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

[Dustin Lazarovici]

Dear Ken,

thank you very much for your comments. It’s nice to get feedback from someone who is so skilled in this area.

Let me try to briefly respond to your points:

1) You are right, the model – as it stands with the 1st order dynamics – is actually not time-symmetric! That’s why I also had to change the title btw. 😉

I don’t think the (appearent) asymmetry in the measurement process is the problem, though, since I believe that, when properly analyzed, (quantum-)measurements turn out to be irreversible in a thermodynamic sense.

Concerning the dynamics, the “solution” that I have in mind – rather than going 2nd order – is that, in the end, you will still need something like a wave-function or quantum state to manifest the structure of entanglement. And this object, whatever it is, may itself have a nontrivial transformation under time-reversal. E.g. in Bohmian mechanics, the guiding equation is first order but the wave-function get’s complex conjugated under time-reversal, compensating for the sign.

Anyway, in order to make the points I was trying to make, it was more convenient to work with a toy model that involves advanced and retarded actions in an asymmetric way. But of Course I believe that if one considers a more serious retrocausal theory, it should be motivated by time-symmetry.

2) You’re right, if you have free fields, the distinction between the advanced and retarded part is somewhat arbitrary. However, I don’t believe in free fields. 😉

3) Yes, absolutely! As I said, the toy-model doesn’t actually explain or account for “entanglement”, i.e. why a pair of particles should be able to interact over arbitrary distances without being disturbed by others. The problem is not the light-cone structure, though. The light-cone structure is a good thing, as it makes the interactions intrinsically relativistic. However, I believe that any more serious theory will need additional ingredients, to account for a structure of entanglement.

4) A colleague of mine is working on such “history space” measures in a somewhat different context. I agree that this is probably the way to go for a statistical analysis of time-symmetric theories, but it’s not that easy. If you have more references on that I’d be very interested.

5) I wasn’t familiar with Gerchberg-Saxton, I’ll look it up! References are very welcome.

6) I agree. The ontological level matters most when assessing whether a theory is “conspiratorial”. In my brief discussion, I was trying to make the connection between the ontological level and the formal “no conspiracy assumption” which enters the derivation of Bell’s inequality. I’m not sure how well I succeeded, though.

Thanks again for your comments!