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

[Dustin Lazarovici]

Hi Travis,

thanks for shedding some light on this background story. I know your scholarpedia article on Bell’s theorem – which is a great article btw – but I didn’t know about the discussion you had with Nathan, of course. I guess Nathan could cite many other sources who neglect the possibility of retrocausal explanations, but I wouldn’t be surprised if he had you in mind, as well. 🙂

Anyway, concerning your other points, I like to emphasize once again that the violation of the Bell inequality in my toy-model does not simply come down to a violation of “no-conspiracy”, i.e. correlation between “lambda” and the “settings”. The issue turns out to be somewhat more subtle (and I think somewhat more interesting).

The relevant lambdas in the (causal) past of the measurement events are not sufficient to “screen off” the correlations – and they are not correlated with the parameter choices. Hence, if you consider only lambdas in the past, the no-conspiracy assumption is formally satisfied, but the locality assumption is violated. It is only when you admit “future common causes” that you can screen off the correlations while (formally) violating no-conspiracy.

Moreover, as primitive as my toy-model may be, it is actually “ontological”. And while the parameter choices are somewhat “outside the system”, I don’t believe that the consistency of the account depends on it.

So it would be very helpful (at least for me) if you could elaborate on your objection to retrocausal accounts of nonlocality. Then I’ll know if I can say anything to help you overcome your hostility. 🙂

By the way: I’d like to emphasize that I’m not an advocate of retrocausation per se. I have some sympathy for it because it is suggested by time-symmetry. Mostly, though, I understand that there is certainly some price to pay if we want to reconcile nonlocality and relativity. And I think that, in the end, retrocausation may not be that much worse than the alternatives. That’s why we should stay open-minded.