[SEQ RERUN] Spooky Action at a Distance

I would be interested to see them, provided they were in physics journals (except for the Foundations of Physics, which often publishes stuff that can only be very charitably described as physics), and not in philosophy journals (my opinion of those happens to coincide with that of Feynman).

Fair enough. I will admit that the only physics journal in which I am published is in fact Foundations of Physics. You may not see this as a worthwhile venue for publication, but I hope that it will at least absolve me of the charge of dilletantism. I'd also point out that while in the past Foundations has had a reputation for encouraging mild crackpottery, its reputation has substantially improved since Gerard 't Hooft (I think probably the most insightful contemporary physicist) took charge. My publication in the journal was in the post-'t Hooft era, for what that's worth. I'd point you to my paper, but I'd like to retain a little anonymity here.

I disagree very strongly with you on the value of (scientifically informed) philosophy, but that is probably a topic on which we genuinely will not get anywhere, so I'll drop it.

That said, your response was (as I predicted) beneficial. I take it you are expressing surprise (or perhaps skepticism) at my claim to some degree of expertise. When I comment on physics, I try to balance signalling that I know what I'm talking about against being informal and accessible. Perhaps I'm not getting the balance right.

As for the immediate reason, I pointed out a rather elementary problem with units, which you did not address. Given that we cannot even agree on how propagation rate is defined, I do not see how we can make further progress.

Your point about units seems irrelevant to what I was saying. My comment was not intended to be non-responsive. I was trying to make clear what I was actually claiming, in the hope that it would also make clear that the units worry isn't a problem. Maybe I'll try again, at slightly greater length.

Look, here's the dialectic: Someone said that in MWI, world-splitting is global and instantaneous. I responded that it isn't, since "world-splitting" is just a consequence of decoherence, and decoherence takes time and relies on system-environment interactions, which are constrained by relativity.

At no point do I commit myself to measuring decoherence rates using units of velocity. I did speak of decoherence propagating, and this is what I meant by it: When a system decoheres its dynamics are no longer unitary. But the dynamics of an informationally isolated system are always unitary, so for a system to decohere, it must exchange information with its environment. Relativity constrains the (spatial) propagation of information.

Whether or not a particular quantum process has led to "world-splitting" is, in a sense, relative to an observer. Say I'm observing a Stern-Gerlach experiment. The spin state of the particle decoheres through interaction with the measuring device, From my perspective, though, this doesn't correspond to a splitting into two worlds with different spin measurements until information from the measuring device propagates to me. This will take a (slight) additional amount of time. To you, standing outside the lab, there wouldn't have been world-splitting until some portion of that information propagates to you; again, at finite velocity. Until that point, the whole particle-measuring device-me system will be in a state of superposition relative to you. So if we are dealing with localized systems, and we assume that information propagates through space at finite velocity, world-splitting can also be (crudely) thought of as propagating through physical space.

But I should emphasize that this in fact a crude approximation. I'm glossing a number of complications in this sketch. But these complications do not affect my main (in fact, sole) point: world-splitting in MWI is not global and instantaneous. Perhaps you think I'm making some bigger claim here, like MWI is a spatio-remporally local or relativistically invariant interpretation. I am not claiming anything of the sort. I don't even think the ordinary notion of spatial locality is well-defined in the MWI framework (since physical space is non-fundamental), and I made a comment above explicitly saying that MWI in its current form cannot be easily reconciled with relativity.