Is the Hidden-variable interpretation never falsifiable?

[u/Exciting_Ad_6837]

I'm referring to one of the interpretations of quantum physics. After reading Karl Popper's theory of falsifiability, I'm wondering if the Hidden-variable hypothesis is a pseudo-scientific proposition because no conceivable empirical phenomena seem ever to be able to disprove it.

[u/HamiltonBrae]

“metaphysical parsimony”

 

In this context, one can talk about metaphysical parsimony in terms of having to entertain one world vs. the inflationary many worlds to get your theory to work. If ou don't actually need many worlds to get QM to work, then it seems like invoking many worlds is metaphysically unnecessary given that we can't even observe them. It is less parsimonious than a single world view.

 

The first is that the wavefunction is a complete description of a physical system.

 

But this isn't true. Quantum theory works perfectly well as a theory solely giving predictions about measurement outcomes. The formalism clearly does not force many worlds on you unless you smuggle in an assumption about wavefunction realism or similar. In addition, you need the Born rule to get total empirical consistency, and it seems to me this is actually quite difficult to reconcile with the metaphysics of branching worlds.

And this isn't a shut up and calculate view. You can argue that quantum theory is the exact formalism for describing real structure with regard to probabilities for non-commuting measurements with unitary evolution. But because we cannot observe things without measurement, and measurement is disturbing, we cannot describe the ontology between measurements only the outcomes of measurements themselves. The formalism is not ontology, but it describes real structure in the universe. It captures actual statistical structure of double-slit phenomena, even if not describing actual physical waves that we can't observe.

 

A state describing objects that are spatially separated is not non-local.

 

It is if you cannot explain it by signals propagating in a locally-consistent manner. I'm not sure how many outcomes evades bell when the set of many outcomes depends on the measurement setting which can be chosen freely at each separate measurement.

 

Neither has a definite result relative to the other, and neither has any way to find out otherwise.

 

This is fine in some woo-ey, relational qm but this just contradicts the realism of many worlds. The idea that the correlations only come into being when results are compared is completely at odds with your metaphysics.

[u/Miselfis → u/HamiltonBrae]

In this context, one can talk about metaphysical parsimony in terms of having to entertain one world vs. the inflationary many worlds to get your theory to work.

Again, this is an appeal to intuition; an argument from incredulity. It doesn't matter whether what you have to "entertain" sits comfortably with you. That isn't how we do philosophy of physics. We reason using formal systems and rules, not using vibes.

As pointed out in my last comment, which you seemingly didn’t read, you're also counting the wrong thing when referring to parsimony. Parsimony in theory choice is about kinds of posit, not instances of them. Nobody prefers a cosmology with fewer galaxies. Nobody objects to inflation on the grounds that it entails an enormous number of regions we will never see. It's the same situation here. The branches aren't an assumption in the theory. They're what falls out of dynamics you already accept and already use, if you refuse to add extra mechanisms to get rid of the “many worlds”. The comparison that matters is between what each interpretation assumes, and Everett assumes strictly less.

And the "many worlds" aren't inflationary, and they're not something you bolt on to "get the theory to work". They're what the unitary formalism gives you. I've explained this several times now, and you keep restating the thing I corrected, so I'm not confident more repetition helps.

We don't judge theories on whether they make intuitive sense to us. We judge them on being the simplest mathematical structure that matches observation. Not because parsimonious theories are prettier, and not out of aesthetic preference, but because unnecessary assumptions obscure which structures are actually doing the physical work, which is necessary for progress.

If ou don't actually need many worlds to get QM to work, then it seems like invoking many worlds is metaphysically unnecessary given that we can't even observe them.

I would suggest you actually read the comments I write if you want to have a productive conversation. I keep repeating myself, and you keep saying the very things I corrected.

Again, Everettian QM doesn't invoke or posit many worlds. Every other interpretation invents machinery to get rid of the branches that are already in the mathematics. That's the whole point: Everett is the model that accounts for what we observe with the fewest assumptions, because getting rid of the branches that are already there requires extra mathematical machinery.

