The mathematically formulated laws of quantum theory show clearly that our ordinary intuitive concepts cannot be unambiguously applied to the smallest particles. — Werner Heisenberg

Heisenberg was correct. That is, the idea of a classical state does not apply to the smallest particles.

Schrodinger took this a step further with his thought experiment. Not only does the classical state not apply to the smallest particles, it does not apply to stones or flowers either. Or cats. The notion of a classical state is untenable for any physical system of any size. (This is the implication of the Heisenberg Uncertainty Principle.)

But it is important to realize that, while the behavior of the smallest particles cannot be unambiguously described in ordinary language, the language of mathematics is still adequate for a clear-cut account of what is going on. — Werner Heisenberg

Yes, the mathematics gives a clear-cut account of what is going on. So we need to add a word to our ordinary language vocabulary, which is "superposition".

We never directly observe a system in superposition. We only ever measure systems in one specific state or another (a particle at slit A or a particle at slit B). But, in quantum mechanics, it is valid for a system to be in a superposition state. Thus, on the premise of realism, quantum mechanics is telling us something about the nature of the world that we are unable to directly observe.

For me, it's just wild and I never seriously considered it, especially since the Bohm model is so tight and was the model that encouraged Bell to develop his theorem on non-locality that has been repeatedly verified in experiments at the macro and micro level. (Bell himself was an advocate for the Bohm model). — Rich

What Bell proved was that hidden variables and locality were incompatible. The Bohm model accepts hidden variables and rejects locality. Whereas the Everett model accepts locality and rejects hidden variables. Which makes it a more natural fit with Einstein's special theory of relativity.

Locality is an uncomplicated principle compared to hidden variables. Per locality, an object can only be directly influenced by its immediate surroundings.

The "field" functions as a mathematical equation which is applied to the described phenomena. — Metaphysician Undercover

Yes and, similarly, the equation of the circle is x²+y²=r². When we say that a car wheel is circular, we are describing the car wheel in mathematical terms.

I think those two points are in conflict. If the particle is said to be 'real and to behave in a consistent way independent of observation', then it's said to be 'a fundamental entity'. — Wayfarer

I mean it in the sense that tables, trees and tigers are emergent not fundamental entities. We don't require that they be identified in our fundamental physical equations. They are nonetheless real and exist independently of observation and measurement (per realism, anyway).

A field is a mathematical construct. So what type of realism are you talking about, Platonic realism? — Metaphysician Undercover

No. It's a way of describing physical phenomena, just as we might describe a car wheel as a circle. A quantized field can be visualized as a mattress with many springs. So an electron that is emitted need not be intrinsically the same electron that hits the back screen. Instead they would be distinct excitations of the electron field.

The Bohm model simplifies everything. — Rich

I would say the same of the Everett model except more so. As I see it, the Bohm model modifies the quantum picture and tries to provide a picture that conforms more with a classical view (using non-local hidden variables). Whereas the Everettian view interprets quantum mechanics on its own terms without needing to modify the equations, introduce non-locality or posit a quantum potential field.

We can also have an option that's realist but that admits ignorance: Namely, particles are something real, but we don't really know their nature very well. The model we have of particles being something like a "chunk of stuff" is wrong--or at least what we're taking to be single particles isn't actually a single particle. — Terrapin Station

Yes we can do that and that is the best approach in the absence of a viable alternative. My claim here is that the Everett model does provide the features that we normally associate with a good scientific theory. However it just happens not to be reducible to a classical picture of the world.

I would say that a non-realist account would revolve around the assertion that there are actually no particles as such, and that what manifests as 'particles' are not actually particles. Realism wants to believe that there are particles which exist whether or not the measurement is taken; this is what is thrown into doubt by the double-slit experiment, which is the godfather of all quantum weirdness. — Wayfarer

A realist account need not require that particles are fundamental entities. A particle can be an emergent feature of an underlying field. So, in the double-slit experiment, an initial ripple in an electron field can then propagate through both slits and interfere on the other side. We identify those ripples as electrons.

But, whether fundamental or emergent, the particles are real and behave in a consistent way independent of observation.

BTW, the video describes the basic experiment well. However at the end it claims that the act of observation or measurement changes the behavior of the particle. That is the interpretational claim at issue.

The realist claim is that a particle's behavior is the same independent of observation and measurement. The observed difference is instead due to the manner in which the observer becomes entangled with the experiment and how this changes their vantage point on what they observe.

