the non-locality of quantum theory is expressed: The measurement of particle 1 influences the experiment on particle 2. The non-locality as a result of correlated many-particle states does not lead to any contradictions to the theory of relativity. Even if a spin component of one particle is measured, that of the other particle is immediately known, no information can be transmitted with it. The correlation can be determined after the measurement. Einstein, Podolsky and Rosen linked this thought experiment with the following argument in favor of a theory of hidden parameters: By measuring particle 1, the result of measuring particle 2 is known in advance. Since the separation of the particles was not affected, the measurement results must have been determined before the measurement. The paradox is that while the consequences of the quantum mechanical state are taken into account, the inherent non-locality is neglected.