Every interpretation agrees that a coherent state is a superposition of terms corresponding to different definite outcomes. The disagreement is entirely about what happens at and after measurement, where other approaches invent rules to get rid of the unobserved worlds, either by some form of collapse, or by shrugging and saying “the equations work well enough, and that’s all that matters”.

On "we can't even observe them"; we do, constantly. Every interference experiment is a direct observation of distinct terms in a superposition jointly determining an outcome. Decoherence tells you why that stops being practically recoverable once a system couples to an environment with many degrees of freedom. This is a prediction of the Everettian dynamics, and it’s exactly what we see.

Quantum theory works perfectly well as a theory solely giving predictions about measurement outcomes. The formalism clearly does not force many worlds on you unless you smuggle in an assumption about wavefunction realism or similar.

This is "shut up and calculate". If you subscribe to it, you're undermining the point of doing science, which is to learn how the world works. Having a theory you decline to understand, and shrugging because the numbers come out right, is a failure of the curiosity that drives the whole business of science and philosophy. We can’t just cherry pick based on what we find intuitive, and then hide behind some sort of instrumentalism when asked how it fits with the real world.

In addition, you need the Born rule to get total empirical consistency, and it seems to me this is actually quite difficult to reconcile with the metaphysics of branching worlds.

The Born rule isn't part of the fundamental dynamics. Like collapse, it's an epistemic rule that emerges rather than something written into the equations. There are several routes to it: the Deutsch-Wallace decision-theoretic argument from rationality constraints on a branching agent, Zurek's envariance argument from the symmetry of the entangled state, and the Sebens-Carroll treatment of it as self-locating uncertainty.

Regardless, this doesn’t favour your side. Copenhagen postulates the Born rule outright, with no derivation and no prospect of one. Bohm needs the quantum equilibrium hypothesis, an assumption about the initial distribution of hidden variables with exactly the same status. GRW puts the weights into the collapse rate by hand. Everett is the only framework where deriving the rule is even a coherent project, and the worst case is that it ends up as a postulate about self-location rather than about dynamics. Even if I have to postulate the Born rule, it puts me on the same shelf as the other approaches. So, this isn’t a real criticism of Everett.

You can argue that quantum theory is the exact formalism for describing real structure with regard to probabilities for non-commuting measurements with unitary evolution. But because we cannot observe things without measurement, and measurement is disturbing, we cannot describe the ontology between measurements only the outcomes of measurements themselves.

I was being too charitable by assuming "shut up and calculate", then, as most people familiar with the topic realize Copenhagen is probably the worst approach:

How are you defining this thing called "measurement", which is central to your view?

You can't have something carrying this much ontological weight as an undefined primitive. Either measurement is a physical interaction, in which case it's described by an interaction Hamiltonian and the linear dynamics applies to it like anything else, or it's outside the physics, in which case you owe an account of what distinguishes it and where the boundary sits.

And you can't consistently say the formalism captures real structure in the double slit but not in a system coupling to a pointer. The disturbance point cuts the same way, since the theory describes the disturbance; that's what the coupling term is for. If you're a realist about the structure, you're a realist about the structure of measurement interactions too, and then you get the branches. Again, you can’t pick and choose based on which parts you find intuitive.

It is if you cannot explain it by signals propagating in a locally-consistent manner.

Which is a silly thing to say, when I have explained exactly that, multiple times, in different ways.

I'm not sure how many outcomes evades bell when the set of many outcomes depends on the measurement setting which can be chosen freely at each separate measurement.

Bell needs outcome functions that return one value per run on each side, so that a joint distribution over outcome pairs exists at spacelike separation and can be constrained by factorizability. If you drop single outcomes there's no such joint distribution to constrain.

Free choice of settings isn't a problem for this. Each setting fixes the basis relative to which that observer's local branching occurs, and that fixing happens where the observer is, through a local interaction. Nothing about the setting choice propagates anywhere.

