The Many Worlds interpretation is rather unconvincing to me for many reasons.

|1| It claims it is "simpler" just by dropping the Born rule, but it is mathematically impossible to derive the Born rule from the Schrodinger equation alone. You must include some additional assumption to derive it, and so it ends up necessarily having to introduce an additional postulate at some point to derive the Born rule from. Its number of assumptions thus always equal that of any other interpretation but with additional mathematical complexity caused by the derivation.

• https://arxiv.org/abs/2402.17066

|2| It claims to be "local" because there is no nonlocal wavefunction collapse. But the EPR paper already proves it's mathematically impossible for something to match the predictions of quantum theory and be causally local if there are no hidden variables. This is obscured by the fact that MWI proponents like to claim the Born rule probabilities are a subjective illusion and not physically real, but illusions still have a physical cause that need to be physically explained, and any explanation you give must reproduce Born rule probabilities, and thus must violate causal locality. Some MWI proponents try to get around this by redefining locality in terms of relativistic locality, but even Copenhagen is local in that sense, so you end up with no benefits over Copenhagen if you accept that redefinition.

• https://arxiv.org/abs/0706.2661v1
• https://arxiv.org/abs/2306.07794

|3| It relies on belief that there exists an additional mathematical entity Ψ as opposed to just ψ, but there exists no mathematical definition or derivation of this entity. Even Everett agreed that all the little ψ we work with in quantum theory are relative states, but then he proposes that there exists an absolute universal Ψ, but to me this makes about as much sense as claiming there exists a universal velocity in Galilean relativity. There is no way to combine relative velocities to give you a universal velocity, they are just fundamentally relative. Similarly, wavefunctions in quantum mechanics are fundamentally relative. A universal wavefunction does not meaningfully exist.

• https://arxiv.org/abs/2012.05712
• https://arxiv.org/abs/1508.05533

|4| You describe MWI as kind of a copying of the world into different branches where different observers see different outcomes of the experiment, but that is not what MWI actually claims. MWI claims the Born rule is a subjective illusion and all that exists is the Schrodinger equation, but the Schrodinger equation never branches. If, for example, a photon hits a beam splitter with a 50% chance of passing through and a 50% chance of being reflected and you have a detector on either side, the Schrodinger equation will never evolve into a state that looks anything like it having past through or it having been reflected, nor will it ever evolve into a state that looks anything like it having past through and it having been reflected. The state it evolves into is entirely disconnected from the discrete states we actually observe except through the Born rule. Indeed, even those probabilities I gave you come from the Born rule.

This was something Einstein pointed out in relation to atomic decay, that no matter how long you evolve the Schrodinger equation, it never evolves into a state that looks anything like decay vs non-decay. You never get to a state that looks like either or, both, or neither. You end up with something entirely unrecognizable from what we would actually observe in an experiment, only connected back to the probabilities of decay vs non-decay by the Born rule. If the universe really is just the Schrodinger equation, you simply cannot say that it branches into two "worlds" where in one you see one outcome and in another you see a different outcome, because the Schrodinger equation never gives you that. You would have to claim that the entire world consists of a single evolving infinite-dimensional universal wavefunction that is nothing akin to anything we have ever observed before.

There is a good lecture below by Maudlin on this problem, that MWI presents a theory which has no connection to observable reality because nothing within the theory contains any observables.

• https://www.youtube.com/watch?v=us7gbWWPUsA

Rovelli also comments on it:

The gigantic, universal ψ wave that contains all the possible worlds is like Hegel’s dark night in which all cows are black: it does not account, per se, for the phenomenological reality that we actually observe. In order to describe the phenomena that we observe, other mathematical elements are needed besides ψ: the individual variables, like X and P, that we use to describe the world. The Many Worlds interpretation does not explain them clearly. It is not enough to know the ψ wave and Schrödinger’s equation in order to define and use quantum theory: we need to specify an algebra of observables, otherwise we cannot calculate anything and there is no relation with the phenomena of our experience. The role of this algebra of observables, which is extremely clear in other interpretations, is not at all clear in the Many Worlds interpretation.

— Carlo Rovelli, “Helgoland: Making Sense of the Quantum Revolution”