Or as Dustin Lazarovici reacted to the paper: "A group of physicists claims to have found experimental evidence that there are no objective facts observed in quantum experiments. For some reason, they have still chosen to share the observations from their quantum experiment with the outside world.

...

In particular, it doesn’t mean that measurement outcomes, once obtained, are not objective. It rather reminds us that a measurement is not a purely passive perception but an active interaction that “brings about” a particular outcome and can affect the state of the measured system in the process."

Basically, the only way we would have to let go the assumption of an objective reality is if we were to insist on locality (against this and other experimental results) in which case nothing really exists but thank God it's local! (pace Tim Maudlin) — Benkei

Here's the DailyNous link for those quotes. The assumption of reality, as defined in Bell test experiments, is simply that there is a definite value for a measurement that has not been performed (counterfactual definiteness).

Almost all QM interpretations reject counterfactual definiteness, with Bohmian Mechanics being the main exception (as it happens, both the individuals you quoted above hold the Bohmian interpretation).

So what experimental results actually show is that counterfactual definiteness, locality and free choice can't all be true. Those interpretations that assume locality are compatible with special relativity.

Quantum particles are even smaller, aka subatomic particles, and not properly described as particles but that's language for you. — Benkei

It's worth noting that superpositions have been created for objects with up to trillions of atoms (as in the case of the piezoelectric "tuning fork"). Probably most physicists would consider QM to be a universal physical theory (i.e., applicable to everything). Which is part of the point of the Schrodinger's Cat and Wigner's Friend thought experiments.

There is no implication of non-physical properties. In QM, light quanta (photons) have physical properties. And QM is consistent with special relativity. — Andrew M

Sure, QM is consistent with special relativity, but I don't think it's correct to call the properties of a photon "physical". A photon has an effect on physical things, and it might have a physical cause, and it is described by mathematics, but according to special relativity, light does not have spatial-temporal properties. The speed of light is the limit to spatial-temporal properties. So how exactly would you describe these "physical properties" which light quanta have? How does something which is only described by mathematics have physical properties? Say for example "2+6=8", that's something only described by mathematics. How does that have physical properties, other than the symbols which represent the mathematical idea?

I'm saying that wave function collapse is a matter of knowledge. — Benkei

Putting this together with your earlier comment that you are not attached to locality, it sounds like you have an affinity to the 'non-local hidden variables' school, of which David Bohm's 'pilot wave' interpretation of QM is perhaps the best-known. In most other popular interpretations, the imprecision about location is not just epistemological.

I like Bohm. I have his book 'Quantum Theory' which is interesting because it was written before the modern Dirac notation for QM with 'bras' and 'kets' became standard.

FWIW Bohm was quite a mystic, and had a famous series of public discussions with Krishnamurti about physics and spirituality.

It occurs to me that the notion of QM undermining the notion of 'objective reality' only makes sense if one insists that only particles, not waves, can be objective.

If we adopt a worldview that the universal wave function IS 'objective reality' - Kant's Noumenon - then we have all the objectivity we could realistically wish for, without even touching on the question of 'whether the moon is there when we are not looking'.

The wave function explains the correlation of observations by independent observers, aka intersubjectivity.

I wonder whether Berkeley would have thought of such a view as confirmation of, or an attack upon, his Idealism. It certainly has no truck with Johnson's rock-kicking approach.

The trouble is that the word 'objective' is inextricably connected to the concept of an 'object'. Whereas the ontological status of the wave-function is a matter of great dispute. Does it really exist in nature? Or is it a mathematical construct through which scientists view the world?

For that matter - are numerical propositions, or even just numbers, objectively real? I would say not. I would say they are deductively valid (or not). So they are used to determine what is objective. If I say there's 6 of something, and you count 5, then I'm objectively mistaken. But, like the nature of the wave-function, the ontological status of number is also a vexed question. (If you doubt it, look at the size of the Wiki entry on philosophy of maths.) Myself, I am inclined to the Platonic view of numbers - that they are real but not materially existent. However, naturalism has problems with that view (see here.)