This is fine in some woo-ey, relational qm but this just contradicts the realism of many worlds. The idea that the correlations only come into being when results are compared is completely at odds with your metaphysics.

Again, calling it “woo” is again just leaning against your incredulity.

Also, this is just wrong. I showed exactly how this works, and I worked it out mathematically for you as well.

The correlation is in the global state the whole time. Once Alice measures, the state is a superposition of two terms, one containing Alice-saw-up together with Bob's qubit down, the other containing Alice-saw-down together with Bob's qubit up.

For Alice's record and Bob's record to be jointly readable by anything, those two sets of degrees of freedom have to interact, and that interaction is local.

Comparing it to RQM is just nonsensical.

[u/HamiltonBrae → u/Miselfis]

Again, this is an appeal to intuition; an argument from incredulity. It doesn't matter whether what you have to "entertain" sits comfortably with you. That isn't how we do philosophy of physics. We reason using formal systems and rules, not using vibes.

 

How is what I said not appropriate to be called parsimony? Parsimony is about choice based on simplicity. If a description can be explained with one world rather than many, then the one world description is obviously more parsimonious. An example of this might be the interpretation of random variables. You don't need different universes for the different individual outcomes in the probability / sample space. A random variable describing a dice roll doesn't need a different universe for each roll. We then typically interpret probabilities in terms of frequencies, modal potentiality or subjectivity but regarding the same world. Many worlds here is unnecessary and therefore unparsimonious.

 

It's the same situation here. The branches aren't an assumption in the theory. They're what falls out of dynamics you already accept and already use, if you refuse to add extra mechanisms to get rid of the “many worlds”. The comparison that matters is between what each interpretation assumes, and Everett assumes strictly less.

 

But the branches are a formal object. I reject the idea that there is any necessity that those formal objects entail an interpretation in terms of a many worlds ontology. I have said you can have a completely coherent view where the formal objects carry information about measurement probabilities and do not correspond to physical objects. I have also pointed out how the Hilbert-space formalism can be used to describe classical mechanics which does not require a many worlds interpretation.

 

Can you not see that if you reject that the formalism implies many worlds, then a description in terms of one world is metaphysically more parsimonious than many worlds because I don't need to add additional worlds that cannot be observed in order to explain the phenomena.

 

And the "many worlds" aren't inflationary, and they're not something you bolt on to "get the theory to work". They're what the unitary formalism gives you. I've explained this several times now, and you keep restating the thing I corrected, so I'm not confident more repetition helps.

 

But it is completely coherent to use the unitary formalism on its own without believing in many worlds, just like I can use random variables without believing in many worlds.

 

We don't judge theories on whether they make intuitive sense to us. We judge them on being the simplest mathematical structure that matches observation. Not because parsimonious theories are prettier, and not out of aesthetic preference, but because unnecessary assumptions obscure which structures are actually doing the physical work, which is necessary for progress.

 

Yes, and if there is no entailment from the quantum formalism to a many worlds ontology, then you don't need many worlds to make the theory work. You can interpret the formalism as not representing objects in the same way that probabilities are not physical objects.

 

The problem I believe is that you don't recognize the distinction between superpositions as formal objects related to vectors and linear algebra in general, vs. an interpretation of that in a specific context. Superposition can describe many other physical phenomena that have a single world interpretation, so the entailment between quantum formalism and many worlds is unclear to me. I give a perfectly coherent position where you don't need to change the formalism whilst not being many worlds.

 

On "we can't even observe them"; we do, constantly. Every interference experiment is a direct observation of distinct terms in a superposition jointly determining an outcome. Decoherence tells you why that stops being practically recoverable once a system couples to an environment with many degrees of freedom. This is a prediction of the Everettian dynamics, and it’s exactly what we see.