The implication is that there are infinitely many universes existing in parallel, in which everything that happens is replicated infinitely many times. — Wayfarer

First, your quote explicitly does not exclude a finite number of branches (i.e., it says "very many"). Second, it's no secret that people variously use "parallel universes", "multiverse", "many worlds" and so on to describe the Everettian idea. So what? It's clear from the context that they're referring to the directly visible universe that is familiar to us all. But the universe proper need not be limited to what can be directly seen. And, thirdly, the Everettian view does include competing versions, some of which may entail finite branching and others infinite branching.

The double-slit scenario I outlined earlier very clearly contains only two branches in superposition - there are no multiple universes or infinite branching implied there. You could try to show how that specific scenario has problems that you think are of concern.

that 'branching' means what it says - every outcome happens in as many universes as there are outcomes - which is infinitely many. It doesn't matter whether you endorse it or not - that is what the theory entails. — Wayfarer

Well, first, branching does not entail more than one universe. A tree has branches, but it is only one tree. Secondly, it does not entail infinite outcomes. Only outcomes described by physical laws occur. A tree can have many branches but there are physical constraints on how many it can and does have.

No - don't rephrase it. Consider the meaning of the three words: 'the universe branches'. Leave the double-slit out of it - just think about what is being claimed. — Wayfarer

What I am claiming is that the universe has a (quantum) branching structure rather than a classical linear structure.

I think that phrase expresses that. But if you think it implies multiple universes, then I don't endorse that claim. Otherwise perhaps you could spell out what you think the claim is.

An interesting notion. A measurement is being taken without a conscious observer. — Rich

The point is just that interactions between systems result in the entanglement of those systems. Observers are not special in this regard.

Just consder what that actually says — Wayfarer

OK, I'll rephrase.

In the double-slit experiment where a single photon is emitted and detected on the back screen, it is represented as being in a superposition of travelling through both slits. On the Everettian view, this indicates two physical branches where one photon travels through one slit on one branch and another photon travels through the other slit on the other branch. These branches then merge into one branch (i.e., interfere) before the photon is detected on the back screen. The detected photon's location on the back screen implies this two-branch history. However there is always only one photon on any particular branch.

The measurement on the back screen results in one point, not a pattern, and only repeated runs reveal such a pattern. — noAxioms

Yes, the observer measures the photon at just one point on the back screen. However if measurement is a linear process (and not a wave function collapsing or reducing process) then there is an observer measuring a photon at each point on the back screen where the wave function gives a probability of finding the photon. Which is what the Schrodinger's Cat thought experiment illustrates.

Let me take a shot if I may, since I have interest and this model seems most plausible to me. I agree with AndrewM's responses except for the double-slit one just above. — noAxioms

Nice post, but I'm curious about where we might disagree.

In the double-slit experiment, the detection of the photon at the back screen is not the only interaction that occurs in the system. It's just the obvious one since it involves someone observing it.

However there are also the distinct photon/slit interactions that occur. These constitute "measurements" between the photon and the apparatus independent of observer interaction and so also result in branching. The observed interference effect when we detect the photon on the back screen just is the interference of those branches (which is quantified as the sum of the wave amplitudes from both branches).

I asked the question, what would a 'branch' be, in relation to 'a universe which branches'. That's a much bigger deal than a 'thought experiment'. — Wayfarer

Consider the double-slit experiment where a single photon produces interference. There is amplitude for the photon going through both slits (all interpretations agree about this). On the Everettian view, this is the universe branching such that a photon goes through the first slit on one branch and another photon goes through the second slit on another branch. Both branches subsequently merge to produce the observed interference pattern on the back screen.

And just what would 'a branch' be, in plain language? — Wayfarer

The familiar example would be Schrodinger's Cat. In the thought experiment, after a while, there is a superposition of a live cat and a dead cat. One branch is the evolution over time of the live cat. The other branch is the evolution over time of the dead cat.

But they do imply an infinite number of parallel universes - which is the only rebuttal I believe necessary. — Wayfarer

They imply one universe with a (possibly finite) number of branches in superposition.

It's a matter of irony that nowadays, the so-called 'realist' interpretations of physics are often said to be the 'parallel universes' of Hugh Everett or the various permutations of the multiverse suggested by string theorists. If you look back at Bohr and Heisenberg's philosophical musings on QM (retrospectively named the 'Copenhagen Interpretation'), they seem lucid - and parsimonious - by comparison. — Wayfarer

Actually the Everettian view is both lucid and parsimonious. The postulates are simply:
1. The universe is described by a quantum state
2. The quantum state evolves according to the Schrödinger equation

These postulates don't imply God playing dice, spooky action at a distance or consciousness-created reality.

