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This question is related to the measurement problem in Quantum Mechanics.

I want arguments in favor or against of these two viewpoints:

  1. The wavefunction is the ontological state of existence of systems in the universe.

  2. The wavefunction refers to the probabilistic knowledge that : (a) one physical system attributes to another physical system or (b) a fundamental element of consciousness attributes to a physical system.

The difference between 2(a) and 2(b) is hard to make out. Both are panpsychist. 1 is distinct from both. 2(a) attributes qualities like "knowledge" to physical systems, so it integrates consciousness with the material. 2(b) takes consciousness as a separate substance from the material.

What arguments support or oppose 1 and 2 respectively?

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  • I think you're asking this in the wrong forum. You're looking for worldbuilding fiction with this question. It's a fundamental failure of reasoning in both philosophy and science.
    – Graham
    Dec 23, 2023 at 10:17
  • 1) is definitely a philosophical question. 2a is even, arguably, a testable physics question mixed with philosophy. A suggestion though: using "wave function" is a very mild red flag for crank. Doesn't matter much when you're clearly in the realm of philosophy, but using "state" instead conveys that you might be more knowledgeable.
    – jchd
    Dec 25, 2023 at 14:12
  • @Graham ???????
    – Ryder Rude
    Dec 26, 2023 at 3:56
  • @jchd the wavefunction is the state in the position or momentum basis. it's not an outdated term
    – Ryder Rude
    Dec 26, 2023 at 3:57
  • @jchd Agreed that 2b is where it falls down. And my issue isn't just the initial philosophical propositions, it's the faulty reasoning after that.
    – Graham
    Dec 26, 2023 at 8:21

6 Answers 6

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Suppose I have N point particles. classically the state of this physical system is described by N 3d position vectors and N 3d momentum vectors. We mathematically represent that state using 6N real numbers.

Quantum mechanically the state of this physical system is described by a normalized complex valued function on 6N dimensional Euclidean space.

Here is a question: Are the 6N numbers that describe the state of the classical particles "the ontological state of existence of systems in the universe"?

For me, the answer to this question is 1:1 with the view on your first viewpoint.

My take is that the 6N numbers that describe the state of the classical particles is not ACTUALLY the state of the particles. It is our description of the state of the particles. The map is not the territory. Nonetheless, the 6N numbers are a description of something that is ontologically real, namely the physical state comprised of particles and their positions and velocities.

I feel the same way about the wavefunction. The wavefunction is not technically the state of the quantum system, it is our description of the state of the quantum system. But, nonetheless, it is a description of something that is ontologically real.

This is technically an argument against your viewpoint 1 because I state that the wavefunction is not the state of the physical system, rather it is merely our description of that state. Nonetheless, I would say it is spiritually an argument in favor of your viewpoint 1 because it maintains that the wavefunction is describing an ontological state.

I can't see the distinction between 2a and 2b. 2a just seems like a way to write the point of view without referring to consciousness, but you are still talking about a physical system being able to "attribute" knowledge so I think it's really the same thing.

I find it hard to argue in favor of 2... every scientific theory we have uses a mathematical framework to describe and explain or predict ontological physical phenomena. It would be strange to now have a theory that is now describing our state of knowledge. But if psi is only our state of knowledge than it is our state of knowledge about what? I suppose such a theory makes more sense if you are an idealist, believing that there is no matter, only mind. But in this I think you would say that the 6N real numbers we use to describe the state of N particles constitute in fact the state of the N particles, and not that they constitute merely our description of the state of the N particles. I would say this is the main argument for point 2. That is, if you believe that there is NO ontology, only knowledge (some kind of Cartesian skepticism or generally idealism), then of course you would argue in favor of a 2-like epistemic theory.

Here's another argument against 2. I think psi-epistemic theories emerged out of QBism which is an outcropping of Bayesianism. And Bayesian analyses tend to use language relying on things like "our state of knowledge about a system". But as far as I can tell, for every Bayesian description of a phenomenon, there is a corresponding frequentist description (and probably vice-versa). The frequentist leans more ontological whereas the Bayesian may lean more epistemic. So my upshot is that the success of Bayesian analyses does not coerce us to start accepting epistemic rather than ontological physical theories.

Another point about viewpoint 2: I don't think we require a panpsychic model of consciousness/knowledge to voice psi-epistemic theories. I think the questions would be just the same if humans were conscious and spiders and molecules were not. In particular, I don't think pan-psychism helps the psi-epistemic arguments at all...

I'll be honest, I'm typically dismissive of the psi-epistemic stuff since it's so different than how I normally think about mathematical/scientific theories. It's likely I haven't given strong philosophical arguments against.