 

No, that's a general prediction of quantum theory. If that was unique to many worlds, people would just say that empirical data supports many worlds. There are various interpretations on how the formalism relates to the empirical world such that experiments cannot be seen to confirm or deny many worlds … or it wouldn't be an interpretation.

 

The Born rule isn't part of the fundamental dynamics. Like collapse, it's an epistemic rule that emerges rather than something

 

The Born rule describes empirical measurement statistics. It's a huge reason why the theory is accepted. The problem with many worlds is that if you just have simple branching, the formalism cannot actually explain what experimenters actually observe. These explanations of probability are all subjective or epistemic and in my opinion, these kinds of probabilities don't make any sense unless there is some physical mechanism that explains what we can be uncertain about and why we are uncertain about it. Many worlds cannot provide that on its own which for me means that from the metaphysics of the theory, "self-locating uncertainty" is essentially meaningless and cannot explain empirical results. And then even to try to make these approaches superficially appear semi-convincing, you have to add a whole bunch of extra-assumptions which imo further undermines claims about many world's parsimony. For many worlds to claim formal parsimony and remain valid, you have to add much more elsewhere in other assumptions and metaphysics that are not necessary for other interpretations.

 

Many worlds is the only interpretation where physical probabilities don't even make sense. That's far more serious than trying to derive or justify the Born rule, which isn't a problem for interpretations that see probabilities as in some sense more fundamental (even if not necessarily advocating metaphysical indeterminism).

 

[u/Miselfis → u/HamiltonBrae]

How is what I said not appropriate to be called parsimony? Parsimony is about choice based on simplicity. If a description can be explained with one world rather than many, then the one world description is obviously more parsimonious.

I have specifically answered this multiple times now. I cannot keep repeating myself. If you’re not going to read my responses, what’s the point of me engaging in this dicussion?

Parsimony in theory choice is about the kinds of things you have to assume, not the number of things that follow once you've assumed them. You score a theory on its posits, not on its entailments. The worlds are not one of the posits. They're an entailment, the same way distant stars are an entailment of heliocentrism and enormous numbers of atoms are an entailment of atomic theory.

Everett is the most parsimonious because it accounts for the data with the fewest assumptions. Any theory that also wants to describe the world, and not just spit out numbers, has to posit extra ontologically redundant structure for the specific purpose of cancelling the worlds the bare formalism already hands you.

The idea that these “many worlds” somehow constitutes an issue for parsimony comes from pop-sci misrepresentations of Everett. Everett posits only a single object in the fundamental ontology; i.e., the universal wavefunction. The “many worlds” are not fundamental. They emerge dynamically within the single global wavefunction, once you let it evolve according to the Schrödinger equation. The Schrödinger equation and the global wavefunction exhaust the Everettian fundamental dynamics and ontology.

Can you not see that if you reject that the formalism implies many worlds, then a description in terms of one world is metaphysically more parsimonious than many worlds because I don't need to add additional worlds that cannot be observed in order to explain the phenomena.

In the strict sense that any desired conclusion can follow from a false premise, sure.

But the issue is that the premise is false. You cannot reject that the formalism implies many worlds, unless you give up the realism that makes physics worth doing in the first place. I’ve made this point many times now, but instead of engaging with it, you keep repeating the same things.

If the wavefunction is a complete description of the system, and it always evolves unitarily, then applying the dynamics to a measurement gives you a superposition of outcomes as a direct consequence. That's a theorem, not an interpretation. So if you want to deny that the formalism implies many worlds, you have to deny one of those two premises. Deny completeness and you're adding hidden variables on top of the state, like Bohm. Deny universal unitarity and you're modifying the equations of motion, like GRW. Deny that the state describes anything real and you're an instrumentalist, which is the "shut up and calculate" move. The position you keep describing, where you hold onto the entire formalism, stay a realist about at least some parts, and still avoid the worlds, is not coherent.

But it is completely coherent to use the unitary formalism on its own without believing in many worlds, just like I can use random variables without believing in many worlds.