Bohr had no answer to this challenge.

Much human mental activity can be described as automatic, inattentive, unintentional, involuntary, uncontrollable (e.g., procedural memory recall, priming effects, intuition, automaticity, schema activation, the application of heuristics, operation of conscience, affect display, etc.). These are semi-conscious activities which are not free or choices.

For example, I can choose when to start or stop walking, but walking itself is automatic. I don't have to make a conscious decision to lift my right foot, I don't have to make a conscious decision to swing it forward in the air, I don't have to make a conscious decision to drop it onto the ground, then repeat the process for my left foot. — Galuchat

That's true. But I'm thinking of occasions where we might spontaneously answer a question or select from some range of options. For example, my normal preference is tea over coffee. If asked, I would normally respond "Tea, thanks" without consciously thinking about it. Yet I would still consider that I had chosen tea.

Similarly, perhaps our world is a fictional one depicted in stories in Harry's world. There can be no test of it, but I was wondering if it was meaningful to ask what it would mean, without asking to what it would be meaningful to. — noAxioms

OK, per hypothesis, there would be a (Many-Worlds) quantum world where a real Harry Potter exists as well as the Harry Potter fictional stories in our quantum world. Similarly, there would be fictional stories in Harry's world that just so happens to describe our world.

In this case it would be true to say (justification aside) that Harry Potter existed somewhere in the universe, though not in our quantum world. We would just need to be careful to keep our claims about the real Harry Potter in the other world distinct from claims about the fictional Harry Potter in our world. Similarly for Harry's claims about us.

I want to agree, but I think where I differ is the claim. If this universe did not exist, I would still be able to point to it. I would just not exist along with it. The universe existing seems not to be a prerequisite to its occupant pointing to it. Harry Potter can point to stuff in his universe despite both their nonexistence. I'm not confused about the difference between the two, but Harry is. Maybe he reads a fiction book about us. — noAxioms

When we say that Harry Potter can point to stuff, we are making a different kind of claim to when we say that we can point to stuff. The latter is understood in a straightforward literal sense, the former assumes we are talking about a work of fiction. That is, our interpretation of those claims already depend on us making a distinction between what exists and what is mere representation. That distinction is enough to provide a usage for the word "exists".

You mention that you are not confused about the difference between the two. But whether Harry Potter is confused is only a question of whether the author represents him as confused or not.

I have a base aversion to idealism, so I stay away from statements implying observation or discourse being the thing distinguishing an objectively actual structure (maybe that's a better word than 'set') from a non-actual one. But it comes up a lot, sort of a circular ontology of mind supervening on the material, but the ontology of the material somehow supervening on that observation. — noAxioms

Per realism, that we can observe or point to something is not what makes that thing exist (which is a separate question), but it is what allows us to claim that it exists. We aren't confused about the ontological difference between our universe and the Harry Potter universe because we can point to the books and the author from where those ideas derive and we understand their history. That gets more complicated with mathematical theories of our universe (or historical and religious accounts) largely due to the fact that some aspects are not readily observable.

Every time we make a choice there is a decision making process — Purple Pond

Not necessarily. Choices can be spontaneous.

There are only two solutions: an infinite regress of decision making processes, or it terminates to factors beyond our control. Since there cannot be an infinite amount of decision making process, our decisions are fundamentally beyond our control. — Purple Pond

Yes, at some level the choice will terminate in factors beyond our control. But in normal cases, those factors will not change our appraisal that a choice was made. Making a choice does not require breaking the laws of physics.

That looks like realism about universals, not a position on the subjective/objective distinction.

Maybe I'm missing something? — Marchesk

Immanent realism reframes the relationship between subject and object. We don't perceive appearances, we perceive things. But an objective representation (or abstraction) of things also depends on our physical sensory systems.

Thus we ordinarily perceive red apples as things in the world that exist independently of us, but their representation as red apples nonetheless depends on our physical sensory systems - other creatures could potentially perceive and represent them differently. So objective representation has a domain of applicability - there is no intrinsic or transcendent representation of things.

Most people are by instinct 'transcendental realists', whereas I tend towards dualism. — Wayfarer

I would suggest a third view that integrates the subjective and objective without transcending the senses. That would be Aristotle's immanent realism.

