The Core of the Einstein-Podolsky-Rosen Experiment
Einstein to Erwin Schrödinger, Old Lyme, 19 June 1935
Einstein explains the core idea behind the Einstein-Podolsky-Rosen essay, which he feels was not explained clearly enough in the paper. As an example of a quantum-mechanical state, Einstein considers two closed boxes. There is a ball in one box, but nobody knows, in which. Is it a physical fact that the ball either is or is not in the first box even if one hasn’t opened any box – as Einstein claims – or is it only a fact after a box is opened – which is Bohr’s position? Einstein argues that Bohr’s position requires that the physical state in one of the boxes changes when the other one is opened. And that contradicts the fundamental intuitions of physics.