QM & Ontology: Why Should We Stay Away?

[Demystifier]

For instance what does it mean to say that position exists, and what does it mean to say the the wave function exists?

I will explain it to you, as soon as you explain to me what does it mean that electron exists.

 

[martinbn]

I will explain it to you, as soon as you explain to me what does it mean that electron exists.

Why!?

 

[Demystifier]

Why!?

Because then I will know what kind of explanation of "existence" do you find acceptable.

 

[evi7538]

My personal preference with respect to the concept of existence is phenomenological empiricism: we can only ascribe the modality of "existence" or "reality" to the phenomena of our direct conscious experience (sensations, thoughts, feelings etc) because we know for a fact that they indeed happened and were experienced by us. Existence of anything else (matter, electrons, wave functions, you name it) for us can only be hypothetical and conceptual. How we particularly ascribe the modality of existence to those conceptual entities is purely a matter of definition and a choice of philosophical platform. For example, if I choose to be a physicalist, I can ascribe the modality of existence to electrons. If I choose zero-world QM interpretation, or if I choose neutral monism as my ontic platform, I do not ascribe the modality of existence to electrons. This view makes all debates about the "true" meaning of the concept of "existence" groundless and futile.

 

[Demystifier]

My personal preference with respect to the concept of existence is phenomenological empiricism: we can only ascribe the modality of "existence" or "reality" to the phenomena of our direct conscious experience (sensations, thoughts, feelings etc) because we know for a fact that they indeed happened and were experienced by us. Existence of anything else (matter, electrons, wave functions, you name it) for us can only be hypothetical and conceptual. How we particularly ascribe the modality of existence to those conceptual entities is purely a matter of definition and a choice of philosophical platform. For example, if I choose to be a physicalist, I can ascribe the modality of existence to electrons. If I choose zero-world QM interpretation, or if I choose neutral monism as my ontic platform, I do not ascribe the modality of existence to electrons. This view makes all debates about the "true" meaning of the concept of "existence" groundless and futile.

My preference is ontology as a thinking tool. We, indeed, can't be sure that the Moon is there when nobody observes it. But if we imagine that it is, then it is much easier to intuitively think about the Moon and eventually to make measurable predictions. Similarly, an adherent of the Bohmian interpretation can't be sure that the particle trajectories are really there, but just imagining that they are makes (for him) intuitive thinking about QM much easier.

 

[martinbn]

Because then I will know what kind of explanation of "existence" do you find acceptable.

I was asking for an explanation of how you use the word. For that you don't need my explanation.

Ok, I will try to explain it. First, of course the word is used in different ways depending on the context. For example it is used in mathematics all the time as in there exist a solution to the equation, there doesn't exist a group with such and such properties and so on. I am talking only in the context of physics. So something exists if it interacts with other things, it can affect them and it can be affected, and it is objective i.e. it is not an abstract mental construct. For example if you study heat in a metal rod, the rod exists. On the other hand if you analyse the heat equation in a certain way, separate variables, Fourier and so on you may talk about an infinite number of harmonic oscillators. Those do not exist. They are part of the map not the territory. And the distinction is important to me when the discussion has a more philosophical nature. Another example, particles exists, they bump into each other and can affect other things. Their positions on the other hand are a prat of the model, of the description. We may use a different, although equivalent mathematically and empirically, model that doesn't use positions.

So, my question is what is the meaning of exists when you apply it to position and the wave function?

 

[Demystifier]

I was asking for an explanation of how you use the word. For that you don't need my explanation.

Ok, I will try to explain it. First, of course the word is used in different ways depending on the context. For example it is used in mathematics all the time as in there exist a solution to the equation, there doesn't exist a group with such and such properties and so on. I am talking only in the context of physics. So something exists if it interacts with other things, it can affect them and it can be affected, and it is objective i.e. it is not an abstract mental construct. For example if you study heat in a metal rod, the rod exists. On the other hand if you analyse the heat equation in a certain way, separate variables, Fourier and so on you may talk about an infinite number of harmonic oscillators. Those do not exist. They are part of the map not the territory. And the distinction is important to me when the discussion has a more philosophical nature. Another example, particles exists, they bump into each other and can affect other things. Their positions on the other hand are a prat of the model, of the description. We may use a different, although equivalent mathematically and empirically, model that doesn't use positions.

So, my question is what is the meaning of exists when you apply it to position and the wave function?

Thanks for the explanation, my understanding of the word "exists" in the context of quantum foundations is different.