Independent, in what sense? We have sensory organs that are adapted to a particular range of stimuli, and intellectual capacities that we are told nowadays are the consequence of biological evolution. So how do we rise above those capacities and see the world 'independently' of those capacities? How do we know we're capable of seeing the world 'in itself' and as distinct from or apart from the categories of understanding through which we view it? I think that 'independence' is really a working assumption that is often treated as a metaphysical principle. — Wayfarer

Independent in the sense that the world was there before humans were around to talk about it.

We don't rise above those capacities (which would be the view from nowhere). The ordinary use of terms like "see" imply that it is the world that is seen (as opposed to a private theater or Platonic cave). You could accurately call it a working assumption.

Color is a lot tricker than taste. Nobody is a taste realist, I take it. Nobody thinks that the apple objectively tastes sweet. It tastes sweet to animals whose taste buds detect a certain amount of sugar content. But what if we didn't' have a sense of taste or smell at all? Maybe we detected chemical content via spectroscopic eyes or some other sensory organ. — Marchesk

Properties like taste and color are pragmatic abstractions. What makes them objective is that they have an ostensive meaning and logic of use. It's not relevant at that level of abstraction what the underlying physical processes are.

These are the basic abstractions that we build on to get to more complex abstractions like "mass". If we had different sense modalities, then we would abstract the world differently at the sensory level. But we would still, in principle, end up with the same fundamental physics.

It is an alternative, but it prevents us from speaking of the world when humans aren't around, which would be most of the time, since humans only occupy part of the surface of one little pale blue dot for the past 50 thousand years or so. — Marchesk

Not at all. We experience and describe the world from a human vantage point but it is still the world, independent of what humans say about it, that we are experiencing and describing.

There was no language about dinosaurs until humans came onto the scene. It doesn't follow that the referents of that language (i.e., the dinosaurs) didn't exist.

The consequence of this reframing is that our reports of our experience are no longer infallible reports of a private theater. They are instead provisional reports of our experience of the natural world. For example, I might report that I saw a red apple but be mistaken about that. It might instead have been a green apple that just appeared red in that lighting. Others can judge whether my report was accurate or not by looking for themselves. And this may lead me to revise my own report of what happened at the time. That is, the correct report of my experience is that I saw a green apple that appeared red due to the lighting.

Nagel's way of putting this is that science provides objective, third person explanations. But experiencing red is first person and subjective. So something is left out with any objective explanation. That explanation can be scientific, mathematical, computational, or functional and it will still leave the experience out, because all of those are objective explanations — Marchesk

I would suggest that objective explanations always directly or indirectly reference experience. For example, you might explain addition by showing how you can group stones together or explain quantum mechanics by pointing out interference patterns on a screen. In other words, there is no view from nowhere.

Which would mean that any explanation of experience would itself need to be in terms of experience.

Perhaps an alternative way of framing the issue to the usual subjective/objective framing.

But they do deny the inner, private part. Experiences can be individual, but not inner or private. — Marchesk

Physicalists don't deny the inner, private part either as meant in the ordinary sense of those terms (e.g., that you can be stoic, hide your true feelings, be misunderstood, etc.). And certainly you could feel something that is unique to you, that no-one else has experienced before or can understand. What is instead denied is a radical privacy - the idea that no-one could experience or understand that feeling in principle, regardless of their physical brain state.

The reason that we can meaningfully talk about red apples is because our physical sensory systems are, in the relevant sense, the same. But they need not be, as considering how one would communicate the idea of red apples to a blind person demonstrates.

Right, but quining qualia amounts to redefining consciousness as having a functional/behavioral role only. Dennett did say in a recent talk I watched on youtube that we are the equivalent of p-zombies. There is nothing going on in our heads in terms of consciousness. — Marchesk

You'll find that he is denying epiphenomenalism, not redefining consciousness. See his paper on The Unimagined Preposterousness of Zombies. In particular, note his analogy with health that concludes:

"Supposing that by an act of stipulative imagination you can remove consciousness while leaving all cognitive systems intact--a quite standard but entirely bogus feat of imagination--is like supposing that by an act of stipulative imagination, you can remove health while leaving all bodily functions and powers intact. If you think you can imagine this, it's only because you are confusedly imagining some health-module that might or might not be present in a body. Health isn't that sort of thing, and neither is consciousness."

Number One would mean that our visual system is reproducing the color that is out there in the world, or somehow directly perceiving it. Thomas Reid is one philosopher who has defended such a position. — Marchesk

I accept color realism, with the qualification that "out there in the world" includes the physical particles in an animal's or person's brain and body as well as the rest of the world (this provides an indexical onto the perceived object, where different creatures perceive an object differently). And, secondly, that normative factors influence the naming of colors (e.g., a blind person should also regard fire engines as being red).