The reason for the controversy is, in my view, because Enlightenment-era science believed it would be able to discover a real, ultimate, material particle or entity of some kind. But if instead if you have to talk in terms of wave-functions, or fields, or mathematical abstractions, or the act of observing, then it's already game over for that effort - there is no ultimate material unit to act in that role. I think that has something to do with the controversies.

The trouble is that the word 'objective' is inextricably connected to the concept of an 'object'. — Wayfarer

I dare say you are right.

Amazingly, I had not hitherto noticed that the first six letters of 'objective' are 'object'. Strange, for somebody with a keen interest in etymology, as I have.

I don't think so; it may well be the other way around, that 'object' derives from 'objective'.

objective
/əbˈdʒɛktɪv/
adjective
adjective: objective

1.
(of a person or their judgement) not influenced by personal feelings or opinions in considering and representing facts.
"historians try to be objective and impartial"
synonyms: impartial, unbiased, unprejudiced, non-partisan, disinterested, non-discriminatory, neutral, uninvolved, even-handed, equitable, fair, fair-minded, just, open-minded, dispassionate, detached, impersonal, unemotional, clinical
"an interviewer must try to be objective"
antonyms: biased, partial, prejudiced
not dependent on the mind for existence; actual.
"a matter of objective fact"
synonyms: factual, actual, real, empirical, verifiable, existing, manifest
"the world of objective knowledge"
antonyms: subjective
2.
Grammar
relating to or denoting a case of nouns and pronouns serving as the object of a transitive verb or a preposition.

noun
noun: objective; plural noun: objectives; noun: the objective; noun: objective lens; plural noun: objective lenses

1.
a thing aimed at or sought; a goal.
"the system has achieved its objective"
synonyms: aim, intention, purpose, target, goal, intent, object, end, end in view, grail, holy grail; More
idea, design, plan, scheme, ambition, aspiration, desire, hope;
the point, the object of the exercise
"our objective is to build a profitable business"
2.
Grammar
the objective case.
3.
the lens in a telescope or microscope nearest to the object observed.
"examine with high power objective"

No mention of 'object' there other than in the sense of 'purpose'.

In what way do you think QM puts an independent reality radically to question? And independent from what? — Benkei

I was thinking how to reply to this but I think Rovelli's presentation here is better than what I'd be able to come up with:

"If different observers give different accounts of the same sequence of events, then each quantum mechanical description has to be understood as relative to a particular observer. Thus, a quantum mechanical description of a certain system (state and/or values of physical quantities) cannot be taken as an “absolute” (observer independent) description of reality, but rather as a formalization, or codification, of properties of a system relative to a given observer. Quantum mechanics can therefore be viewed as a theory about the states of systems and values of physical quantities relative to other systems.

A quantum description of the state of a system S exists only if some system O (considered as an observer) is actually “describing” S, or, more precisely, has interacted with S. The quantum state of a system is always a state of that system with respect to a certain other system. More precisely: when we say that a physical quantity takes the value v, we should always (explicitly or implicitly) qualify this statement as: the physical quantity takes the value v with respect to the so and so observer.

Therefore, I suggest that in quantum mechanics “state” as well as “value of a variable” – or “outcome of a measurement–” are relational notions in the same sense in which velocity is relational in classical mechanics. We say “the object S has velocity v” meaning “with respect to a reference object O”. Similarly, I maintain that “the system is in such a quantum state” or “q = 1” are always to be understood “with respect to the reference O.” In quantum mechanics all physical variables are relational, as is velocity". (source)

[Quick note, by this point in the paper Rovelli has already set out very clearly that an 'observer' is nothing more than a 'physical object having a definate state of motion', and may well be something as benign as a table lamp. I mention this to head off any waffling idiot who equivocates on 'observer' to mean 'consciousness' or any such trash].

And 'objective' comes from the latin objectivus, which was used in the exact opposite way in which it is used now, because Kant fucked things up for everyone:

"The word “objectivity” has a somersault history. Its cognates in European languages derive from the Latin adverbial or adjectival form obiectivus/obiective, introduced by fourteenth-century scholastic philosophers such as Duns Scotus and William of Ockham. (The substantive form does not emerge until much later, around the turn of the nineteenth century.) From the very beginning, it was always paired with subiectivus/subiective, but the terms originally meant almost precisely the opposite of what they mean today. “Objective” referred to things as they are presented to consciousness, whereas “subjective” referred to things in themselves.

