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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.

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 rule, 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.

|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.

arxiv.org/abs/2012.05712

|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. 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. 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.

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”

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bitcrafter@programming.dev ⁨8⁩ ⁨hours⁩ ago
First, working in terms of decoherence is significantly simpler than worrying about whether something has been measured or not at every single step of the evolution of a system, because I have observed that when people do the latter they tend to get headaches contemplating the meaning of the “quantum eraser” when there is no need to. Second, you actually can observe Born’s rule in action by modeling the evolution of a system with an experimenter performing measurements and watching it emerge from the calculation.

The only way that the two sides of the EPR pair know that they agree or disagree is by communicating with each other and comparing results, which can only happen through local interactions.

I have no idea what you even mean by this. What makes the (terribly named) Many Worlds Interpretation nice is precisely that you can just treat everything as a wave function, with parts that might be entangled in ways you don’t know about (i.e., decoherence, modeled via density matrices).

The fact that you are even making this claim is why I have trouble taking the rest of your comment seriously at all, because I specifically said, “However, it is important to understand that the concept of branches is just a visualization; it is nothing inherent to the theory, and when things get even slightly more complicated than the situation I have described, they do not meaningfully exist at all.”
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- bunchberry@lemmy.world ⁨8⁩ ⁨hours⁩ ago
  Not sure what this first point means. To describe decoherence you need something like density matrix notation or Liouville notation which is mathematically much more complicated. For example, a qubit’s state vector grows by 2^N, but if you represent it in Liouville notation then the vector grows by 4^N. It is far more mathematically complicated as a description. Your second point also agrees with me. We know the Born rule is real because we can observe real outcomes on measurement devices, something which MWI denies exists.
  
  This is also true in Copenhagen. Again, if that’s your criterion for locality then Copenhagen is also local.
  
  I think you should read Everett’s papers “‘Relative State’ Formulation of Quantum Mechanics” and “The Theory of the Universal Wave Function” to see the difference between wavefunctions defined in a relative sense vs a universal sense. You will encounter this with any paper on the topic. I’m a bit surprised you genuinely have never heard of the concept of the universal wavefunction yet are defending MWI?
  
  That quotation does not come one iota close to even having the air of giving the impression of loosely responding to what I wrote. You are not seriously engaging with what I wrote at all.
  
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  - bitcrafter@programming.dev ⁨6⁩ ⁨hours⁩ ago
    
    A simpler way of stating my point is that entanglement is sufficient to understand measurement, and more importantly, what phenomena are “measurement-like” and which aren’t. Also, you missed my point regarding the Born rule. You can write down a mathematical model of an experimenter repeating an experiment and recording their measurements, turn the crank, and see the probabilities predicted by the Born rule fall out, without any experiment ever having taken place.
    
    I am confused, then, about what we are supposedly even arguing about here. (Are you sure you are even arguing with me, rather than someone else?)
    
    I did some searching and I think that what you are calling “relative states” is an older term for what we now call “entangled states”. Being entangled with another system implies (by definition) that there is a greater system containing you and the other system, and so on, which is how you end up with a universal system that contains everything. However, we do not actually believe that reality is dictated by quantum mechanics but by quantum field theory, which is manifestly built on top of special relativity and posits a single field for each kind of particle for the entire Universe, and describes the microscopic behavior so well that it is absurd. Of course, the next step is figuring out how to reconcile this with general relativity, but that isn’t something Copenhagen helps you out with either.
    
    First you criticize the way that I talked about branches, which I only mentioned briefly as a sort of crude visualization and explicitly called out as being such. Now you are claiming that I am “denying the physical existence of real-world discrete outcomes”?
    
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    bunchberry@lemmy.world ⁨5⁩ ⁨hours⁩ ago
    
    Entanglement is just a mathematical property of the theory. If it is sufficient to explain measurement then there is not anything particularly unique about MWI since you can employ this explanation within anything. You also say I missed your point by repeating exactly what I said.
    
    You’re the one giving this bullet point list as if you are debunking all of my points one-by-one. If you agree there is nothing especially “more local” about MWI than any other interpretation then why not just ignore that point and move on?
    