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  • thanks for the detailes answer. indeed the subjective-psi model can only be valid if an objective ontology isn't part of the model, as hidden variable psi-epistemic theories are almost ruled out by Bell and PBR. The reason I have to make the second viewpoint panpsychist is to avoid Solipsist viewpoints like only your own mind exists, or drawing arbitrary lines like "only humans are conscious". But yeah, it's hard/impossible to make a distinction between these scenarios
    – Ryder Rude
    Dec 23, 2023 at 8:15
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In practice the wave function is a mathematical model which we can use to calculate certain properties of systems, subject to a variety of approximations. What the wave function is modelling (ie what the 'real' thing is that the wave function approximates) is an open question in physics. Some physicists say the wave function is an abstraction that allows the behaviour of point-like quantum objects to be calculated. Others say that the wave function is the quantum object (or objects), and that we get the impression of particles only when interactions between the objects happen at a point. The obvious weakness of any argument which suggests that consciousness has anything to do with the status of the real thing represented by the wave function is that matter and energy behave in accordance with quantum theory whether or not there is anything obviously conscious around to contemplate the behaviour.

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  • You mention two viewpoints : 1 the wavefunction is modeling something more concrete like in statistical physics or 2 the wavefunction is the concrete thing in the universe. both of these are objective collapse viewpoints, right? That collapse happens in the universe as "events in spacetime". These events are when things one of the eigenstates becomes real, and these events are objective, as in, no observer can deny it.
    – Ryder Rude
    Dec 22, 2023 at 13:54
  • 2
    Yes, I think that's right. Whatever the wave function is modelling is actually out there in some way, and objectively there are interactions which cause the wave function after the interaction to differ from the wave function before, whether anyone is around to contemplate that or not. Dec 22, 2023 at 14:04
  • Thanks. I want to respond to : "whether or not there is anything obviously conscious around to contemplate the behaviour." This doesnt apply to 2(a) and 2(b) as those are panpsychist positions. For e.g. in 2(a), the wavefunction is the probabilistic knowledge that one physical system holds about another. So it can be any physical system.
    – Ryder Rude
    Dec 22, 2023 at 15:19
  • I'd generally group myself with the Others who Say, but I'd express it as: "Particles point to categories of verbs that reality does (ways that systems can interact), equations of state that describe particles are adverbs, equations of state that describe systems are adjectives. Adjective reality is that reality which does verbs in the manner of the corresponding adverb."
    – g s
    Dec 22, 2023 at 18:40
  • That is to say: trying to construct a non-tautological is statement about reality without a that which does phrase misconstrues the meaning of the word is, and any sentence that uses equations of state or collections of quantum numbers as nouns, rather than adjectives or adverbs, is a sentence about abstract mathematical objects, not reality. I will, of course, promptly become a hypocrite when I'm not devoting great effort to being precise in my phrasing...
    – g s
    Dec 22, 2023 at 18:43
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The 2022 Nobel physics award was given for the continuing work showing local hidden variable theories are ruled out. There is a small contingent of physicists and philosophers who still say the wave function is probabilistic (psi-epistemic). These two are in opposition to each other. There are two main camps to the probabilistic wavefunction interpretation side, the Qbists (quantum Bayesians), and the more general psi-epistemists like Matt Leiffer and Rob Spekkens, who look for entirely new causal models somehow left out, among other possibilities.

There was the PBR theorem a decade earlier which also strongly shows the probabilistic account is impossible for similar reasons. The 2022 Nobel is a culmination of a half century or more of work on these ideas.

Bell wanted to show non-locality of the wave function, a non-epistemic notion, and the Nobel is a huge acceptance and of his original goal.

Entailed in “non-locality” is the impossibility of a purely epistemic wave function.

However, opponents can say physics is mistaken about what has made science so successful in the past. It could all have been a grand conspiracy or experiments can’t be done in the lab as they don’t isolate from the human agent’s consciousness which is confounding results.

Basically, epistemicists have to zoom way out, to the very notion of science and its inception and lineage since Aristotle say, to find their refutation. It’s not out of the question, but their battle is now upon the armor of science writ large.

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  • 1
    While psi epistemic hidden variable. theories are pretty much ruled out, this does not include non hidden variable relational psi theories like relational quantum mechanics (which is a concrete example of 2a)
    – Ryder Rude
    Dec 23, 2023 at 3:04
  • I think there still is a possibility for non-local deterministic and non-local non-deterministic models. And it's not clear how to rule out one of the two.
    – rus9384
    Dec 23, 2023 at 15:00
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    @rus9384 correct those are not ruled out, but they are both “psi-ontic”
    – J Kusin
    Dec 23, 2023 at 15:19
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    @RyderRude epistemic theories fall outside of the scientific paradigm we’ve enjoyed for millennia. We can control for our human subjective vision not being able to see at night—we know how the experiment was carried out in darkness, we can remove our agency. If Galileo dropped his two objects at night there’s no issue. I’m leveraging the mainstream scientific pedigree as my main argument, as psi-epistemic theories fall outside it, as they are changing the status quo when other traditional interpretations don’t, like pilot wave (psi-ontic)
    – J Kusin
    Dec 23, 2023 at 15:26
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I’m sceptical that one of the two alternatives hits the point how to interpret the wavefunction from quantum mechanics (QM).

