in 1935, einstein, podolsky and rosen published a thought experiment with which they hoped to expose the incompleteness of the copenhagen interpretation of quantum mechanics in relation to the violation of local causality at the microscopic scale that it described. afterwards, einstein presented a variant of these ideas in a letter to erwin schrödinger, which is the version that is presented here. the state and notation used here are more modern, and akin to bohm's take on epr. the quantum state of the two particles prior to measurement can be written as