Reply To: Retrocausality is intrinsic to quantum mechanics

Daniel Rohrlich

Thank you again!

1. Dear Aurelien Drezet, it is very appropriate that you invoke Bohr in this connection since, in my story about Alice, Bob and the first loophole-free test of Bell’s inequality (above in my answers to Travis and Ken) I subject quantum phenomenology to a classical description, as Bohr did. But I don’t see that I have committed myself to any ontology. If we only talk about “preparation” and “measurement” of states, what ontological commitment have we made? (And I certainly don’t consider time machines or exceeding the speed of light.)

Likewise, in returning to Travis’s question (about whether my claim applies to all interpretations of quantum mechanics), it seems to me that interpretations are not the issue. The issue is the description of quantum phenomenology in natural language. Thus, on the one hand, my claim is stronger because it is not tied to any particular interpretation. On the other hand, it is weaker because I don’t prove that quantum mechanics is intrinsically retrocausal; I argue that it is more natural and apt to include Jim’s choice in the preparation of Alice and Bob’s state, than to be tied to any microscopic account.

2. Dear Miroljub, please be patient with me; I found your question a bit cryptic. If I misunderstood, please try again!

Neither “A reasonable thing that just might work” nor my earlier paper “PR-box correlations have no classical limit” mentioned Planck’s constant h. Your question suggests to me that it might be worth examining how the two limits h → 0 and N → ∞ (where N is the number of systems in an ensemble) fit together. Making the role of h explicit might clarify that the transition from “quantum” to “quasi-classical” (or “semi-classical”) in my proof is indeed smooth.

Best regards,

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