Two would be a Dennettian approach. We don't actually experience color anymore than p-zombies do. It's an illusion. As such, the physical facts leave nothing out, and there is nothing more to explain. — Marchesk

From my own reading of him, I think Dennett also accepts color realism. In "Bacteria to Bach and Back" he says, "Yet another option insists that colors do indeed exist; they just aren't what the folk think they are. I have followed that path throughout my career, insisting that not only colors are real but also consciousness, free will and dollars."

In this case, the illusion is not that colors are experienced (they are), the illusion is the idea that objects have color independent of any perceptual experience that would give meaning to color terms.

When a rock is dropped on my foot and I say that it hurts, I certainly don't mean the resulting behavior, I mean the felt pain. Similarly, when I comment on the redness of a rose, I don't mean the wavelength of light. — Marchesk

Yes and your usages of "pain" and "red" here are the ordinary usages.

So, per physicalism, this is where the impossibility of p-zombies becomes apparent. You can't have two atom-by-atom identical beings where one is sentient (experiences pain, sees redness) and one is not.

There is definitely a private, unshareable aspect to our being. — Marchesk

So the philosophical question is whether this is an in-principle radical privacy, or whether language and technology can potentially bridge this. This is really a version of the Mary's Room thought experiment.

But what does a behaviorist mean when when they say that dropping a rock on your toe "hurts"? If they mean you hop up and down and yell, then that's not consciousness. That's simply behavior. It they mean certain nerves are firing resulting in that behavior, it is again not consciousness, it's neurological activity. — Marchesk

They presumably mean the same thing as you do. They're not redefining the term "pain" as hopping and yelling even in the absence of any felt sensation. Instead, they're attempting to provide an explanatory account of such experiences in terms of observable behavior or neural activity.

I would agree, however, that the behavioral project fails since I think it rests on a category mistake. Mental terms don't reduce to physical terms.

In any case, my argument would be that some of our concepts are subjective and not behavioral or physicalist. When I say that it hurts or the rose is red, I mean my experience of feeling pain and seeing red, not howling and jumping around, or a scientific account of optics and reflective surfaces. — Marchesk

I basically agree, though I would note that we mean roughly the same thing when we say that dropping a rock on our foot hurts or that the rose is red, or else ordinary communication would not be possible.

So while mental terms don't reduce to physical terms, they are still grounded in ordinary observation.

It's easier to see this is not the case if we avoid the word consciousness and stick with qualia and behavior.

It's clear that when speaking of qualia we are not talking about behavior, and vice versa. A behaviorist would deny the existence of qualia, not say that qualia is actually behavior, because that makes no sense. — Marchesk

The analogy with "qualia" here is to "hole in the sky". We can deny that there are holes in the sky while affirming that stars exist. Similarly, a behaviorist or physicalist can deny the existence of qualia, while affirming that dropping a rock on your toe hurts and that roses are red. For them, qualia serves no explanatory purpose.

In other words, a behaviorist or physicalist can affirm that consciousness is real, but deny the dualist explanation of consciousness.

Does it, though? What if ten people in the crowd had a red shirt? Does the statement fail to refer to them?

I've certainly listened to speakers use a general you to address some people in the crowd.

Maybe the problem is expecting that ordinary language propositions necessarily rely on bivalence. In the case of QM, the truth value can depend on which branch, if one adopts MWI. — Marchesk

There's actually no problem making (bivalent) truth apt statements for quantum scenarios. The problem is only that statements that are truth apt in a classic scenario may not be truth apt in a quantum scenario because there is no one-to-one translation. The conceptual schemas are different and so just require different statements to be made.

The red-shirt statement does not refer if understood in its literal, singular sense in the crowd context. But, as you point out, it could be interpreted as referring to a subset of the crowd which could be expressed as, "some of you have red shirts". Alternatively, each person could interpret the singular statement as talking about them personally, in which case there would be multiple propositions with potentially different truth values.

This is really what is going on with the quantum counterfactual. Interpretations are being made that depend on the conceptual schema that the person holds. My view is that the wave function is real and that our conceptual schema should reflect this. On this view, there can be no fact about what would have happened to you (singular) if you had opened the Schrodinger's Cat box at an earlier time.

This conclusion is actually the basis of Bell's Theorem, where "fact" above is equivalent to "hidden variable" or "element of physical reality". Bell proved that the classical picture was incompatible with Einstein's principle of locality.

