Responses to the excellent responses you all gave to my previous post…
Dustin: I think I agree with everything you wrote about measurement being special in some thermodynamic sense, this being bound up with our sense of agency and macro-causality, etc. Obviously this is tricky stuff and nobody would claim to have a solid grasp on how it all fits together. In any case, I agree that in general it’s fine for a toy model to treat measurement/observation differently. My worry is that it “feels” to me (somewhat vaguely) as if it should be considerably harder to generalize from a toy model to a qtwo in a theory with retro-causation. I’m not sure I can say what that’s based on. Maybe it’s just the sheer mathematical issues associated with even defining appropriate initial data / boundary conditions.
Ken: yes, constructing a theory of exclusively local beables is a goal we share, but I’m really not at all convinced that retro-causality is going to somehow help on this front. (In the discussion with Rod S, we never even really got into this part, but I’ll just say that I couldn’t understand at all how he thought his theory was going to achieve this.)
Then re: your “one last point”, I guess I am pretty impressed by how Bell’s (time-asymmetric) notion of local causality rules out a certain class of phenomena, namely, those that violate Bell’s inequality. So you’re right to pick up on some element of that kind of hope in my comments. But really I didn’t mean to insist on something like that. I’m more just concerned that (if — like Nathan but apparently unlike Dustin? — you want to use retro-causality to preserve some notion of locality) you can’t use that notion of locality to cleanly and finally diagnose different candidate theories as either consistent, or inconsistent, with the notion in question. So it seems rather empty.
Good point about confounding factors. So maybe my China/Boston example isn’t probative. But then it was just kind of a joke anyway. I’ll have to think more about whether your response here really undercuts my whole worry, or just shows that this silly example isn’t the best one for expressing it.
I look forward to any further elaborations/comments you have on my point (4)!
Nathan: thanks for chiming in and nice to hear from you. I am, as I said before, genuinely sorry that our final edit of the scholarpedia thing fell off the back burner. (I also quite liked that expression when it occurred to me!) And I was sorry to hear that you felt frustrated by all the discussions about this stuff. Probably you can appreciate that we also felt somewhat frustrated by them (and that this probably contributed to our non-excitement to make final changes afterwards). I guess it would be fair to summarize by saying that, for you, retro-causality is a really important and central issue that you thought should be addressed and acknowledged and made into a really important and central thread in the article… whereas, for all of us, really, it’s not that important and basically the kind of thing that, sure, ought to be explicitly acknowledged, at least once (but maybe just in a footnote), but needn’t be made a big deal of. In any case, given this basic disagreement over what is really crucial and important and what isn’t, there was bound to be some mutual frustration there. And I bet if you had asked for less you would have ended up getting more, if that makes sense. But, that’s all behind us (at least until we fish the thing out from behind the stove and put it back on the back burner).
As to your actual comments here… I didn’t really understand your parsing of the old Bell vs Shimony et al thing. I think it’s clear that everybody was just taking for granted that there was no backwards-in-time causation. That’s a pretty standard and normal and reasonable assumption, in most contexts, you have to admit! And then given that assumption, the discussion was about the assumption that Bell makes explicitly in deriving the inequality (the independence of settings and lambda. I think Shimony et al didn’t really understand at first that there was a separate assumption here (separate from local causality) so they thought they were refuting the claim that violation of the inequality proved violation of local causality, period. Whereas in response Bell had the opportunity to clarify that there is just an additional assumption here. Anyway, I think — assuming no retro-causation — calling this additional assumption “no conspiracies” is completely reasonable (certainly better than “free will”). So I guess I see that exchange as just a normal and perfectly reasonable and comprehensible working out of the fact that there’s an additional assumption here. You are obviously upset that none of these guys ever bothered to question the even-further-in-the-background assumption of no retro-causation. But your comments almost read as if you see this whole thing as something like a deliberate attempt to wrongly diagnose retro-causation as conspiratorial. I just don’t see that at all. Retro-causation (often or maybe) violates the assumption that gets called “no conspiracies”, and quite reasonably so, by people who don’t really take retro-causation too seriously. When you press us we’re happy to acknowledge that that classification is somewhat misleading since there may not be anything “morally” conspiratorial (to use Dustin’s apt description) in such models. But that doesn’t necessarily make us take retro-causality any more seriously than we did before.
I’m glad to hear we’re in agreement about the ultimate need for a qtwo.
I was confused by your remark that “locality should be understood as … a notion which is to be defined only in the context of discussions in which the causal arrow of time has already been accepted.” Did you say, two posts up, that (unlike Dustin) you “would like to use retrocausality to save locality”? Did you mean a time-symmetric notion of “locality” in the earlier comment (which, for maximum confusion, I quoted second just now)? I’d be very interested to know how you might define/formulate this symmetric notion of “locality”.
And then finally, you asked me if I’m persuaded? Of what, exactly? I certainly didn’t find anything in all these comments (nice and thought-provoking though they were) that fundamentally changes my attitude about retro-causality, if that’s what you meant.
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