  1. First I would like to ask you: What do you mean by this overloaded sentence “ontological state of existence of systems in the universe”? What does mean an “ontological state of existence”? I have a certain understanding of the three terms ontology, state, and existence, but no understanding of the fusion of the three terms in your sentence.

    Secondly, you emphasize that both positions 2a and 2b are panpsychist positions. I have no argument, why panpsychism is useful in QM. Hence I exclude position 2a/b from my further consideration.

  2. IMO currently the most convincing interpretation of the wavefunction is the probability interpretation (Born) as part of the radical view of the Copenhagen interpretation (Bohr, Heisenberg): The wavefunction, via the square of its modulus, formalizes a world of possibilities as the result from preparing the state of the microphysical system e.g., of an electron. The next observation makes the transition from the possible worlds to the real world. The observation realizes one of the possibilities.

  3. Due to the explanation in your comment you consider position 1 a modified Copenhagen interpretation, stating: “This IS the physical object upto isomorphism, nothing more".
    I know the concept of an isomorphism in mathematics. If the wavefunction were the domain of the isomorphism, then what is the image of the map? Apparently not a second mathematical object.

    I am sceptical that speaking about “what really exists” is meaningful. Caveat: In the background lurks the Kantian thing-in-itself.

Note. Corrected and expanded after the OP’s comment.

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  • Thanks for the answer. 1. By "ontological state of existence", I meant "what really exists", the mathematical structure we can point to and say "This IS the physical object upto isomorphism, nothing more". Now, we cant really know the answer to questions like this, but we strive to make answers that are good enough to think about
    – Ryder Rude
    Dec 22, 2023 at 15:00
  • About the second point of your answer, you describe what the wavefunction is in terms how it's used. My question is about how we carry that over to deduce "what really exists", or an ontology.
    – Ryder Rude
    Dec 22, 2023 at 15:02
  • One example of an ontology would be : "A universe where two kids of entities exist : objects that are measurement devices and objects that are wavefunctions. The former collapse the latter." This is a terrible ontology as it draws an arbitrary divide in the material world, without making the distinction mathematically clear. So I have not included this ontology in my post's options. My 1. is Copenhagen modified to not have an arbitrary divide, while 2(a) and 2(b) are panpsychist.
    – Ryder Rude
    Dec 22, 2023 at 15:10
  • Regarding this update, I want to say that by "isomorphism", I mean the asbtract structure of the physical system in existence is isomorphic to the structure of the mathematical model we've made. I agree that "what really exists" is not super meaningful. Basically, the 1st viewpoint in the post tries to construct an objective account of the universe, which doesnt reference any observer or measurement. One concrete example of such a theory would be a gravitational collapse theory (a speculative theory right now)
    – Ryder Rude
    Dec 22, 2023 at 17:53
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Pro 1) Pure quantum states are unique among any description of reality in that they (in the orthodox, local view) contain all information about a system, ie they capture everything god knows about the system. If any model gets to be called "real" it would be the one that can make (and back up) such a claim.

Anti 1) The map is not the territory, even if it's the perfect map.

Pro 2a) No one should/does disagree with this. Our brains are physical systems, and they can use the square of a quantum state to get probabilistic information about the outcome of measurements. Of course the state (even on paper) doesn't only contain probabilistic information, it generally needs its imaginary component, which goes away when calculating probabilities, but it's still important dynamically.

Anti 2a) A pure quantum state (as opposed to mixed) doesn't have anything to do with what a human brain knows about it. At least not more so in QM compared to any other field. My mathematical representation of a physical state could be correct or incorrect, but that's not at all related to the inherently probabilistic nature of measurement outcomes. Additionally, if I know my knowledge is imperfect, density matrices can be used to capture that lack of knowledge about the pure state - this just amounts to incorporating classical statistics when needed, as you'd do with any other model.

These are kinda dry philosophically, but I think it's representative of how a lot of physicists would respond.

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It's very easy to rule out 2b by the fact that experiments are run remotely. Humans design the experiments, but do not directly move atoms themselves.

2a does not exclude 1. The wave function can describe the behaviour of individual quantum entities, and from there you can model how they interact.

Regarding your statement about 2a and "knowledge", you have made a fundamental error in assuming "knowledge" equals "consciousness", by reifying a descriptive term. "Knowledge" in this context merely means the state of one entity affecting the state of another entity, and does not require the entities to be conscious.

As an analogy, consider throwing a ball at a tin can. On impact, the speed of the ball affects the movement of the can, so the can has "knowledge" of the ball's speed. Neither the ball nor the can needs to be conscious for this to take place.

Linking quantum mechanics with consciousness makes for some great sci-fi/fantasy novels but with very bad science.

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