I like the analogy with magic tricks. For instance, consider a magician that pulls out a rabbit from the hat. Obviously, the magician, the rabbit and the hat exist. But that's trivial and not of any serious interest. What I am interested about is an explanation of the trick from the point of view of a spectator who does not know how the magician performs the trick. Any reasonable explanation assumes that there exists something more than meets the eye. For instance, one possible explanation is that there exists a hidden compartment in the hat in which the rabbit was sitting. Another possible explanation is that there exists ... well, I'll leave to your imagination to figure out what else might explain the trick. The point is that any explanation assumes that there exists something for which there is no direct evidence of its existence. In other words, we do not know whether it exists or not, but if it exists, we understand that it would explain how the magic trick works. By contrast, if we do not assume existence of anything which we don't see, then we cannot explain the trick. We can describe it, we can even predict the outcome (e.g. the rabbit will appear when the magician removes the curtain), but we cannot explain it.

Similarly, in QM the Bohman interpretation assumes that particles have positions all the time, even when we don't measure them. We assume that those positions exist. We don't know whether it is true, but if it is, it explains, for instance, how the Stern-Gerlach apparatus produces a definite outcome. Without that assumption, and without any additional assumptions beyond the minimal textbook QM, we can make predictions on the outcomes, but we cannot explain them.

 

[martinbn]

Thanks for the explanation, my understanding of the word "exists" in the context of quantum foundations is different.

I like the analogy with magic tricks. For instance, consider a magician that pulls out a rabbit from the hat. Obviously, the magician, the rabbit and the hat exist. But that's trivial and not of any serious interest. What I am interested about is an explanation of the trick from the point of view of a spectator who does not know how the magician performs the trick. Any reasonable explanation assumes that there exists something more than meets the eye. For instance, one possible explanation is that there exists a hidden compartment in the hat in which the rabbit was sitting. Another possible explanation is that there exists ... well, I'll leave to your imagination to figure out what else might explain the trick. The point is that any explanation assumes that there exists something for which there is no direct evidence of its existence. In other words, we do not know whether it exists or not, but if it exists, we understand that it would explain how the magic trick works. By contrast, if we do not assume existence of anything which we don't see, then we cannot explain the trick. We can describe it, we can even predict the outcome (e.g. the rabbit will appear when the magician removes the curtain), but we cannot explain it.

This is perfectly fine.

Similarly, in QM the Bohman interpretation assumes that particles have positions all the time, even when we don't measure them.

This is also perfectly fine, and clear what it means.

We assume that those positions exist.

This is where the confusion might arise. Why do you need to say that they exist, when you mean that they have a value at any given time? I can see that the values exist in the mathematical sense. But you insists that the positions themselves exist in the ontological sense. That to me this is mixing different levels and unnecessary. I believe, and I don't think this is just me I think this is the common meaning, that ontology is concerned with the existence in the sense I described it, not in the sense that you use it.

We don't know whether it is true, but if it is, it explains, for instance, how the Stern-Gerlach apparatus produces a definite outcome. Without that assumption, and without any additional assumptions beyond the minimal textbook QM, we can make predictions on the outcomes, but we cannot explain them.

This is also fine.

 

[Demystifier]

Why do you need to say that they exist, when you mean that they have a value at any given time?

For instance, positions in the path integral formulation of QM also have values at any given time, but those paths have a different ontological status than trajectories in the Bohmian interpretation. The path integral formulation is just a computational tool, its utility does not require that the paths really exist in the physical-ontological sense. The Bohmian interpretation, by contrast, is not just a computational tool. If the Bohmian interpretation is true, then those trajectories really exist in the physical-ontological sense. So saying that they exist makes a difference. If it's still not clear, I can try to rephrase it.

 

[atyy]

I used to be one of them. Now I don't play tennis any more, due to problems with the tennis elbow.

Sorry to hear that, it would have been better to develop ontology knee than tennis elbow.

 

[Demystifier]

Sorry to hear that, it would have been better to develop ontology knee than tennis elbow.

Sport injuries develop when you do things improperly, which shows that my work on ontology is proper.

 

[martinbn]

For instance, positions in the path integral formulation of QM also have values at any given time, but those paths have a different ontological status than trajectories in the Bohmian interpretation. The path integral formulation is just a computational tool, its utility does not require that the paths really exist in the physical-ontological sense. The Bohmian interpretation, by contrast, is not just a computational tool. If the Bohmian interpretation is true, then those trajectories really exist in the physical-ontological sense. So saying that they exist makes a difference. If it's still not clear, I can try to rephrase it.

