In the first article of this series, we studied Einstein, Podolski and Rosen’s paper, where we encountered for the first time the concept of quantum entangled stated and their “spooky action at a distance”.
As EPR realized, when particles interact (under certain conditions), a new type of non-classical correlations arise. In order to understand why entangled states concerned Einstein so much, you have to remember that he never accepted the Copenhagen Interpretation of Quantum Mechanics. Among many other things, this interpretation postulated that physical observable properties arise only when they are measured, meaning that the interaction of an apparatus (observer) and the systems causes the wave-function to collapse. A perfect example of this is the so-famous Schrödinger Cat Experiment (I won’t get into any detail as I’m sure you all know what I’m talking about).
However, Einstein believed that in any complete physical theory, one should be able to predict the values of observable properties of quantum systems without the need to perform any measurement or to disturb the system. Said otherwise, physical observable properties should exist and have definite values independent of observation, they should be elements of reality. This is stated explicitly in EPR’s criterion of reality and sometimes recalled in Einstein’s quote:
“I, at any rate, am convinced that He (God) does not throw dice.”
In this article we will continue to study quantum entangled states and we will see what was the fundamental mistakes that Einstein, Podolski and Rosen made in the derivation of their “paradox”. Also, we will show why it is that we say that these correlations between quantum systems cannot be accounted for by any classical theory.