# Non-locality in MWI

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## Main Question or Discussion Point

In another thread, @Nugatory asked
"how exactly does it turn out that both spacelike-separated observers end up in the same world without some appeal to non-locality?"

Worlds are defined by their observed macroscopic values. They separate through decoherence. It is a physical process. Since decoherence is so fast, it effectively propagates as a sphere expanding at the speed of light. Thus the whole universe does not split, the splits are in the future light cones from the two observations. Only where the cones intersect have both worlds split. Alice can know that Bob's worlds have separated, her world is, however, not yet split by anything over at Bob's place.

## Answers and Replies

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DrChinese
Science Advisor
Gold Member
Worlds are defined by their observed macroscopic values. They separate through decoherence. It is a physical process. Since decoherence is so fast, it effectively propagates as a sphere expanding at the speed of light. Thus the whole universe does not split, the splits are in the future light cones from the two observations. Only where the cones intersect have both worlds split. Alice can know that Bob's worlds have separated, her world is, however, not yet split by anything over at Bob's place.
Bob's world splits into 2 when he measures: one where you get (say) spin up and and another where you get spin down.

Alice's world splits into 2 when she measures: one where you get (say) spin up and and another where you get spin down.

Presumably these 2 expanding spheres (as you describe them) eventually meet. How do they know to sync up and produce the expected statistics? Especially since there must be many other expanding spheres meeting up all the time that are not entangled particles?

zonde and Mentz114
Demystifier
Science Advisor
Gold Member
Locality and non-locality are properties of the 3-dimensional world. But according to MWI, there is no 3-dimensional world. According to MWI, the only object that exists is the wave function, which does not live in a 3-dimenional world. It lives in a 3N-dimensional world, where N is the number of "particles". Hence MWI is neither local nor non-local. It is alocal. [https://arxiv.org/abs/1703.08341 Sec. 3.1]

According to MWI, the 3-dimensional world is an illusion, very much like a 2-dimensional shadow is an illusion emerging from a real 3-dimensional object. Our 3-dimensional world sometimes look local (as in the classical limit), sometimes non-local (due to quantum entanglement), but both locality and non-locality are mere illusions, low-dimensional shadows of the multi-dimensional reality.

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Matter Times, AlexCaledin, bhobba and 2 others
Bob's world splits into 2 when he measures: one where you get (say) spin up and and another where you get spin down.

Alice's world splits into 2 when she measures: one where you get (say) spin up and and another where you get spin down.

Presumably these 2 expanding spheres (as you describe them) eventually meet. How do they know to sync up and produce the expected statistics? Especially since there must be many other expanding spheres meeting up all the time that are not entangled particles?
I don't understand you. The correlations already exist.

Locality and non-locality are properties of the 3-dimensional world. But according to MWI, there is no 3-dimensional world. According to MWI, the only object that exists is the wave function, which does not live in a 3-dimenional world. It lives in a 3N-dimensional world, where N is the number of "particles". Hence MWI is neither local nor non-local. It is alocal. [https://arxiv.org/abs/1703.08341 Sec. 3.1]

According to MWI, the 3-dimensional world is an illusion, very much like a 2-dimensional shadow is an illusion emerging from a real 3-dimensional object. Our 3-dimensional world sometimes look local (as in the classical limit), sometimes non-local (due to quantum entanglement), but both locality and non-locality are mere illusions, low-dimensional shadows of the multi-dimensional reality.
Non-local wavefunctions are trivially expressible as a superposition of local, separable ones. So whilst "the" wavefunction may be non-local, it is always a superposition of 3-dimensional local states - ones that eventually go to make up the phenomenal worlds of MWI.

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Lord Jestocost
Gold Member
So, as Nick Herbert has remarked: “The moral of Everett’s tale is plain: if you don’t want to split, stop looking at attribute-laden systems.

bhobba
martinbn
Science Advisor
...According to MWI, the only object that exists is the wave function, ...
I must say i still don't understand what this means. What does it mean to exist when it refers to a wave function?

DrChinese
Science Advisor
Gold Member
I don't understand you. The correlations already exist.
There is no requirement of that. Entangled particles need not share a common past. They don't even need to have existed at the same time. But they will eventually have a future overlapping light cone.

