Quantum mechanics: does many-worlds interpretation remove nonlocality? Bell-type experiment

Trying to make sense of many-worlds interpretation and see if it removes nonlocality or solves other problems. It appears to me that branching is equivalent to collapse... so trying to see what I'm misunderstanding.

If we do a Bell-type experiment with a pair of correlated particles... we have two observers Alice and Bob that agree to measure spin in a particular direction. We know the spins will always be opposite.

So we start with the wave function... Alice measures the spin on her particle. It can be spin up or spin down. By many worlds, the wave function branches into two at this measurement? And in one branch Alice measures spin up and bob measures spin down... and the other branch Alice measures spin down and Bob measures spin up.

Now, isn't this branching itself a nonlocal effect due to the fact that a measurement takes place? This branching as I'm understanding it encompasses both Alice and Bob, what I mean is that Alice learns which branch she's on... and Bob is on that same branch measuring opposite spin... ie: Alice and Bob's states are both fully determined at this point. They weren't determined prior to measurement... so the phenomena seems equivalent to collapse to me. I can't really see the difference... Unless the branches existed prior to measurements happening... then Alice is just finding out which branch she was always on.

So as I'm understanding it... with many worlds... all branches continue to exist... in the Copenhagen interpretation only one branch exists when measurement happens.

So what exactly is the advantage of postulating the existence of these other branches? What I mean is... from an empirical point of view... we observe a measurement... now we could have split into several branches... but we can never observe this for sure. But we do know what we measured in our branch... Why bother asserting these other branches exist post-measurement?

So it seems in one interpretation we have a nonlocal effect of collapse with one world. In the other interpretation we have a nonlocal effect of branching, as well as a bunch of parallel worlds. Can you clarify the advantages of MWI over Copenhagen?

• In many worlds you can define the probability of Alice measuring something according to Bob, but you cannot ask what is the probability of Bob's measurement by itself. That makes it very complicated to formulate Bell's theorem. This blog post by one of the members of this community goes into detail: mateusaraujo.info/2016/08/02/… Nov 5, 2022 at 12:52

"It appears to me that branching is equivalent to collapse... so trying to see what I'm misunderstanding."

The idea of branching universes in the popular picture is indeed equivalent to collapse. However, the Everett Interpretation does not have branching universes; it says that when a quantum observer interacts with a system they jointly evolve into a superposition of orthogonal (and hence mutually non-interacting) states, in each of which the observer sees one outcome. From the point of view of the quantum observer, it is as if they were in separate worlds.

Back in the 1970s when Bryce DeWitt was trying to raise awareness of Everett's idea with the public, he used this 'Many Worlds' image to explain how it would appear to someone living in such a universe. But it's not how the physics works, and taking it too literally causes endless confusion.

"So we start with the wave function... Alice measures the spin on her particle. It can be spin up or spin down. By many worlds, the wave function branches into two at this measurement?"

You start with two particles which are jointly in a superposition, so their up/down spins are correlated. When Alice interacts with her particle, her wavefunction becomes correlated with that of the particle, and Alice herself becomes a superposition of an Alice seeing $$\left|\uparrow \right>$$ and an Alice seeing $$\left|\downarrow \right>$$. Nothing happens to the other particle, no changes travel outside the locality - whether faster-than-light or otherwise.

Using the 'Many Worlds' analogy, when the particles are prepared in a superposition of up/down states, this is when the particles first split across two worlds. When Alice measures her particle, she splits to join it, one version of her moving to one of the particles' worlds, the other version of her moving into the other.

Either before or after Alice makes her measurement, it doesn't matter which, Bob measures the partner particle, and he too enters a superposition of a Bob seeing $$\left|\downarrow \right>$$ and a Bob seeing $$\left|\uparrow \right>$$.

Bob's state is correlated to his particle, which is correlated to Alice's particle, which is correlated to Alice. So Bob and Alice are now correlated in exactly the same sort of way as the original particles were correlated.

And so when Alice and Bob get back together to compare notes, the version of Alice that saw $$\left|\uparrow \right>$$ can only see/interact with the version of Bob that saw $$\left|\downarrow \right>$$, and vice versa. They have both split and joined the two particles in their two 'worlds', and each only sees the Alice or Bob in the same 'world' as themselves.

"I can't really see the difference... Unless the branches existed prior to measurements happening... then Alice is just finding out which branch she was always on."