    A relative state is not an entangled state. Again you need to read the papers I linked. We are talking about observer-dependence in the sense of how the velocity of a train in Galilean relativity can be said to have a different value simultaneously for two different observers. I drew the direct comparison here in order to explain that in my first comment. This isn’t about special relativity or general relativity, but about “relativity” in a more abstract sense of things which are only meaningfully defined as a relational property between systems. The quantum state observer A assigns to a system can be different from the quantum state observer B assigns to the system (see the Wigner’s friend thought experiment). The quantum state in quantum mechanics is clearly relative in this sense, and to claim there is a universal quantum state requires an additional leap which is never mathematically justified.
    
    Please for the love of god just scroll up and read what I actually wrote in that first post and respond to it. Or don’t. You clearly seem to be entirely uninterested in a serious conversation. I assume you have an emotional attachment to MWI without even having read Everett’s papers and getting too defensive that you refuse to engage seriously in anything I say, so I am ending this conversation here.
    
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Natanael@infosec.pub ⁨6⁩ ⁨hours⁩ ago
Irrational probabilities makes MWI impractical unless you interpret the branching much like a continous graph (as a visualization, see phase of matter graphs) with an ever increasing number of dimensions. And yes continous branching is weird

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- bitcrafter@programming.dev ⁨6⁩ ⁨hours⁩ ago
  Again, as I said in my comment, the branches in MWI are just a visualization of the very simplest possible case, not a literal description of reality. It is unfortunate (though understandable) that people have latched on to them as if they were the central idea of MWI.
  
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gbzm@piefed.social ⁨5⁩ ⁨hours⁩ ago
My understanding might be a bit superficial, but I thought the whole point of the MWI was to make explicit the fact that states are relative? To me the rationale was that states are relative and if we simultaneously describe relative states and their observers we can translate the shrödinger+born-rule in a density-operator+partial-trace-rule and make the wave function collapse physical (aka unitary) through branching and decoherence, even though that’s mathematically tedious and in practice people will keep using projectors (1). States being relative means their physical reality is somewhat broken but locality is mostly saved (2), so then we postulate that they derive from a universal wave function to rehabilitate some form of physical realism (3). As to (4), isn’t it solved if you assume that Schrödinger’s equation is actually the less fundamental formalism since it’s only valid for systems that are unrealistically isolated?

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- bunchberry@lemmy.world ⁨4⁩ ⁨hours⁩ ago
  MWI very specifically commits to the existence of a universal wavefunction. Everett’s original paper is literally titled “The Theory of the Universal Wavefunction.” If you instead only take relative states seriously, that position is much closer to relational quantum mechanics. In fact, Carlo Rovelli explicitly describes RQM as adopting Everett’s relative-state idea while rejecting the notion of a universal quantum state.
  
  MWI claims there exists a universal quantum state, but quantum theory works perfectly well without this assumption if quantum states are taken to be fundamentally relative. Every quantum state is defined in relation to something else, which is made clear by the Wigner’s friend scenario where different observers legitimately assign different states to the same system. If states are fundamentally relative, then a “universal” quantum state makes about as much sense as a “universal velocity” in Galilean relativity.
  
  You could arbitrarily choose a reference frame in Galilean relativity and declare it universal, but this requires an extra postulate, is unnecessary for the theory, and is completely arbitrary. Likewise, you could pick some observer’s perspective and call that the universal wavefunction, but there is no non-arbitrary reason to privilege it. That wavefunction would still be relative to that observer, just with special status assigned by fiat.
  
  Worse, such a perspective could never truly be universal because it could not include itself. To do that you would need another external perspective, leading to infinite regress. You never obtain a quantum state that includes the entire universe. Any state you define is always relative to something within the universe, unless you define it relative to something outside of the universe, but at that point you are talking about God and not science.
  