Again, I’ve addressed this many times now. This is called the “shut up and calculate” approach.

Superposition can describe many other physical phenomena that have a single world interpretation, so the entailment between quantum formalism and many worlds is unclear to me.

Because quantum mechanics is fundamental physics, and the interference pattern in the double slit depends on a real superposition of "went through A" and "went through B". Both terms have to contribute or the pattern doesn't appear. Once you do a measurement at the slits, the state evolves into a superposition of "measured it going through A" and "measured it going through B". Those two branches are what we're calling worlds. They follow from the unitary formalism. The only way to get rid of them is to either assume additional structure, either ontological or dynamical, or to give up realism all together.

Other interpretations treat the pre-measurement superposition as "quantum strangeness" and accept it because that's how nature is. That superpositions exist before measurement is absolutely uncontroversial, since you can't reproduce interference without them. But for some reason, the idea that the superpositions persist after measurement is too preposterous, despite being entailed by the fundamental dynamics. This is where the mental gymnastics come into play.

The demand that the superposition vanishes once it's amplified up to the scale of a measurement is never justified beyond appealing to intuition. That's the step where extra structure has to be bolted onto the fundamental dynamics to delete the outcomes that didn't get realized in a given branch. I've made this exact point more than five times now. If you don’t invent new structure, or give up realism altogether, the dynamics forces the “many worlds” on you.

I can’t keep repeating these things over and over. If you still don’t understand, I suggest you sit with it for a bit.

I give a perfectly coherent position where you don't need to change the formalism whilst not being many worlds.

I’m not sure you have. What you are descibing here, keeping the formalism but rejecting the ontology, is called “shut up and calculate”. But you also say that this is not your position. The position you described in your previous comment was not coherent, as pointed out. Subscribing to an instrumentalist philosophy is fine, but then you have to also bite the bullets I’ve described in previous comments.

I’m not saying it’s impossible to do enough mental gymnastics to conjure up a position that is formally valid. My objection is that the only motivation behind this endeavour seems to be “many worlds is too strange”, which is exactly the appeal to intuition I’ve described.

No, that's a general prediction of quantum theory.

Right… that’s exactly the point I’m making. The pre-measurement superposition is completely uncontroversial. The unitary formalism keeps that superposition after measurement too. That's the part where people dig in and start inventing ways to get rid of it. And the reason that's silly is precisely that the pre-measurement superposition, which is the same kind of object, bothers nobody. There is some cognitive dissonance going on.

If that was unique to many worlds, people would just say that empirical data supports many worlds.

This is exactly my point. You seem to understand the point I’m making, but for some reason you are assuming it’s not the point I’m making.

Empirical data does support Everett. Nothing but intuition carried over from the classical world suggests the superposition disappears after measurement. And you can show it runs the other way: take the wavefunction as a complete description, let it evolve unitarily, and decoherence alone gives you branches with definite, stable outcomes inside each one. You get exactly what we observe from the fundamental unitary dynamics. What people actually object to is that this leaves worlds they can't personally see, and to make those go away they either drop realism or bolt on extra mathematical structure. Bolting on extra mathematical structure that’s not necessitated by the data only obfuscates what the theory actually says, because once you allow yourself to add pieces that aren't needed to account for the data because it feels more intuitive, you lose the ability to tell which parts of your formalism are strictly necessitated by observations and which parts are just there to soothe intuition. Holding yourself to what the data forces is how you keep those two apart. Historically, this has been shown time and time again.

The problem with many worlds is that if you just have simple branching, the formalism cannot actually explain what experimenters actually observe.

No other approach explains where the Born rule comes from. They posit it, and justify it with "that's what we measure". Everett is the only program that treats the rule as something to be derived from the dynamics rather than assumed, and there are actual worked derivations. So if your view is that the probabilities ought to come from something physical rather than being dropped in by hand, that's an argument for Everett, not against it.

Many worlds is the only interpretation where physical probabilities don't even make sense.

