Schrödinger’s observation that entanglement shows up throughout quantum physics only deepened the problem for the Copenhagen interpretation. For any entangled system, Einstein’s choice applied: either the system is nonlocal, or quantum physics can’t fully describe all the features of that system. And Schrödinger had just shown that nearly any quantum interaction would result in an entangled system. Thus, the challenge the EPR paper posed wasn’t limited to some tiny corner of quantum physics—it was deeply embedded in the fundamental structure of the theory.