The 'branches' existed prior to the measurements happening, but Alice only split and entered the two branches when the measurement was made. She was not 'on' either branch until that point. The idea of branching universes only really makes sense when there is only a single observer in the universe. As soon as you try to deal with multiple observers, the analogy falls apart. The split is observer-specific. One observer may be split without affecting any other observers.

"So what exactly is the advantage of postulating the existence of these other branches?"

Philosophical elegance and simplicity. Superpositions already exist and are required in all consistent-with-experiment theories of quantum mechanics - all we are doing is extending the same theory unchanged to the macroscopic level. It's the same theory but without collapse, so it's simpler. It's also local, deterministic, reversible, conserves information, and realist. We don't need to explain how collapse works or what triggers it. We don't need to deal with having separate 'quantum' and 'classical' parts of the universe, operating according to different rules, and a mysterious and badly-defined boundary between them. There is the potential to explain features of quantum mechanics in terms of underlying mechanisms that would otherwise simply have to be asserted as axioms, and removes the need to explain other features (like non-determinism) entirely. It's more explanatory.

"What I mean is... from an empirical point of view... we observe a measurement... now we could have split into several branches... but we can never observe this for sure."

Correct. The 'unitary evolution' part of quantum mechanics predicts a superposition of states, almost all of which we do not and cannot see. Copenhagen says all but one of them disappears at some unspecified time during a measurement, by some unexplained mechanism. There is no way (if quantum mechanics is correct) to distinguish between them experimentally. The Scientific Method is entirely agnostic on the point.

It all comes down to how you want to wield Occam's Razor. Do you want to remove the gazillions of unobservable states as unnecessary, or the unexplained and unobservable mechanisms by which 'wavefunction collapse' invisibly deletes all the unseen entities that the simpler theory predicts, that you could never have seen anyway? Neither is 'necessary', given the alternative of the other.

• "We don't need to explain how collapse works or what triggers it." But how do you explain any measurement then? You have this wavefunction with all the possibilities... this is reality itself... what within the wavefunction explains why when a measurement is done by an experimenter, he measures just one outcome? Why does an observer see himself/herself as a single body when the reality is this wavefunction which encompasses all possibilities? I don't understand when you say you don't need to explain how collapse works... there has to be some explanation by which distinct measurements happen. Nov 5, 2022 at 17:17
• The mechanism by which only one outcome is seen is a separate question, but see physics.stackexchange.com/q/731652 (which was already linked in the answer above) for a very rough outline of how we think it works. Or read Everett's thesis (also linked). Nov 5, 2022 at 17:24
• Then isn't many worlds exactly the same as relational interpretation? The observer sees one outcome from their own frame, but is in a superposition in a different frame. Nov 5, 2022 at 17:36
• Yes. I believe Everett's name for it was the "Relative State Formulation of Quantum Mechanics". I think modern relational interpretations are a bit different to Everett in their details, but they share much of the same spirit. Nov 5, 2022 at 17:41
• Then I see some extreme misunderstanding of many worlds, in even physicists' description of it. The world branching makes it sound like a hidden variable theory. The wiki article of Bohmian mechanics also claims similarities with Many worlds Nov 5, 2022 at 17:52

The many worlds interpretation (MWI) involves working out the implications of quantum mechanics and taking what it says seriously as a description of reality. One of the implications of quantum mechanics, according to the MWI, is that each system exists in multiple versions. When a system $$S_1$$ measures another system $$S_2$$, information about $$S_2$$ is present in $$S_1$$ and may spread to other systems that interact with $$S_1$$. When information from $$S_2$$ is copied into multiple other systems, the different results of that measurement can no longer undergo quantum interference. As a result, the different versions of that system that arise as a result of that measurement act independently of one another, as do all the records of those different versions. So as information spreads locally the different versions of all the systems that receive the information about $$S_2$$ are sorted into layers and each of those layers acts approximately like the universe as described by classical physics: a collection of parallel universes. See

https://arxiv.org/abs/quant-ph/0104033

https://arxiv.org/abs/0707.2832

I said that the world acts approximately like a collection of parallel universes, but that approximation isn't perfect because "classical" systems can carry quantum information and they do in experiments such as the EPR experiment and other experiments that are often wrongly described as evidence for non-locality:

https://arxiv.org/abs/quant-ph/9906007

https://arxiv.org/abs/1109.6223

The Copenhagen interpretation (CI) involves using quantum mechanical equations of motion while ignoring the implications of those equations for how the world actually works. As such, the CI doesn't say anything about whether the laws of physics are local, or about anything else. The MWI by contrast implies that the laws of physics are local if the relevant quantum mechanical equations of motion are local, and they are.