Reply To: Retrocausality is intrinsic to quantum mechanics

David Miller

Hi Daniel Rohrlich

Firstly and with respect, I am very enthusiastic about your idea of using “the existence of a classical limit” as an axiom. Apart from the fact that you have shown it to be fruitful, I think it strengthens physicists’ “grip on reality” which seems to be being progressively loosened.

I have a few questions/comments about your retrocausality argument which I hope you might consider responding to.

(1) In your GHZ example, Jim steers a bipartite system into a mixture of either product or entangled states. In an EPRB experiment, Jim can steer a single quantum system into a mixture of polarization states of his choice. Does the involvement of entangled states in the GHZ case add anything to the argument? It “feels” like it does (the paper of Timpson and Brown may be relevant here).
(2) In the DCES case, Jim steers into states which span the Hilbert space while in the GHZ case, Jim steers into a subspace. Is there a material difference?
(3) In our submission to this forum, Matt Farr and I try to draw a distinction between “correlation” and “causation”. In our terms, “correlation” applies when joint probabilities are changed but marginals are not. “Causation” is reserved for changes in the marginal probabilities. Relativistic causality means we are talking about “correlation” in this sense in the relevant cases. Perhaps your suggestion about retrocausality in A reasonable thing that just might work attracted criticism because the term unconsciously implies in the minds of readers that a (retro)change in marginal probabilities is involved. Perhaps a better and less alarming term is “retrocorrelation”. But if the marginal probabilities are not changed can there be a distinction between “correlation” and “retrocorrelation” (except by bringing in the direction of causation when the marginals are changed)?

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