Reply To: On the single-particle Bohmian account

[Ken Wharton]

Hi Travis,

Thanks for the thoughtful response! You’re right that not much “needs explaining” concerning my original concern, and your last bit is a good point (where you point out that later position measurements can be used to measure other non-position quantities at earlier times). It somewhat bothers me that (say) the energy measured in such a manner isn’t *really* the energy of the particle, etc. But since it matches what a past-energy-measurement would have made, I guess you make a fair point that the single-particle account is a bit more general than I implied.

Concerning your preferred ‘jumping off point’, I absolutely agree that the approach you outline here is worth pursuing, but of course there are other jumping off points as well, also, based in 3D space (4D spacetime). Namely, classical fields.

One of the things that most bothers me about particle-based approaches in general, and dBB in particular, is that one ends up with quite strange stories for straightforward cases that map to classical wave interference. I would imagine there are even cases of classical E+M fields, of just the right intensity, that work perfectly fine even in the few-photon limit (say, perfectly tuned interferometers) but which would would require “dynamical nonlocality” in a dBB framework (assuming a model of particle-like photons, which I think you once told me about…). Of course, my field models look quite strange when applied to cases that clearly look like classical particles! But I at least wanted to mention that there’s more than one reasonable jumping-off-point to get back to spacetime-local beables.

I think you have my reaction just about right, concerning my dislike of “dynamical nonlocality”, and I don’t think that this is in any way equivalent to retrocausality (at least not the sort of models that interest me). You might take a look at the new afterword of my “The Universe is Not a Computer” essay, where I compare/contrast to Rob Spekkens concern about swapping the nonlocality between the dynamics and the kinematics. Here I tried to make it clear how Rob’s concern doesn’t really apply to all-at-once Lagrangian style accounts, hopefully setting retrocausal stories apart from those with true dynamical nonlocality (fundamental action-at-a-distance).