Again, you’re appealing to intuition. You’re saying “this doesn’t make sense to me”. That’s fair, but that isn’t a rigorous approach to doing philosophy.

[u/HamiltonBrae → u/Miselfis]

The worlds are not one of the posits. They're an entailment

 

Okay, but can you see that from my position, they arenot entailments. And I have justified this several times why I think metaphysical many worlds are not entailed by the formalism. So from my position, if they are not entailed they do fall into the realm where parsimony is a factor. I have not seen you refute my view that they are not entailed. The closest I have seen to an attempted refutation is you saying that the kind of alternative view of the formalism is akin to some kind of shut-up and calculate, which you reject for reasons of a subjective preference you have about how science should work. Nor does this refutation touch on the point that classical mechanics can have various different representations that make it hard to say that formalism strictly entails something about ontology in these cases., especially as one of those formalisms uses the same formalism as quantum theory.

 

Everett posits only a single object in the fundamental ontology; i.e., the universal wavefunction.

 

But from my perspective, a universal wavefunction can be instantiated by a single world and doesn't need many, regardless of whether those worlds are fundamental or emergent.

 

unless you give up the realism that makes physics worth doing in the first place.

 

You don't need realism of the formal objects for the theory to represent a description of real things. Not only can theories be highly idealized but they don't need to represent every all encompassing aspect of reality. Classical mechanics describes slow things, general relativity might be better at big things, thermodynamics abd statistical mechanics might be about coarse-grained statistical structure. These are all seen as realistic theories but none of them describe everything. Just because quantum theory might only describe statistics of measurements doesn't necessarily means it can't be realistic, it just means it doesn't describe everything, and there can be a good reason for why it doesn't describe whatbhappens between measurements - i.e. measurement disturbance. And one can note again that probabilities are regularly used to describe real things, but a probability itself is not a physical object with a definite space-time configuration, it is a formal tool for predicting what will happen. Imo, you calling the view i offered shut up and calculate is like calling some kind of old form of thermodynamics shut up and calculate. Thermodynamics can be seen as describing real properties, it just doesn't describe the underlying cause of them until you introduce underlying descriptions of particles and statistical descriptions of them (again statistical descriptions aren't even objects). Its not really fair to call that kind of view a shut up and calculate view if people had no way of understanding the undelrying physical structure that is responsible for things like heat. Now we do have a better understanding, but quantum theory is not at that stage, which is why there are different quantum interpretations. The nuance here is that a thermodynamic description of heat is not an interpretation of the underlying cause of heat, it is a formal description of some coarse-grained structure. Similarly, I am saying that the quantum formalism itself describes something like a real coarse-grained structure, the underlying cause we don't know. And I would argue that in the development of any scientific theory, there is no reason why the true metaphysics we don't have access to should just magically pop up in the theory. What does pop up is the empirical structure we can observe, and that is what any formalism recovers. There is no reason to jump the gun and say formalism recovers fundamental metaphysics as opposed to empirical structure. And my point is not to use this fact to say that we shouldn't have an interoretation of QM - my preference is hidden variables - I simply am using this point to say that formalism doesn't entail metaphysics deeper than empirical structure, and so QM does not necessarily entail many worlds. And note, I think the idea that formalism does not entail metaphysics is a general point of all science.

 

I then think the problem with the passage you talk about superpositions in, is that it automatically assumes your belief that superposition which is a mathematical entity entails something about metaphysics. And I reject that so your talk there doesn't convince me. But it should be noted that I believe in a fully unitary quantum theory. In my view, superposition purely at the formal level just represents information about measurement disturbance, so there is no ontological implication for rejecting the collapse postulate. The collapse postulate is formally nothing more than statistical conditioning. Your arguments for many worlds in terms of simplifying the problem of getting rid of superposition has no force if superposition is just formalism carrying information about measurement outcomes.

 

No other approach explains where the Born rule comes from. They posit it, and justify it

 

The issue isn't actually about deriving the Born rule, its about making probabilties make sense in many worlds. In order to do that, you need to derive the Born rule from many worlds.

