I QM & Ontology: Why Should We Stay Away?

[entropy1]

I was thinking that QM and QM practicioners tend to stay away from the ontology part of QM, and see the QM framework as useful for predicting measurement outcomes. But if I see the properties of wood or plastic, they are explained by the properties of the particles they are "made up from", and I consider these materials ontic, so the particles and the laws they obide should have some ontology about them too, right? So why should we stay away from this ontology part?

 

[andrewkirk]

So why should we stay away from this ontology part?

Ontology is like tennis. There is no rule saying that physicists must not do it. It's just that physicists don't count either as part of physics. Very many physicists do both ontology and tennis in their spare time.

There is no universally accepted rule for what counts as part of physics and what doesn't. But the most commonly accepted rule is that of Karl Popper, regarding the boundary of science. Popper suggested that if a proposition is not falsifiable then we would do best not to treat it as science. Popper's main aim was to argue that Sigmund Freud's psychological theories should not be treated as science. But it applies equally well to ontology.

Popper's rule has been treated as a blurry boundary in recent years, because - at least according to Lee Smolin and Mike Woit - much of string theory is unfalsifiable, and yet is conducted in and funded by physics departments.

 

[Demystifier]

Very many physicists do both ontology and tennis in their spare time.

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

 

[Demystifier]

So why should we stay away from this ontology part?

Idealistically, I think we shouldn't. Realistically, if we should that's because
(i) ontological statements cannot be directly tested experimentally
(ii) it irritates most physicists because it involves a philosophical way of thinking which most physicists don't like.

 

[romsofia]

I don't think people stray away from the ontology part. At my university, I've had many talks with professors and fellow grad students a like on the ontology of QM! However, they stay there -as talks-.

A lot of advice I've been given is that while it's fun to think about, if you want a career in academia, you can't really focus on it until you're a more established physicist. Which, sadly, I've learned to accept.

 

[Morbert]

So why should we stay away from this ontology part?

There's no a priori reason to, in the sense that if a physicist can compute expectation values by starting with a primitive ontology of fundamental elements and deducing observable properties, they are free to do so. But instead they normally work with a model Hamiltonian/Lagrangian and state space because it's easier/more fruitful to do so. QM has a great track record so there's little incentive to work on an alternative ontology-based method.

 

[martinbn]

So why should we stay away from this ontology part?

Who do you mean be "we"? What do you mean by "stay away"? What do you mean by "should"?

 

[timmdeeg]

due to problems with the tennis elbow.

I got rid of that with Diclofenac, nothing else helped.

 

[evi7538]

One of the things with ontology in physics is, once you add ontology to mathematical models of physics, the number of possible ontological interpretations quickly multiply like fruit flies and physicist quickly lose any common ground and split into interpretation-biased groups. This split not only involves the variety of known QM interpretations, but also a bunch of ontological schemes from the realm of philosophy like materialistic monism, idealistic monism, variants of dualism and panpsychism etc. And needless to say, each of those ontic variants are unfalsifiable and basically become a matter of faith, just like religions. It is not necessary a bad thing, people (both physicists and non-physicists) do it anyway, we all have certain ontic beliefs or assumptions. It's just that in professional field physicists tend to avoid such discussions just like people tend to avoid political or religious discussions in a workplace.

 

[Demystifier]

One of the things with ontology in physics is, once you add ontology to mathematical models of physics, the number of possible ontological interpretations quickly multiply like fruit flies and physicist quickly lose any common ground and split into interpretation-biased groups. This split not only involves the variety of known QM interpretations, but also a bunch of ontological schemes from the realm of philosophy like materialistic monism, idealistic monism, variants of dualism and panpsychism etc. And needless to say, each of those ontic variants are unfalsifiable and basically become a matter of faith, just like religions. It is not necessary a bad thing, people (both physicists and non-physicists) do it anyway, we all have certain ontic beliefs or assumptions. It's just that in professional field physicists tend to avoid such discussions just like people tend to avoid political or religious discussions in a workplace.

