"The wavefunction never collapses"

[Roberto Pavani]

The wavefunction itself is not a local variable.

So MWI has a non-local variable (the wavefunction) that contains all outcomes from the start. Isn't that structurally a non-local hidden variable theory, similar to Bohmian mechanics?

 

[PeroK]

So MWI has a non-local variable (the wavefunction) that contains all outcomes from the start. Isn't that structurally a non-local hidden variable theory, similar to Bohmian mechanics?

I don't understand that comparison at all. Bohmian mechanics is the complete opposite of MWI!

 

[Roberto Pavani]

I don't understand that comparison at all. Bohmian mechanics is the complete opposite of MWI!

Both have a non-local universal wavefunction that contains all outcome information before measurement. Bohm selects one outcome, MWI keeps all. But the non-local predetermination is the same, the difference is only in how many outcomes you keep.
This is all following the answers to my questions on this thread

 

[PeterDonis]

does the branching occur at A's measurement, at B's measurement, or at both independently? And if both, in which order? (Since they are spacelike separated, the order is frame-dependent.)

If you're going to bring in relativity, you can't use non-relativistic QM; you have to use QFT. So you need to go find a reference that analyzes a situation like this using QFT. I'm not sure if MWI proponents have settled on a single version of how the MWI works with QFT, but my general sense is that they view "branching" as occurring at each measurement independently and spreading at the speed of light. So there is no "time order" required; where the future light cones of A's and B's measurements meet is the only relevant factor, and that's an invariant.

 

[PeterDonis]

MWI has a non-local variable (the wavefunction)

In QFT there is no wave function, there are only quantum fields. How QFT captures nonlocality is more complicated than it is for non-relativistic QM.

 

[PeterDonis]

Both have a non-local universal wavefunction that contains all outcome information before measurement.

No, this is not correct for Bohmian mechanics. In Bohmian mechanics, outcome information is contained in the unobservable particle positions. The wave function is part of the nonlocal equation of motion for the unobservable particle positions. Bohmian mechanics explains single outcomes by attributing them to the unobservable particle positions--each particle has just one position. Those positions are nonlocal hidden variables; the fact that they're nonlocal (because their equation of motion includes the nonlocal information in the wave function) is what allows them to produce correlations that violate the Bell inequalities.

 

[Roberto Pavani]

I'm not sure if MWI proponents have settled on a single version of how the MWI works with QFT

Thank you.
If there's no settled version, isn't the branching structure underdetermined?

In Bohmian mechanics, outcome information is contained in the unobservable particle positions.

Thank you for the correction on Bohm. But the structural point remains: in MWI it's which branch you're in.
Both are inaccessible to the observer, both contain the outcome information. The specific variable changes, but the epistemological structure is the same.

 

[PeterDonis]

the structural point remains: in MWI it's which branch you're in.

But in the MWI, all branches correspond to actually occurring outcomes. In Bohmian mechanics, only one does. That's a big difference.

 

[PeterDonis]

If there's no settled version, isn't the branching structure underdetermined?

I don't think so; AFAIK there isn't any question about which branches there are. The question is about how MWI explains the branching process in the context of QFT, as opposed to non-relativistic QM.

 

[Roberto Pavani]

But in the MWI, all branches correspond to actually occurring outcomes. In Bohmian mechanics, only one does. That's a big difference.

The question is about how MWI explains the branching process in the context of QFT, as opposed to non-relativistic QM.

On the first point: agreed, the ontological claim is different. But epistemologically, the observer's situation is the same, inaccessible information determines the experienced outcome.

On the second: if there's no question about which branches exist, then for my earlier example of the entangled singlet, are there 4 branches (all spin combinations) or 2 (only those allowed by angular momentum conservation)? If 2, what eliminated the other 2?

 

[PeroK]

On the first point: agreed, the ontological claim is different. But epistemologically, the observer's situation is the same, inaccessible information determines the experienced outcome.

On the second: if there's no question about which branches exist, then for my earlier example of the entangled singlet, are there 4 branches (all spin combinations) or 2 (only those allowed by angular momentum conservation)? If 2, what eliminated the other 2?

Where did you get the idea that MWI does not respect entangled states? And might allow impossible uncorrelated outcomes?

 

[Roberto Pavani]

Neve said so, I'm just embracing the MWI and asking questions based on the answers received here on the thread. The post you quoted was just asking a number: 4 vs 2

 

[bhobba]

The question is about how MWI explains the branching process in the context of QFT, as opposed to non-relativistic QM.

David Wallace's book, The Emergent Multiverse, discusses its extension to QFT.

Thanks
Bill

 

[bhobba]

It collapses all the way down!

With my mentor's hat on, please refrain from quoting long posts.

Thanks
Bill