[xdarnold]

> Decoherence is a superposition effect, is it not? Entanglement with the environment, i.e. a superposition of system-environment states.

The point I'm trying to make is that "splitting", while sometimes identified with decoherence, isn't superposition (or any other well defined traditional QM phenomenon). It's a term peculiar to MWI and it importantly has no clear canonical technical definition. It generally refers to something just considered abstractly: the branching of a single _universe_ into multiple. If you use "split" and "entanglement" or "superposition" or any other QM term interchangeably, you are bound to invite misunderstanding.

> ...illuminate their question about superpositions propagating...

Agreed... if that was their question. But their question didn't reference superposition at all, it was about a split propagating:

> ...a quantum event occurs here and the universes split, that split propagates out at the speed of light...

Which is why I responded as I did. It is understandably confusing to wonder what it means for propagating split universes to meet years later, if you start talking about splits in this way. Propagating superposed particles? Much easier to make sense of.

[n4r9]

Thank you for bearing with me for so long. I think I understand the point of contention, i.e. that "split" is a slightly nebulous term which depends not only on splitting but somehow on there being a negligible likelihood of future interference between branches. In this context I agree it doesn't make sense to speak of a split being spatially localised.