Yes, but it is possible to study ontology on a professional level. See e.g. https://en.wikipedia.org/wiki/PBR_theorem

 

[martinbn]

Yes, but it is possible to study ontology on a professional level. See e.g. https://en.wikipedia.org/wiki/PBR_theorem

Strictly speaking the theorem is not about ontology, but about the ontic-ness of the state.

 

[martinbn]

One of the things with ontology in physics is, once you add ontology to mathematical models of physics, the number of possible ontological interpretations quickly multiply...

How so? I usually see only one ontology. What are the others?

 

[evi7538]

Yes, but it is possible to study ontology on a professional level. See e.g.

Oh sure, there is a philosophy of physics and philosophical ontology as professional academic disciplines. And some people manage to be both professional physicists and philosophers.

 

[Demystifier]

I usually see only one ontology.

What do you mean by that?

 

[Demystifier]

And some people manage to be both professional physicists and philosophers.

Yes. Moreover, often those are not two parallel activities, but a single activity that happens to be relevant to both physics and philosophy. Which should not be strange at all, because, after all, there is no sharp borderline between the two. Perhaps most professional physicists would be happy if there was such a sharp borderline, but there isn't.

 

[martinbn]

What do you mean by that?

He said that the number of ontological interpretations multiplies. I am just asking him what he means by that. I cannot see more thatn one.

 

[evi7538]

He said that the number of ontological interpretations multiplies. I am just asking him what he means by that.

As I said, in philosophical ontology there is large variety of platforms such materialistic, idealistic or neutral monism, variants of dualism, panpsychism, phenomenalism and so on with multiple sub-variants in each category. Many of them are compatible with QM interpretations, others call for different non-standard QM interpretations, so the number of combinations of ontic philosophical platforms paired with QM interpretations becomes quite large.

As an example, the works of M. Silberstein and M. Stuckey represent an interesting combination of neutral monism with QM.

 

[martinbn]

As I said, in philosophical ontology there is large variety of platforms such materialistic, idealistic or neutral monism, variants of dualism, panpsychism, phenomenalism and so on with multiple sub-variants in each category. Many of them are compatible with QM interpretations, others call for different non-standard QM interpretations, so the number of combinations of ontic philosophical platforms paired with QM interpretations becomes quite large.

As an example, the works of M. Silberstein and M. Stuckey represent an interesting combination of neutral monism with QM.

Let's take an example. Say you study the spin of an atom, say textbook case of silver atoms. The only ontology that I can see is that the silver atoms exist. What else?

 

[evi7538]

The only ontology that I can see is that the silver atoms exist. What else?

Oh, no, there is a large variety of ontic interpretations of the experimental data from experiments with silver atoms. Your ontology represents physicalism, a particular branch of materialistic monism in philosophical ontology. You really need to study philosophical ontology to learn about other alternatives, if you have any time and interest in it of course. Physicalism is very popular among physicists (for obvious reasons), but not all physicists subscribe to it. To avoid philosophical debates, my disclaimer is that physicalism is non-verifiable and non-falcifiable, which makes all debates about its validity or falsity ultimately futile. Some simple examples: what if we live in a computed virtual reality (VR, Matrix)? Then silver atoms are nothing more than virtual events in the computed VR. What if the reality is only-consciousness (idealistic monism)? Then the silver atoms are nothing more then mental constructs of consciousness.

 

[martinbn]

Oh, no, there is a large variety of ontic interpretations of the experimental data from experiments with silver atoms. Your ontology represents physicalism, a particular branch of materialistic monism in philosophical ontology. You really need to study philosophical ontology to learn about other alternatives, if you have any time and interest in it of course. Physicalism is very popular among physicists (for obvious reasons), but not all physicists subscribe to it. Some simple examples: what if we live in a computed virtual reality (VR, Matrix)? Then silver atoms are nothing more than virtual events in the computed VR. What if the reality is only-consciousness (idealistic monism)? Then the silver atoms are nothing more then mental constructs of consciousness.