Which is precisely why I am asking how this works.

There is no requirement of that. Entangled particles need not share a common past. They don't even need to have existed at the same time. But they will eventually have a future overlapping light cone.
Which is precisely why I am asking how this works.
I don't see what the origin of the entanglement has to do with the correlations.

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I must say i still don't understand what this means. What does it mean to exist when it refers to a wave function?
It means that something exists that is fully described and not over-described by the wavefunction. For economy we equate the real entity with its description.

martinbn
Science Advisor
It means that something exists that is fully described and not over-described by the wavefunction. For economy we equate the real entity with its description.
How is that specific to MWI?

How is that specific to MWI?
I have no idea. Is it?

DrChinese
Science Advisor
Gold Member
I don't see what the origin of the entanglement has to do with the correlations.
I'll try to say it another way. There are correlations in some worlds of Alice and Bob that match statistical expectations. How do the worlds know to match up properly when Alice and Bob's "expanding spheres" meet up? We have agreed that the source of the entanglement doesn't matter.

zonde
zonde
Gold Member
How do the worlds know to match up properly when Alice and Bob's "expanding spheres" meet up?
I would say that MWI wolds need a "snap" feature in addition to "split".
If I remember correctly there was something about it in this paper http://philsci-archive.pitt.edu/504/2/cracow.pdf

I'll try to say it another way. There are correlations in some worlds of Alice and Bob that match statistical expectations. How do the worlds know to match up properly when Alice and Bob's "expanding spheres" meet up? We have agreed that the source of the entanglement doesn't matter.
They are already "matched up". Nothing new happens when they meet.

I would say that MWI wolds need a "snap" feature in addition to "split".
If I remember correctly there was something about it in this paper http://philsci-archive.pitt.edu/504/2/cracow.pdf
TLDR. Please make your point clearly as I have no idea what a "snap" feature is

DrChinese
Science Advisor
Gold Member
1. They are already "matched up". Nothing new happens when they meet.
Nugatory asked: "how exactly does it turn out that both spacelike-separated observers end up in the same world without some appeal to non-locality?"

You answered: "2. They separate through decoherence. It is a physical process. Since decoherence is so fast, it effectively propagates as a sphere expanding at the speed of light. Thus the whole universe does not split, the splits are in the future light cones from the two observations. "

Your explanations contradict. One says they start "matched up" (which is nonlocal); the other says they meet in a future light cone (where the speed of light is a limiting factor).

So, which?

The Deutsch-Hayden interpretation of WMI shows that it's local in spacetime. There have been some criticisms over the years, but after a thorough review of this topic, I'd say it's well established by now. Deutsch addresses most of the criticisms in his most recent paper on the topic.

The main idea behind WMI is that the wave function is what's real, so you must look at it's evolution to see how the splits match up (how they interfere).

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"matched up" (which is nonlocal)
Why on earth do you say that? Please don't tell me Bell's Theorem!

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PeterDonis
Mentor
2019 Award
It would probably be helpful to this discussion to look at the math. Say we have two entangled qubits in the singlet state, and two measuring devices A and B (which stand for the obvious names everybody always uses in these scenarios). The initial state of the system is then

$$\Psi_i = \frac{1}{\sqrt{2}} \left( |\uparrow>_1 |\downarrow>_2 - |\downarrow>_1 |\uparrow>_2 \right) |R>_A |R>_B$$

where the meanings of the kets should be reasonably obvious ("R" stands for "ready to measure").

The two measurements are realized by two unitary operators ##U_1## and ##U_2## (for simplicity, we assume that the Hamiltonian is the identity other than these two operators, i.e., that time evolution outside the measurements leaves all states the same) which induce the following entanglement transitions

$$U_1: \ \ |\uparrow>_1 |R>_A \rightarrow |\uparrow>_1 |U>_A \ \ ; \ \ |\downarrow>_1 |R>_A \rightarrow |\downarrow>_1 |D>_A$$

$$U_2: \ \ |\uparrow>_2 |R>_B \rightarrow |\uparrow>_2 |U>_B \ \ ; \ \ |\downarrow>_2 |R>_B \rightarrow |\downarrow>_2 |D>_B$$

The final state of the system will therefore be

$$\Psi_f = \frac{1}{\sqrt{2}} \left( |\uparrow>_1 |\downarrow>_2 |U>_A |D>_B - |\downarrow>_1 |\uparrow>_2 |D>_A |U>_B \right)$$

(I have left out decoherence in all this by not including any kets representing the environment; that could be put back in but would not add anything useful to the analysis for this discussion. If you like, treat the final state as the state after everything has decohered.)