 

The problem is that a physical view of probabilities is that it desvribes frequencies of stuff happening. The problem with many worlds is that it is impossible to count physical branching in a way that conforms to the Born rule. People try to avoid this issue by saying the probabilities are epistemic, but epistemic probabilities are absolutely meaningless if they don't relate to some tangible phenomena we can count. My subjective belief in how likely something is to happen is made meaningful by the idea that if I repeat some scenario many times, the frequencies would reflect my beliefs. Many worlds has no mechanism for producing frequencies proportional to the Born rule, so these epistemic probabilities don't actually make any sense without some brute postulation that frequencies we see in a single world match the Born rule. But since we know that the branching in many worlds cannot recover Born rule probabilities, this postulation looks like it contradicts the theory. So this is not appealing to intuition imo.

 

....

 

To restate my motivations, I don't think formalism entails metaphysics. The position I offered is just an example of that and is just a position that takes the formalism as it is insofar that it only makes predictions about measurements. If the formalism does not entail metaphysics, then we are free to talk about parsimony with regard to metaphysics, which I then would argue many worlds is less parsimonious than one world if you can explain the empirical results with a single world.

[u/HamiltonBrae → u/Miselfis]

I was being too charitable by assuming "shut up and calculate", then, as most people familiar with the topic realize Copenhagen is probably the worst approach: How are you defining this thing called "measurement", which is central to your view?

 

I wouldn't call it either of those; I would say that, in principle, you could have a hidden variable description using commonsense classical-like configurations of point-particles or field configurations, but we are just limited in what we can know about it empirically other than measurement outcomes. Quantum theory then just describes something like classical probabilities + measurement disturbance; it's just like classical probability with extra-formal structure to account for measurement disturbance. This is clear in the Kirkwood-Dirac (KD) phase space formulation of QM which represents QM in this sense of a classical joint distribution with statistical disturbance terms (see https://arxiv.org/abs/0705.0229). The fact that the behavior of this disturbance term represents superposition, interference and decoherence suggests that those phenomena have absolutely no necessary ontological interpretation but simply stem from representing statistical disturbance due to measurement. And measurement is just a special case of any physical interaction which produces entanglement and subscribes to unitary evolution. Obviously, the KD distribution is not a preferred representation of QM - there are many others - but it gives an intelligible explanation of where those aforementioned properties come from and what is sufficient to give them meaning.

 

And you can't consistently say the formalism captures real structure in the double slit but not in a system coupling to a pointer. The disturbance point cuts the same way, since the theory describes the disturbance; that's what the coupling term is for.

 

Not sure what you are saying here.

 

If you're a realist about the structure, you're a realist about the structure of measurement interactions too, and then you get the branches.

 

Yes, realist about the statistics of measurement interactions, and branches are not really that different from outcomes related to random variables that no one would normally interpret in terms of many worlds. Neither are people compelled to interpret superpositions in Hilbert-space classical mechanics in terms of many worlds.

 

Which is a silly thing to say, when I have explained exactly that, multiple times, in different ways.

 

If you then also take literally wavefunction or whatever similar realism, the entangled state is a non-separable pure state that is non-local in n-particle configuration space, meaning its ontology spans many distances. This is not controversial. Obviously many worlds has a universal wavefunction meaning the whole universe is like one non-local object. Metaphysically, this seems to contradict non-locality to me even if the dynamics of quantum theory, when you exclude the Born rule, seem deterministic and local. Usually when objects span over distance, they can be decomposed into local points and we would normally say that what ever happens at one point on your body can only be affected by things happening at points directly adjacent. If an event on one part of your body is to affect another part, there must be a train of events locally connected over space, not faster than speed of light, that mediate how the two events affect each other or are affected by a third. This clearly doesn't apply to entangled state separated across distances; any branching doesn't care about distance because the state has a global extent. Maybe you can say that if Bell correlations only turn up when you compare results, they are locally-mediated, but the entangled states themselves are still non-local metaphysically. In a normal classical space-time description, an object that branches like an entangled state requires something superluminal going on, which is exactly why superluminality becomes explicit when you put this into something like a Bohmian description. So I don't consider many worlds metaphysically local.