This misses the point. If I don't exist, but I am a part of some simulation or any other such idea, then the whole question of ontology is unanswerable and we can do something more interesting. But if I exist, or if I don't but take me as a measure of existence i.e. something exists if it exists in the same way I do. Then what are the alternative ontologies to the existence of the atoms?

 

[evi7538]

This misses the point. If I don't exist, but I am a part of some simulation or any other such idea, then the whole question of ontology is unanswerable and we can do something more interesting. But if I exist, or if I don't but take me as a measure of existence i.e. something exists if it exists in the same way I do. Then what are the alternative ontologies to the existence of the atoms?

The whole question of ontology is unanswerable anyway, but I personally still find it interesting, regardless whether such entity as "I" exists or not .

The only thing that we know for a fact about ourselves and about the world is the existence of our conscious experience. Everything else beyond this, including the existence of "me" as a subject of conscious experience, or the existence of the atoms, is a hypothesis.

"Even the concept of the “real external world” of everyday thinking rests exclusively on sense impressions" (Einstein)
"Reality is merely an illusion, albeit a very persistent one." (Einstein)

I knew that it will end up with philosophical debates.

Physicalism

 

[evi7538]

Here is another example of alternative ontology in physics related to the so-called "zero-world" interpretation of QM. In such interpretation there are no real physically existing atoms whatsoever:
The Ithaca Interpretation of Quantum Mechanics
What is quantum mechanics trying to tell us?
"Correlations have physical reality; that which they correlate does not."
"... the first pillar of the Ithaca Interpretation is that correlations are the only fundamental and objective properties of the world ..."

I know it sounds a little bizarre for an average physicist, but there seems to be nothing wrong or inconsistent with it.

 

[Demystifier]

He said that the number of ontological interpretations multiplies. I am just asking him what he means by that. I cannot see more thatn one.

Bohmian interpretation is one ontological interpretation, many-world interpretation is another ontological interpretation, objective collapse interpretation is yet anoher ontological interpretation, and so on. This is what is meant by many ontological interpretations.

 

[Demystifier]

I know it sounds a little bizarre for an average physicist, but there seems to be nothing wrong or inconsistent with it.

The only inconsistency is that Mermin (and other adherents of this or a similar interpretation) often both claim and deny this interpretation, sometimes within the same paper. For instance, Mermin and co in http://de.arxiv.org/abs/1311.5253 say
"But quantum mechanics itself does not deal directly with the objective world; it deals with the experiences of that objective world that belong to whatever particular agent is making use of the quantum theory."
So here they seem to be saying that objective world does exist, but quantum mechanics just doesn't deal with it.

 

[Demystifier]

Let's take an example. Say you study the spin of an atom, say textbook case of silver atoms. The only ontology that I can see is that the silver atoms exist. What else?

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?

 

[martinbn]

Bohmian interpretation is one ontological interpretation, many-world interpretation is another ontological interpretation, objective collapse interpretation is yet anoher ontological interpretation, and so on. This is what is meant by many ontological interpretations.

Well, for me ontology is about what exists, as the word suggests. So all of these are different intepretations but not about the ontology. They all have the same ontology.

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?

No, properties don't exists in the same way as the atoms. The things that exist have properties, but the properties do not have ontology. I agree that it is not enough to say that the atoms exist. Of course it is important to say what properties atoms have, but that is not ontology anymore.

 

[Demystifier]

The things that exist have properties

So what properties an atom has?

 

[martinbn]

So what properties an atom has?

That is irrelevant here, it is not an ontological question.

 

[Demystifier]

That is irrelevant here, it is not an ontological question.

Well, your understanding of the concept of "ontology" differs from the meaning usual in quantum foundations.

Here is an example. When you say "electron exists", for you that's an ontological statement. But for others, it's not. Instead, ontological statements are electron's position exists (for a Bohmian) or wave function exists (for an adherent of objective collapse interpretation).

