How Does the Many-Worlds Interpretation Address Nonlocality?

[JuneSpring25]

Hello,

I'd appreciate some help with understanding how the MWI deals wtih nonlocality.

My understanding is most versions of MWI do not include non-locality.

To start off this is my understanding of a potential entanglement expeirment:

Two particles A and B are entangled then the particles are moved to seperate labs around the world.

When measured seperately in space and time, their paired properties such as spin will always be correlated so if A is spin up, B is spin down.

In experiment 1 particle A is observed to be spin up.

In experiment 2 an hour later, particle B is observed to be spin down.

They are then measured again.

In experiment 3 particle A is observed to be spin up again.

In experiment 4 an hour later, particle B is observed to be spin down again.

This could carry on for a series of experiments over time so A might be measured as up, up, down, down, up and B would have to be measured as down, down, up, up, down, etc.

*

My understanding is MWI says this is because observers find themselves on the same branch for each experiment in sequence. Is that right? So some observers will see particle A as spin down and will then see particle B will be spin up, some observers will see particle A and spin up and will then see particle B as spin down. No observers will be on a branch were they see particle A spin down and then experience particle B as spin down as well.

If this is right so far, am I right in assuming something is believed to happen at the point of entanglement that sets out how the sequence of spin outcomes will turn out? Is that where MWI differs from non-local interpretations which suggest that the two particles are influencing each other in the moment of each experiment?

Answers appreciated!

 

[PeroK]

Hello,

I'd appreciate some help with understanding how the MWI deals wtih nonlocality.

My understanding is most versions of MWI do not include non-locality.

That's a fundamental misunderstanding. MWI is a interpretation of QM.

To start off this is my understanding of a potential entanglement expeirment:

Two particles A and B are entangled then the particles are moved to seperate labs around the world.

When measured seperately in space and time, their paired properties such as spin will always be correlated so if A is spin up, B is spin down.

In experiment 1 particle A is observed to be spin up.

In experiment 2 an hour later, particle B is observed to be spin down.

As long as the spins are measured about the same axis.

They are then measured again.

The entanglement is broken by the first measurement, so you can stop here.

In experiment 3 particle A is observed to be spin up again.

In experiment 4 an hour later, particle B is observed to be spin down again.

This is not correct. The particles are no longer entangled. The rest of your post appears to be conjecture based on this fundamental misunderstanding.

 

[PeterDonis]

The entanglement is broken by the first measurement

Not in the MWI. See below.

My understanding is most versions of MWI do not include non-locality.

As @PeroK says, that is wrong.

Two particles A and B are entangled then the particles are moved to seperate labs around the world.

When measured seperately in space and time, their paired properties such as spin will always be correlated so if A is spin up, B is spin down.

In experiment 1 particle A is observed to be spin up.

In experiment 2 an hour later, particle B is observed to be spin down.

They are then measured again.

In experiment 3 particle A is observed to be spin up again.

In experiment 4 an hour later, particle B is observed to be spin down again.

Your story here violates the MWI, because you only gave one result for each measurement. In the MWI, measurements have all possible results.

The correct MWI telling of your story is as follows:

The initial wave function, as expressed in the spin up/down basis (we are assuming, as @PeroK says, that all the spin measurements are about the same axis), is an equal superposition of "particle A up, particle B down", and "particle A down, particle B up". To be complete, since there are four measuring devices involved, we should also throw in the "ready" state of all of them, but that is a product state and does not show any entanglement with anything (although that will change, see below).

After experiment 1 is done, the wave function is now an equal superposition of

"particle A up, measuring device #1 shows A up, particle B down"

and

"particle A down, A measuring device #1 shows A down, particle B up".

with the "ready" states of measuring devices #2, #3, and #4 multiplied in. So now measuring device #1 is included in the entanglement--but A and B are still entangled as well (though the entanglement is no longer maximal since there is a third system involved).

Experiments 2, 3, and 4 then add more measuring devices into the entanglement, but since the results of 1 and 3, and 2 and 4, must match, that limits the number of terms involved, so the final wave function is an equal superposition of

"particle A up, measuring device #1 shows A up, measuring device #3 shows A up, particle B down, measuring device #2 shows B down, measuring device #4 shows B down"

and

"particle A down, measuring device #1 shows A down, measuring device #3 shows A down, particle B up, measuring device #2 shows B up, measuring device #4 shows B up"

So now all of the systems involved are included in the entangled wave function.

 

[PeterDonis]

The particles are no longer entangled. The rest of your post appears to be conjecture based on this fundamental misunderstanding.

You are correct that the OP made a mistake, but it wasn't this one. See my post #3.

 

[PeroK]

You are correct that the OP made a mistake, but it wasn't this one. See my post #3.

Ah, I see what was intended.

 

[JuneSpring25]

Thank you both for the comments.

To follow up, David Deutsch argues for a local interpretation here. I note this is an old paper. https://arxiv.org/pdf/quant-ph/9906007

Does anyone know if he has changed his mind on this or this has been superseded by new information?

 

[jbergman]

Thank you both for the comments.

To follow up, David Deutsch argues for a local interpretation here. I note this is an old paper. https://arxiv.org/pdf/quant-ph/9906007

Does anyone know if he has changed his mind on this or this has been superseded by new information?

I don't remember the episode, but Sean Carroll disagrees with this and says that MWI is fundamentally non-local. But obviously we have to be more precise as using words like non-local inly have precise meanings if you define clearly what you are talking about and have different meanings in different contexts.

He also mentions it here, https://www.preposterousuniverse.com/blog/2015/08/03/hypnotized-by-quantum-mechanics/

"It’s neither manifestly local nor Lorentz-invariant; those properties should emerge in appropriate situations, as often happens in physics."

 

[Morbert]

Many "Oxford Everettians" (Deutsch, Wallace, Vaidman et al) hold a local (in the sense of no spooky action) account of MWI.

 

[JuneSpring25]

This might be daft question, but in the entanglement experiment, (if measured on the same axis) why can't the two world banches be:

A spin up
B spin up

A spin down
B spin down

why must be they be opposite according to Many Worlds?

 

[PeroK]

why must be they be opposite according to Many Worlds?

Because MWI is an interpretation of QM!

 

[PeterDonis]

This might be daft question, but in the entanglement experiment, (if measured on the same axis) why can't the two world banches be:

A spin up
B spin up

A spin down
B spin down

why must be they be opposite according to Many Worlds?

Not just according to Many Worlds. The wave function in the experiment we are discussing is not a superposition of those two states. It is a superposition of "A up, B down" and "A down, B up". So those are the only results that are possible. That's just basic QM, independent of any interpretation.