What one word does most to capture the central new lesson of the quantum? ‘Uncertainty,’ so it seemed at one time; then ‘indeterminism’; then ‘complementarity’; but Bohr’s final word ‘phenomenon’ – or, more specifically, ‘elementary quantum phenomenon’ – comes still closer to hitting the point. [...] In today’s words, no elementary quantum phenomenon is a phenomenon until it is a registered (‘observed’ or ‘indelibly recorded’) phenomenon, ‘brought to a close’ by an ‘irreversible act of amplification’. (Miller and Wheeler, 1984)57Brian Greene comments on the view that the essence of the quantum is entanglement. Maybe superposition is second. But he says that when he was in school, no one made a big deal out of entanglement.
I don't think any of these are so central to quantum mechanics. Yes, in a quantum mechanical system, particles are usually entrangled with others. But would you say that the essence of solar system gravity is that every planet exerts a force on every other? Maybe, but I doubt it.
Scott Aaronson has weighed in on the Bohr-Einstein debates. I added this comment:
If you believe in MWI, then both Bohr and Einstein completely missed what quantum mechanics is about. What makes QM unusual is not measurement, or entanglement, or superposition, or probability, or complementarity. It is the continuous splitting of the universe into parallel worlds, where essentially everything happens somewhere.Aaronson notes that von Neumann's assumptions have been criticized. Yes, that is true, but not the conclusion. Conventional wisdom has been since 1932 that a theory of classical variables will not work. So I do not think that Bell's theorem would have affected Bohr or Einstein at all.
While Bohr and Einstein did not find Bell’s Theorem, they were probably aware of von Neumann’s 1932 textbook with a theorem that had similar conclusions. That is, under certain hypotheses, QM cannot be recast as a theory of classical variables.
The bigger issue is my first point. Aaronson and many others now say that they subscribe to many-worlds theory (MWI). If so, why is he even talking about these other issues? MWI is so bizarre and counter to science that it makes all the other issues trivial.