Reply To: Are retrocausal accounts of nonlocality conspiratorial? A toy model.

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OK, friends,

Now I’ve read the rest of the discussion again, and I see that Ken has already given good answers to your questions, Travis. I’d like to add just a few more words:

(a) First, it seems that we will all remain unhappy unless we have a quantum theory without observers. That was the main point in my discussion back when I wrote my article: We had Bohmian mechanics, which was represented in Bell’s 1964 article by a simplistic nonlocal toy model; subsequently, simplistic retrocausal toy models were found to work as well, but we haven’t found a retrocausal analogue of Bohmian mechanics yet. We should strive to find one (or else, prove that it’s impossible, which isn’t going to be easy to do – as you say, there’s no obstacle in sight).

(b) I think locality should be understood as synonymous with local causality (those are the words Bell himself used in his later publications), i.e., it is a notion which is to be defined only in the context of discussions in which the causal arrow of time has already been accepted.

(c) For retrocausal models, in order to describe the physics of the world in which we live (i.e., in order to develop a qtwo) we will need to break time-reversal symmetry: we will need to somehow include the fact that the entropy was low in the past in our description. It is perhaps possible that this will be accomplished just by treating initial conditions differently from final conditions, as in classical mechanics. One could then hope to develop a theory in which “no signalling” and “information causality” are manifest. That would rule out the absurdities you allude to in your Boston/China example (I don’t call them conspiratorial, because you haven’t denied the free-variable status of any device settings). So here my reply is different from Ken’s – I would not rely on time symmetry in this case.

Overall, I don’t think it’s going to be that easy to find a retrocausal qtwo with strictly local beables. Just constructing a description of a dissipative measurement would require a lot of work, perhaps akin to the well-known Legget-Caldeira description. But I certainly see it as well worth pursuing.

Travis, are you by any chance persuaded? Can you bring up further points, or do you just feel outnumbered?

Best, Nathan.

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