MWI and the entangled photon experiment

[Morbert]

For what it's worth I think that "global branching vs local" can also be settled by being careful with what you mean. (I am moderately familiar with both ideas and know some of the reasoning behind them, though I'd have to grovel to Google to check whether it's what your authors mean.) The instantaneous branching presumably reflects the way the global wave function branches; the local branching presumably reflects the expansion of the region which has actually interacted with the system in decoherence and where information about the interaction has reached. Totally different things, therefore no confusion - as long as you make sure you don't use the same word. No idea what people mean when they talk about different kinds of realism. K.I.S.S. is what I say!

Vaidman describes his stance re/ Sebens and Carroll here

Contrary to our analysis, Sebens and Carroll work under the assumption that “branching happens throughout the wavefunction whenever it happens anywhere". [...] Consequently, “observers here on Earth could be (and almost surely are) branching all the time, without noticing it, due to quantum evolution of systems in the Andromeda Galaxy." [...] Sebens and Carroll concede that this global branching picture is psychologically unintuitive (p11). But it also goes against the spirit of the many worlds interpretation, which involves removing as much nonlocality as possible. Thus, after removing the nonlocality of collapse, they reinsert a different kind of nonlocality.

Vaidman hopes to find a separable description of the wavefunction, but accepts that it doesn't exist at the moment. He accepts this kind of nonlocality. But he sees the Sebens and Carroll account as elevating nonseparability to a stronger nonlocality, where doing something here instantly affects something there. I don't see how this disagreement can be attributed to a confusion due to imprecise use of words.

 

[kered rettop]

That's how I understand the two descriptions as well.

One of the references in another thread on "Is the MWI local" had what it claimed to be a completely local description, but this interpretation involved having each qubit carry with it a potentially unbounded amount of information about all of its past interactions (this information would play the same role as the global wave function in the "instantaneous branching" interpretation).

We seem to be agreeing to an unnerving degree today, Peter. I think I'd better quit while I'm winning,

 

[kered rettop]

A large number of untrackable degrees of freedom...

Fair enough. I was not trying to define the third postulate precisely, only to point out there has to be one. Happy to leave the details to people who are capable.