EINSTEIN–PODOLSKY–ROSEN THOUGHT EXPERIMENT

Thought experiment introduced in 1935 by these three authors in their Physical Review paper ‘Can Quantum-Mechanical Description of Physical Reality be Considered Complete?’ Suppose two quantum systems 1 and 2 that briefly interact, and are then separated and in no kind of physical contact. Then a measurement on system 1 for property P will yield some definite value, p ₁ for 1. Corresponding to this can be calculated a value of P, p ₂, for system 2. Similarly if we measured a property Q yielding a value q ₁ for 1, we can calculate the value q ₂ for 2. So definite magnitudes can be assigned to properties of 2 from measurements that do not affect 2. But if we now consider conjugate pairs (see Heisenberg uncertainty principle ), such as the position and momentum of a particle, this makes the existence of states of 2 depend upon which processes of measurement we choose to carry out on 1, although there is no signal betwen them. Attributing an antecedent determinate nature to 2 that can be revealed by a measurement on 1 contradicts the Heisenberg uncertainty principle, if that is taken to apply to reality itself, rather than to mere indeterminacies of measurement. The contentious conclusion of the paper was that the quantum mechanical description of physical reality is not complete. See also Bell's theorem.