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

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Dustin Lazarovici

Hi Travis,

thank you very much for your very detailed feedback! That’s a lot to think about and I certainly won’t be able to give a satisfying answer to all of your questions/objections right away. For now, I can just add a few remarks.

You’re right that the discussion in the final section would have been different with Bell’s 1990 picture in mind. That’s a good point! I guess I just used the definition of locality/conspiracy most suitable to the point I was trying to make. Anyway, in the end, the relevant question is not whether “no-conspiracy” is formally violated, but whether the physical account is morally conspiratorial in a way that undermines its explanatory power or even the whole idea of empirical tests. I believe that as far as my toy-model is concerned, this is rather not the case or at least it’s up for debate.

My goal is not to save or restore locality, at least I wouldn’t put it that way. Let’s agree that nonlocality is a fact of nature. Period. Advanced interactions are merely one possible way to implement/understand/explain nonlocality. And it may be a way to implement nonlocality that is more compatible with relativity than other approaches. That’s one good reason to keep an open mind about it.

Your second point is a very good one, too. You’re right, as a metatheoretical concept, nonlocality à la Bell is a much more fruitful than the “absence of direct space-like interactions”. I didn’t mean to dispute that, though. People who are sympathetic to retro-causation rather like to point out that when you assume time-symmetry as an a priori, some form of nonlocality were to be expected. They/we then usually see that as one (very important) explanatory success of the hypothesis. But then again, not everyone is impressed.

I agree that, in the end, “measurements” should be part of the theory. For a time-symmetric/retro-causal theory, it would thus be ok to require final boundary conditions for the entire universe. It would not be ok to assign a special status to individual measurement results. On this issue, I might stand closer to you than to Rod and Ken (as far as we understand their position correctly).

I believe, however, that in the end, when you have a complete theory, the process of measurement and preparation will turn out to be somewhat special in that they’re irreversible in a thermodynamic sense. And I believe (although my ideas on this are still very vague) that they will turn out to have a special role in defining “macro-causality”, because they have to do with our sense of agency. Under this premise and under this premise alone I think it’s legitimate to treat “measurement” and “observation” on a somewhat different footing when discussing simplified models (such as mine).

In other words, to the degree that “measurements” have a special status in retrocausal models, this should not prevent the models from being – at least in principle – embedded into a complete qtwo. Rather, a complete qtwo should be able to explain (away) the specialness.

In any case, I would never claim that, as of today, any retrocausal model is superior to or less mysterous than nonlocality in Bohmian mechanics (let’s say.) I would only maintain that such an approach is not a priori absurd or doomed to fail. And unless we have a complete relativistic qtwo, we should stay open minded towards different possibilites. There is no cheap way of reconciling quantum non-locality and relativity, that’s for sure.

Best, Dustin

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