 

[evi7538]

The only inconsistency is that Mermin (and other adherents of this or a similar interpretation) often both claim and deny this interpretation, sometimes within the same paper. For instance, Mermin and co in http://de.arxiv.org/abs/1311.5253 say
"But quantum mechanics itself does not deal directly with the objective world; it deals with the experiences of that objective world that belong to whatever particular agent is making use of the quantum theory."
So here they seem to be saying that objective world does exist, but quantum mechanics just doesn't deal with it.

I don't see any inconsistency here, he is just being careful not to draw too far-stretching conclusions based on his QM interpretation and not to deny any possibility of existence of any objective reality behind/beyond the non-objective non-reality of the world that we are experiencing (that he calls "toy universe").

For example, for the sake of a simple illustration, if we live in a simulated reality, then the world we are experiencing is not objective, it's just computer-calculated based on QM-like equations, and what we are dealing with is just flows of quantum information. But that does not mean that there is not a "real" objective reality behind this where the computer itself is located and where this simulated reality is being created.

See for example
Constraints on the Universe as a Numerical Simulation
Computer-Simulation Model Theory

Of course, one can attempt to go further and, starting from the Ithaca interpretation (IIQM), stretch it to the proposition of denial of existence of any objective or material world whatsoever. But one thing is that the IIQM interpretation by itself does not exclude the existence of objective world beyond the "toy universes", and another thing is that such far-stretching proposition would place this kind of QM interpretation into the realm of idealistic ontologies, which would be a hard sell in the community of scientists and physicists.

On the other hand, if one is to choose the idealistic monism to be his/her ontic platform of preference (and there is nothing wrong with that), then the IIQM seems to be the only currently existing QM interpretation (that I'm aware of) that is fully compatible with it.

 

[martinbn]

Well, your understanding of the concept of "ontology" differs from the meaning usual in quantum foundations.

And why do they use it differently? My way is the standard way it is used in philosophy.

Here is an example. When you say "electron exists", for you that's an ontological statement. But for others, it's not. Instead, ontological statements are electron's position exists (for a Bohmian) or wave function exists (for an adherent of objective collapse interpretation).

And what does it mean for position to exist? Or for the wave function to exists? It seems that it is just muddling the water.

 

[martinbn]

The only inconsistency is that Mermin (and other adherents of this or a similar interpretation) often both claim and deny this interpretation, sometimes within the same paper. For instance, Mermin and co in http://de.arxiv.org/abs/1311.5253 say
"But quantum mechanics itself does not deal directly with the objective world; it deals with the experiences of that objective world that belong to whatever particular agent is making use of the quantum theory."
So here they seem to be saying that objective world does exist, but quantum mechanics just doesn't deal with it.

That only seems inconsistent to you, because you (and others) use the word exists in a strange and inconsistent way.

 

[evi7538]

That only seems inconsistent to you, because you (and others) use the word exists in a strange and inconsistent way.

There is no single commonly accepted and consistent way to interpret the term "existence" even among professional philosophers, let alone the ordinary people.

"Existence remains, then, itself a serious problem in philosophy of language, metaphysics, and logic and one connected to some of the deepest and most important problems in those areas."

But if you happen to have a consistent definition of "existence" then please share it with us.

 

[Demystifier]

That is irrelevant here, it is not an ontological question.

According to wikipedia https://en.wikipedia.org/wiki/Ontology the ontological questions include:
- How do the properties of an object relate to the object itself?
- Do physical properties actually exist?
Can you cite some source which claims that those are not ontological questions?

 

[martinbn]

According to wikipedia https://en.wikipedia.org/wiki/Ontology the ontological questions include:
- How do the properties of an object relate to the object itself?
- Do physical properties actually exist?
Can you cite some source which claims that those are not ontological questions?

That is in the section Further examples of ontological questions. Where it says that citations are needed! Can you cite an actual source to support that these are meaningful ontological questions?

In any case I would be willing to expand my understanding about what ontology includes if you can explain to me what the meaning of those statements is. For instance what does it mean to say that position exists, and what does it mean to say the the wave function exists?

 

[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.