Let me explain why dislike the terminology. By itself it is ok of course, as long as you define that by position exists you mean that position has values at any time, all is good. The problem is that people, you included, tend to say that in an interpretation, where position doesn't have a value at any time, the existence of the particle is denied, not just position. Why? Then there is the wave function. You didn't say what you meant by it exists. Is the the same as for position? That it has a value at any given time. But that is just the definition of a function of $t$. It is true in all interpretations (in the Schrodinger picture). Or do you mean something else? You see how easy it is to get into the twilight zone. Sometimes you say that according to some interpretations only the wave function exists. What does that mean?

 

[Demystifier]

The problem is that people, you included, tend to say that in an interpretation, where position doesn't have a value at any time, the existence of the particle is denied, not just position. Why?

It depends on what one means by "particle". If one means a "classical" particle, namely an object with a well defined position at all times, then indeed the existence of particles is denied in non-Bohmian interpretations. On the other hand, if by particle one means something else, then it may exist in all QM interpretations, but then the source of confusion is the terminology because the same word "particle" has very different meanings in classical and quantum physics.

Then there is the wave function. You didn't say what you meant by it exists. Is the the same as for position? That it has a value at any given time. But that is just the definition of a function of $t$. It is true in all interpretations (in the Schrodinger picture). Or do you mean something else? You see how easy it is to get into the twilight zone. Sometimes you say that according to some interpretations only the wave function exists. What does that mean?

I mean something else. Let me take an example. Let $S(x,t)$ be the Hamilton-Jacobi function (a solution of the classical Hamilton-Jacobi equation), and let $f(x,t)$ be the amplitude of the water wave (a solution of the water-wave equation). I claim that $f(x,t)$ is ontic (it "exists") and $S(x,t)$ is not ontic. What does that mean? It means that $S(x,t)$ is just an auxiliary mathematical tool to compute the classical particle trajectories (which are ontic according to classical mechanics), while $f(x,t)$ is an actual material height of the water wave. If in this example you still don't understand the difference between ontic and non-ontic, then I really don't know how to explain it to you.

Now, assuming that you understand the difference, the question is whether the quantum wave function $\psi(x,t)$ is more like $S(x,t)$ or more like $f(x,t)$? Different interpretations of QM have different answers to that question.

 

[martinbn]

It depends on what one means by "particle". If one means a "classical" particle, namely an object with a well defined position at all times, then indeed the existence of particles is denied in non-Bohmian interpretations. On the other hand, if by particle one means something else, then it may exist in all QM interpretations, but then the source of confusion is the terminology because the same word "particle" has very different meanings in classical and quantum physics.

The word particle is not the problem here because you do that even without it. For example in BM an electron exists and its position exists too i.e. has a value at any time. In other interpretations the position doesn't have a value at any time, which you call doesn't exit. But then you go further by saying that in those interpretations the electron doesn't exits.

I mean something else. Let me take an example. Let $S(x,t)$ be the Hamilton-Jacobi function (a solution of the classical Hamilton-Jacobi equation), and let $f(x,t)$ be the amplitude of the water wave (a solution of the water-wave equation). I claim that $f(x,t)$ is ontic (it "exists") and $S(x,t)$ is not ontic. What does that mean? It means that $S(x,t)$ is just an auxiliary mathematical tool to compute the classical particle trajectories (which are ontic according to classical mechanics), while $f(x,t)$ is an actual material height of the water wave. If in this example you still don't understand the difference between ontic and non-ontic, then I really don't know how to explain it to you.

That is clear but the terminology is strange.

Now, assuming that you understand the difference, the question is whether the quantum wave function $\psi(x,t)$ is more like $S(x,t)$ or more like $f(x,t)$? Different interpretations of QM have different answers to that question.

In which interpretations is the wave function more like $f(x,t)$? After all the wave function, unlike the height, is complex valued and is not a function of $(x,t)$ if you have more than one particle.

 

[Demystifier]

In which interpretations is the wave function more like $f(x,t)$? After all the wave function, unlike the height, is complex valued and is not a function of $(x,t)$ if you have more than one particle.

That's a good question. The wave function is somewhat like $f(x,t)$ in the many-world interpretation and objective-collapse interpretation. Of course, it's not exactly like $f(x,t)$, due to the differences you mentioned. The fact that $\psi$ is complex is not such a big difference, because you can always think of it as two real functions. The fact that it is not a function of $(x,t)$ in the many-particle case is a much more serious conceptual problem, which indeed is one of the reasons why many people don't like interpretations in which $\psi$ is ontic.