So in other words all counterfactuals are trivially true (if physically possible) because we can simply stipulate that they refer to the possible worlds in which the described events happen? — Michael

Yes.

No, we have to abandon the idea that the statement meaningfully refers to something. Suppose I was speaking to a crowd of people and I said, "You have a red shirt." That statement lacks a truth value unless I'm addressing a specific person.

The problem is not the principle of bivalence, it's the presence of ambiguity.

This is ambiguous. Are you say that it doesn't have a truth value, or only that we can't determine what that truth value is? If the former then we've abandoned the principle of bivalence. If the latter then we need to refer to something other than the laws of nature to explain its truth value. — Michael

Under unitary quantum mechanics, the statement, "if I had opened the box at earlier time t I would have found the cat to be dead" doesn't have a truth value because it doesn't unambiguously pick out a specific branch of the wave function.

In other words, the statement is not properly grounded.

For instance, how can one know what is and isn't 'affecting' the observers? — mcdoodle

The ESP seems to me just a version of a common-sense principle that we apply in everyday life. Suppose we are predicting whether it will rain this afternoon. We look at local factors and ignore irrelevant or remote factors.

There are also strong constraints in play, such as speed-of-light limits and isolation between branches.

Of course, in all of these derivations, only unitary evolution occurs, and in the end you get something ontological - i.e. branch weights. — tom

I think you may have misunderstood my comments on the Born rule (or I could have expressed them better) - I certainly agree that it should be derivable in principle from the ontology of QM. Anyway, I've just read through Carroll's blog post on his and Seben's derivation and it makes sense to me.

Basically the Born probabilities are explained in terms of self-locating uncertainty just after a measurement has been taken and decoherence has occurred. For branches with equal amplitudes, we should be indifferent about the branch we find ourselves on and so assign them equal probabilities. Whereas (as Zurek shows) branches with unequal amplitudes can be mathematically reduced to many more branches with equal amplitudes (which, as it happens, will be in proportion to the square of the amplitude). So that seems like an intuitive result.

For sure, there could be states like frames in a movie. But this is a completely different premise, not consistent with QM, it's a completely different ontology. — Metaphysician Undercover

Quantum states are fundamental to QM. The Schrodinger equation describes how the quantum state of a quantum system changes with time.

Don't you find it bizarre that you can (supposedly) go from an ontic state in Reality, to an epistemic state in a mind, just by taking the modulus squared? — tom

I see the Born rule as an empirical result that is useful for making predictions about the state that will be observed. But I don't have an explanation for the rule. Do you?

It's all mathematics, therefore it's all epistemology — Metaphysician Undercover

The mathematics of QM describes the world. That is what is being modeled. It is not a description of people's knowledge.

This is a misrepresentation though. What is being referred to is an active system. So you cannot represent this as a case of the coin having been tossed, and the result is hidden under your hand. The system is an active system, so there are no states, there is no result. The ontology of the system must be represented as if the coin is in the air. That is your mistake, you want to represent states, talk about states, when there are no real states in an active system, it is inherently active. — Metaphysician Undercover

It's difficult to discuss this when you won't accept a simple, familiar example like the outcome of a coin toss. Of course there can be states in an active system, just as there can be frames in a movie.

The uncertainty principle demonstrates that this is not the case, there are no definitive positions of particles — Metaphysician Undercover

The uncertainty principle does not demonstrate this. It shows that an object can't have a precisely-defined position and precisely-defined momentum at the same time.

I think my version of 'realism' is different from yours but I understand what you're saying. I'm sceptical that we can know 'what nature is really like', which is why I keep asking for the agnostic option in science: what is the minimum ontological commitment involved in such and such a proposition? It feels as if people of a scientific bent sometimes drift from the minimum to a greater 'metaphysical' realism that is to my mind just a metaphysical claim, not something that's necessary to agree on the proposition in question. — mcdoodle

As it happens, Many-Worlds does involve the minimum ontological commitment (albeit more of the same ontology we are already familiar with) - it's bare-bones QM without extra postulates. Whereas it requires extra effort to take the branches out of QM. Other approaches either change the philosophy (Copenhagen, instrumentalism) or change the physics (Bohmian Mechanics, Dynamical Collapse theories).

Regarding knowing 'what nature is really like', this is where translating mathematical models into understandable language is useful. QM is generally regarded as being not merely difficult, but inherently confusing and incomprehensible. Which is a philosophical issue.