  The analogy to Galilean relativity actually is too kind. Galilean relativity relies on Euclidean space as a background, allowing an external viewpoint fixed to empty coordinates. Hilbert space is not a background space at all; it is always defined in terms of physical systems. You can transform perspectives in spacetime, but there is no transformation to a background perspective in Hilbert space because no such background exists. The closet that exists is a statistical transformation to different perspectives within Liouville space, but this only works for objects within the space; you cannot transform to the perspective of the background itself as it is not a background space.
  
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  - gbzm@piefed.social ⁨3⁩ ⁨hours⁩ ago
    Ah so I think I sort of conflated RQM and MWI because I thought it was all about Everett’s other paper “relative state formulation of qm”.
    
    I thought on top of an ad hoc rehabilitation of physical realism, the universal state also did something for the consistency. Something like all the density operators may be expressed as partial traces of the operator describing the their systems’ union, in order for everything to be consistent, and the ‘largest’ operator describes the state of the universe or something. I’ll check out your sources next insomnia
    
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    bunchberry@lemmy.world ⁨2⁩ ⁨hours⁩ ago
    Depends upon what you mean by realism. If you just mean belief in a physical reality independent of a conscious observer, I am not really of the opinion you need MWI to have a philosophically realist perspective.
    
    For some reason, everyone intuitively accepts the relativity of time and space in special relativity as an ontological feature of the world, but when it comes to the relativity of the quantum state, people’s brains explode and they start treating it like it has to do with “consciousness” or “subjectivity” or something and that if you accept it then you’re somehow denying the existence of objective reality. I have seen this kind of mentality throughout the literature and it has never made sense to me.
    
    Even Eugene Wigner did this, when he proposed the “Wigner’s friend” thought experiment, he points out how two different observers can come to describe the same system differently, and then concludes that proves quantum mechanics is deeply connected to “consciousness.” But we have known that two observers can describe the same system differently since Galileo first introduced the concept of relativity back in 1632. There is no reason to take it as having anything to do with consciousness or subjectivity or anything like that.
    
    (You can also treat the wavefunction nomologically as well, and then the nomological behavior you’d expect from particles would be relative, but the ontological-nomological distinction is maybe getting too much into the weeds of philosophy here.)
    
    I am partial to the way the physicist Francois-Igor Pris puts it. Reality exists as independently of the conscious observer, but not independently from context. You have to specify the context in which you are making an ontological claim for it to have physical meaning. This context can be that of the perspective of a conscious observer, but nothing about the observer is intrinsic here, what is intrinsic is the context, and that is just one of many possible contexts an ontological claim can be made. Two observers can describe the same train to be traveling at different velocities, not because they are conscious observers, but because they are describing the same train from different contexts.
    
    The philosopher Jocelyn Benoist and the physicist Francois-Igor Pris have argued that the natural world does have a kind of an inherent observer-observed divide but that these terms are misleading being “subject” tends to imply a human subject and “observer” tends to imply a conscious observer, and that a lot of the confusion is cleared up once you figure out how to describe this divide in a more neutral, non-anthropomorphic way, which they settle on talking about the “reality” and the “context.” The reality of the velocity of the train will be different in different contexts. You don’t have to invoke “observer-dependence” to describe relativity. Hence, you can indeed describe quantum theory as a theory of physical reality independent of the observer.
    
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  - bitcrafter@programming.dev ⁨3⁩ ⁨hours⁩ ago
    
    The analogy to Galilean relativity actually is too kind. Galilean relativity relies on Euclidean space as a background, allowing an external viewpoint fixed to empty coordinates. Hilbert space is not a background space at all; it is always defined in terms of physical systems, what is known as a constructed space. You can transform perspectives in spacetime, but there is no transformation to a background perspective in Hilbert space because no such background exists. The closest that exists is a statistical transformation to different perspectives within Liouville space, but this only works for objects within the space; you cannot transform to the perspective of the background itself as it is not a background space.
    
    …which is why eventually you need to switch to the grown-up version of Quantum Mechanics, Quantum Field Theory, is defined in terms of relativistic fields with a single “universal” field for each flavor of particle.
    
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- bitcrafter@programming.dev ⁨4⁩ ⁨hours⁩ ago
  For what it’s worth, you’ve done a fairly good job describing my own understanding of MWI quite succinctly.
  
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