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

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Nathan Argaman

Hi again,

I’m sorry for not paying attention for so long, but looking back now I think I still need to respond to some of Travis’ comments of a week ago. There are two things I would like to clarify. One is the issue of “locality.” I said I would like to see retrocausation used in a way which restores some kind of locality to QM, and as I also said that Bell used “locality” as a short form of local causality (of the forward-in-time variety), that’s obviously not going to work. What I mean is that, like Ken, I would like to have a QTWO where all the ontic variables are “local beables,” i.e., defined in space-time, and not in some configuration space or Hilbert space. I hope that resolves it.

The second issue is really just a choice of words, but words can be important I think, and from your paranthetic remark ‘(certainly better than “free will”),’ it seems that you agree. Words are especially important for the novices who might be reading a review such as a scholarpedia article, and much less so for the experts who know precisely what each phrase stands for. Now, I agree with most of what you said. It is natural to make the causal-arrow-of-time assumption, and even though I might find it frustrating that it’s not mentioned explicitly, with the causal arrow of time taken for granted, it is completely reasonable for the assumption that the device settings are independent of lambda to be called “no conspiracies,” and its negation may be called “conspiratorial.” In the disussion which ensued at the time, such conspiracies (those that deny the status of free variables from device settings which are ostensibly free) were said to be of the type that would undermine any scientific enquiry, and rightly so.

Now, things become completely different in contexts in which one is explicitly considering retrocausality, i.e., the assumption of the causal arrow of time is not made. In this case, it is not the device settings but lambda which becomes a dependent variable (the device settings are still free variables). I think it is highly misleading (albeit not to the experts who know all of this well) to continue to use the same name for that. Clearly, you should just call this option retrocausal, or denial of the arrow of time or something like that. Even if you don’t like it and are completely skeptical about the possibility of it leading to something useful, you shouldn’t want to get away with hinting that this option can be disposed of by the same arguments that were used to argue against conspiratorial descriptions. You realize, of course, that retrocausality requires a completely different discussion in order for a novice to be able to decide whether or not to adopt the opinion that it is or isn’t worth pursuing.

Given that retrocausality is supposed to apply to variables which are hidden also in the usual quantum description, such as which-path variables (e.g., those which determine which slit a particle went through in a delayed-choice interference experiment), the assumption of no retrocausality is clearly the less obvious one. Thus, in a brief discussion, the assumption which it makes sense to take for granted is not the arrow of time assumption but the “no conspiracies” assumption (in the sense that the device settings are indeed free variables, a sense which is meaningful in both causal and retrocausal discussions).

It is this argumentation about the wording which I hope you find convincing. And I hope that Dustin too finds it convincing… Of course, there is also a lot about which we must agree to disagree, and there’s no reason to be particularly frustrated about that.

Cheers, Nathan.

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