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Hi Nathan and Dustin —

Nathan, when I read this paragraph…

“I was also dismayed by the fact that retrocausation had been charaterized as conspiratorial (when I learned about it). Clearly, those who adopt this charaterization are taking the causal arrow of time for granted. Apparently their intuition is so firmly grounded in the macroscopic world that they just can’t really think in any other way. Unfortunately, even in those discussions which aim to carefully lay out all the assumptions involved in Bell’s theorem, this simple fact – that they take the causal arrow of time for granted – is not pointed out. However, if you ask them – which I am bent on doing – whether or not they’re assuming the causal arrow, they’re usually happy to admit that they are.”

…I couldn’t help but suspect you were thinking of the scholarpedia article on Bell’s theorem here! =) Maybe it will help Dustin and other readers to explain the situation there a bit? So Shelly Goldstein, Nino Zanghi, Daniel Tausk, and I wrote, at some point a few years back now, a big review article on Bell’s theorem for the website scholarpedia. We were lucky enough to get Nathan as a referee, and he made (among other comments/suggestions) the point that we don’t acknowledge, as explicitly as we might, that we are assuming no retro-causation. One of the effects of this (un- or at least under-acknowledged) assumption is then that the types of retro-causal models you guys are discussing here would violate what we call the “no conspiracies” assumption in that article — even though, as I think you are both entirely correct to point out here, such models may in fact not involve anything “conspiratorial” in the everyday sense of that term.

In any case, the sad truth (for which I can only apologize rather than offer any good explanation) is that going back and making a few small wording changes to that scholarpedia article, in response to Nathan’s good suggestions, has been on our joint to-do list for, well, about 5 years now. Somehow we just never got around to tweaking it (after some good in-person conversations about this stuff in Sesto a few years ago) and then it sort of fell completely off the back burner and is now hidden completely in a pile of dust and dead bugs behind the stove. Anyway, that little piece of history/sociology maybe explains what might otherwise appear as a slightly puzzling tone in Nathan’s paragraph that I quoted above. Is that fair Nathan? =)

Regarding the actual issues under discussion here, as I said, I agree with you guys (1) that there need really be nothing conspiratorial about a correlation between “lambda” and the “settings”, in the context of a retro-causal model and (2) that the assumption of no retro-causation should be made more explicit when it is being made, to avoid miscommunication and false impressions. That being said, I think I am among the “hostile” people that Dustin mentioned in his previous comment. That is, I just can’t really get myself to take retro-causality very seriously. Part of the stumbling block, for me, is that I can only even really understand what retro-causality *means* in the context of these sorts of toy models that treat different kinds of variables (like “settings”) in different ways. My sense is that somehow the very idea of retro-causation sort of crumbles away to dust in your hands as soon as you imagine instead a true “quantum theory without observers”, i.e., a theory that treats the whole universe in a consistent and uniform way (without, for example, any ad hoc “settings” that are treated as outside the system being described by the theory). I’d be happy to elaborate the thinking (half-baked though it remains) behind this sense, if it’s not at all clear why I’d say something like that.

And if it is at least somewhat clear, but you disagree with it, I’d be interested in hearing arguments intended to persuade me out of my hostility…

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