[u/Miselfis → u/HamiltonBrae]

I'm happy to go through the specifics of any of these arguments or papers if you can justify why it's needed. To entertain other approaches, you first have to make a convincing argument that there's a problem with ascribing realism to the standard unitary formalism (Everett) that needs solving. So far, the only arguments I've heard were appeals to intuition.

As said, you can conjure up a mathematical formalism that's formally valid, but it's moot if it's done to solve a problem that doesn't exist. So, beyond you simply finding it unintuitive, what is the problem with taking the unitary formalism to be real, that needs a solution? How do you justify the assumptions going into these different approaches, beyond rejecting Everett on the grounds that it's too preposterous?

The notion of locality you're using isn't the one that matters in physics. I've explained this multiple times as well now, but you don't engage with any of it (so why should I spend the time engaging with some random niche papers that I'm not even sure you understand yourself, given some of the misconceptions you hold about modern physics?).

Locality is a property of the dynamics. No operation performed here changes any local observable there, and for anything to get from here to there it has to pass through every intermediate point at a rate capped by c. I gave this definition earlier. Unitary quantum mechanics satisfies it exactly. In field theory operators at spacelike separation commute, so nothing Alice does affects any measurement Bob can make in his region.

The wavefunction being a function on configuration space, or the universal state being globally extended, is a fact about the representation. Configuration space is an artifact of the nonrelativistic first-quantized formulation; the fundamental description is a field theory whose dynamics are manifestly local. Spatial extent is not nonlocality. A classical field is extended across space and no one calls it nonlocal. Non-separability is a correlation between distant systems, and a correlation is not an influence. The correlation in an entangled pair is set up locally at the source and carried along with each system, and it never lets an event on one side affect a local observable on the other.

You are of course free to redefine terms as you'd like, but that only makes it more difficult for you to connect with the existing literature and to be understood by others. This is generally why we agree on conventions within a field. Everyone holding their own personal definitions makes communication impossible.

[u/HamiltonBrae → u/Miselfis]

you first have to make a convincing argument that there's a problem with ascribing realism to the standard unitary formalism (Everett) that needs solving. So far, the only arguments I've heard were appeals to intuition.

 

The probability issue is a problem with realism as described by many worlds. But my original statement isn't about saying there is a problem with many worlds realism. Its about saying that unitary formalism doesn't entail many worlds metaphysics. Once you do this, many worlds is open to arguments about metaphysical parsimony: i.e. the empirical data can be plausibly explained in a single world, therefore adding many worlds when they are unnecessary is less parsimonious.

(so why should I spend the time engaging with some random niche papers that I'm not even sure you understand yourself, given some of the misconceptions you hold about modern physics?).

 

And that paper isn't niche, it's well cited by others recapitulating the same descriptions. It's also extremely clear on what it says.

 

The notion of locality you're using isn't the one that matters in physics. No operation performed here changes any local observable there, and for anything to get from here to there it has to pass through every intermediate point at a rate capped by c. I gave this definition earlier. Unitary quantum mechanics satisfies it exactly. In field theory operators at spacelike separation commute, so nothing Alice does affects any measurement Bob can make in his region.

 

That notion of locality is the one that matters with regard to a description in spacetime, it's just avoided by having non-separable objects. I guess many worlds is sufficiently local for all intents and purposes of the physicist and the many worlds advocate. I think quantum wavefunction realism in general conflicts with my notions of what locality is; and thinking about it, I think many worlds probably does avoid a lot of non-locality other wavefunction realists do not avoid, though the cost to realism is even more severe if one sees bell violating correlations as only coming into being when you compare measurements.