Note also that the PBR theorem proves that $\psi$ is "ontic", but in this context "ontic" has a somewhat different meaning. In fact, that may be the only context in which "ontic" is defined precisely, but the price payed for this precision is that it does not exactly correspond to the more traditional meaning of "ontic" in quantum foundations.

 

[martinbn]

Does this conceptual problem have a solution? What is it? If not, then there are no interpretation in which the wave function exists/is_ontic in this sense of the word.

Wouldn't it have been better to use new and well define terminology. Most arguments, at least on forums like this one, wouldn't even begin.

 

[Demystifier]

Does this conceptual problem have a solution?

It depends on whom you ask.

What is it? If not, then there are no interpretation in which the wave function exists/is_ontic in this sense of the word.

I would suggest you to take some more detailed text on many worlds or objective collapse and decide by yourself whether their solution is satisfying.

Wouldn't it have been better to use new and well define terminology. Most arguments, at least on forums like this one, wouldn't even begin.

Yes, it would be better. Unfortunately, nobody yet has found a good substitute for the word "ontic" with a precisely defined meaning. If you have a proposal, I am going to listen.

 

[martinbn]

I would suggest you to take some more detailed text on many worlds or objective collapse and decide by yourself whether their solution is satisfying.

What about Bohmian mechanics?

 

[Demystifier]

What about Bohmian mechanics?

There, the wave function is very much like the Hamilton-Jacobi function.

 

[Paul Parnell]

Hmm, pointing out from the bleachers that Ernst Mach didn't believe in atoms. Nor space.

 

[haushofer]

Imo, the boundary between philosophy and physics is as artificial as the boundary between classical and quantum. It's just that many physicists feel more comfortable calculating than interpreting.

It's not just quantum mechanics. I also think about my own struggles with the meaning of general covariance or interpreting coordinate transformations actively and passively. Doing the calculations is so much easier than carefully interpret the subtle issues.

 

[physika]

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The things that exist have properties.

Right.

"properties are characteristic qualities that are not truly required for the continued existence of an entity but are, nevertheless, possessed by the entity."
..........Aristotle.

properties are just atributes, qualities.
Predicates.

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[Demystifier]

All non-ontological interpretations of quantum mechanics are alike; each ontological interpretation is ontological in its own way.

 

[user30]

each ontological interpretation is ontological in its own way.

Including hidden variables or not. Those who advocate hidden variables insists that the universe is empirically explicable at all scales.

Had it not been for classical physics predating quantum mechanics, I'm fairly certain physicists would be content with reproducibility.

That is to say this "Reproducibility means obtaining consistent computational results using the same input data, computational steps, methods, code, and conditions of analysis.".

To state that this needs to be accompanied with empirical explicability is a form of induction. It cannot be deduced regardless of whether we live in a world that has reproducibility or not. It's an aestethic requirement, completely independent of the process.

 

[physika]

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Ontic:

Relating to entities (things, objects) and the facts about them.

Entities : example; Electrons (be a wave, a particle...)
Facts: Properties (attributes, qualities, features, characteristics...)

facts are secondary, I.E. derived.

Entity: a thing with distinct and independent existence.

Objects are Subjects;
Properties are Predicates.

Predicates: what is said about objects (things, entities)

"Existence is not a predicate"
......Inmanuel Kant..
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[A. Neumaier]

Entities : example; Electrons (be a wave, a particle...)
Facts: Properties (attributes, qualities, features, characteristics...)

facts are secondary, I.E. derived.

? One cannot even define entities without stating their defining properties, i.e., some facts about them.

 

[physika]

? One cannot even define entities without stating their defining properties, i.e., some facts about them.

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What do you say ? that if you don't define it, it doesn't exist?
without definition, there is no existence ??

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[A. Neumaier]

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What do you say ? that if you don't define it, it doesn't exist?
without definition, there is no existence ??

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Without a sufficiently precise definition of a concept X it is undetermined what the statement that X exists means.

 

[physika]

Without a sufficiently precise definition of a concept X it is undetermined what the statement that X exists means.

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Objects (thing, entity) exist whether or not has a definition. To imagine they don't exist before we make such a definition but they do after we make the definition is nonsense.

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[physika]

Saying that the atom exists means nothing unless you specify what properties of the atom exist. For instance, does its spin (before one measures it) exist?

and how you can talk about properties without that atom.
no object, no property.
.