Now, in MWI terminology, the two terms in ##\Psi_f## represent two "worlds". But there is nothing to "match up" in these two worlds: they already include, by construction, the correlations between the A and B measurements, because each term already says that the two results are opposite. So there is nothing else that needs to happen for those correlations to be there; they're there because of how the unitary operators that realize the measurement interactions affect the state. And each of those operators is local: it only acts on the part of the state that is at the measurement location (A or B).

Given the above, I think the answer to @Nugatory's original question is that the unitary evolution of the overall state enforces the correlation between the A and B measurements; each "copy" of A and B must find out (once an ordinary light-speed or slower communication has happened between the two) that the other measurement got an opposite result to theirs, because that's built into the term in ##\Psi_f## that describes their "world". And the process that produces this is perfectly local. What it isn't, at least in the terminology of Bell's Theorem, is "realistic"; at least, that's the usual description of how the MWI evades the "non-locality" horn of the dilemma posted by theories that violate the Bell inequalities.

DrChinese, bhobba and Derek P
bhobba
Mentor
I must say i still don't understand what this means. What does it mean to exist when it refers to a wave function?
Now that is something a philosopher would argue about for a lifetime.

All I will say is its like probability - does probability exist? For a roulette wheel as found out by some enterprising physics students it is an objective property of the roulette wheel:
https://en.wikipedia.org/wiki/The_Eudaemonic_Pie

But like interpretations of QM who really knows. You can just about take any view you like. In many worlds it's real in some sense - but I have to say having studied that interpretation from Wallace I could not explain to you in what sense. Strange.

Thanks
Bill

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Demystifier
Science Advisor
Gold Member
I must say i still don't understand what this means. What does it mean to exist when it refers to a wave function?
Do you know what does it mean that a particle "exists" in classical mechanics, or that electromagnetic field "exists" in classical electromagnetism, or that spacetime curvature "exists" in general relativity? It means that it is there in the ontological sense, even if we neither measure nor calculate it. According to MWI, the claim that wave function "exists" in QM means exactly the same.

Michael Price, Derek P and bhobba
martinbn
Science Advisor
Do you know what does it mean that a particle "exists" in classical mechanics, or that electromagnetic field "exists" in classical electromagnetism, or that spacetime curvature "exists" in general relativity? It means that it is there in the ontological sense, even if we neither measure nor calculate it. According to MWI, the claim that wave function "exists" in QM means exactly the same.
That is what I understand by exist, but it requires that you have a space-time first. Look, you say that it means that it is there in the ontological sense. And earlier you said that according to MWI there is no three dimensional world. Also all of the above, particles and fields, can have energy and momentum and can interact with each other and so on. None of that makes sense about the wave function.

I suppose my main problem is that the statement "object A exists" means that it exists in space-time and interacts with other objects. What does all that mean for the wave function?

Demystifier
Science Advisor
Gold Member
That is what I understand by exist, but it requires that you have a space-time first. Look, you say that it means that it is there in the ontological sense. And earlier you said that according to MWI there is no three dimensional world. Also all of the above, particles and fields, can have energy and momentum and can interact with each other and so on. None of that makes sense about the wave function.

I suppose my main problem is that the statement "object A exists" means that it exists in space-time and interacts with other objects. What does all that mean for the wave function?
You are right, the wave function does not exist in space-time. And according to MWI, at the fundamental level there is no space-time, there is only wave function. If it doesn't make sense to you, then blame MWI, not me.

Anyway, when I try to make sense of MWI, I use the analogy with Plato's cave where the things one observes are mere shadows of the true things.

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Michael Price and bhobba