...Even eighteenth-century dictionaries still preserved echoes of this medieval usage, which rings so bizarrely in modern ears: “Hence a thing is said to exist OBJECTIVELY, objective, when it exists no otherwise than in being known; or in being an Object of the Mind.”" (Daston and Galison, Objectivity).

Sure, QM is consistent with special relativity, but I don't think it's correct to call the properties of a photon "physical". A photon has an effect on physical things, and it might have a physical cause, and it is described by mathematics, but according to special relativity, light does not have spatial-temporal properties. The speed of light is the limit to spatial-temporal properties. So how exactly would you describe these "physical properties" which light quanta have? — Metaphysician Undercover

The physical properties of a photon are able to be measured in the same way as for any other particle. If you want to know a photon's position or speed, you set up an experiment and measure it.

See https://en.wikipedia.org/wiki/Photon#Physical_properties

I have a rough time with this distinction. Something not real can still be used to describe a real thing. It just isn't the actual thing. — noAxioms

I believe that if you say that something 'not real' describes 'a real thing' you're just re-asserting a realist/representational view. The 'unreal'/'non-representational' view of the wave-function advocated by Rovelli, Bitbol etc is that the wave-function does not describe anything. It is just a tool.

This is very compatible with the view that collapse is due to an increase of knowledge (i.e. an 'epistemic', not 'ontic' view). But, of course, if the wave-function is not representational in any sense, it is difficult to explain why QM predictions are very good, for instance. In fact, a non-representational view of the wave-function implies that we cannot know anything about the unmeasured objects and the 'collapse' mechanism.

In other words, 'real' and 'representational' should be taken as synonyms (or very close to that) - the point is that there is a biunivocal correspondence between mathematical formalism and reality. In MWI all outcomes occur because you consider the wave-function as real. This is true for the objective collapse theories. In CI there is a spectrum. In dBB theory too this is a controversial issue, actually.

For instance, many dBB-supporters do not consider the wave-function as a 'real field' but as 'nomological'. The point is that the wave-function is a 3N-dimensional function, while particles or (normal) field configurations live in the 3-dimensional space. This paper explains some possible views of this 'nomological' camp: https://arxiv.org/pdf/1406.1371.pdf. The authors distinguish between two possible meaning of 'nomological'. The first is called 'Humeanism' - it is actually a reductive approach: there is no real 'reason' why particles move in the way that move. The wave-function is just a useful tool to 'describe' (or, maybe better, to calculate) the evolving configuration of particles (at least as I understand it) - it is more or less 'non-representational'. This is somewhat ironic because dBB is usually chosen to understand what's going on. Indeed, another 'nomological' view is dispositional, quoting from the article (p.16):

On this view, the universal wave-function Ψt of the system of particles at a given time is a mathematical object that represents the disposition to move in a certain manner at that time. This disposition is a holistic property of all the particles in the universe together – that is, a relational property that takes all the particles as relata. It induces a certain temporal development of the particle configuration, that development being its manifestation. In other words, given a spatial configuration of the particles (actual or counterfactual) and the disposition of motion at a time as represented by the wave-function as input, the Bohmian law of motion yields the velocities of the particles at that time as output.

This is entirely different from Humeanism. Now the wave-functions is not indeed a 'physical field'. Yet it represents some physical property. So I would say that the wave-function is indeed 'representational'. It represents a physical, real property - it's not just a tool like in Humeanism. The point is that in Humeanism, the 'wave-function' does not add anything to ontology whereas in dispositionalism, it is related to a physically real disposition.

The point in bringing this distinction between two different 'sub-interpretations' of dBB was simply to explain better what I mean by 'real'. If you think that the wave-function has some ontological meaning, then the wave-function is real/representational. If not, it is just a tool of some sorts.

Edit: a somewhat related discussion on physics forums can be found here: https://www.physicsforums.com/threads/the-reality-of-configuration-space.554543/

That doesn't follow from either interpretation of the wave function. It seems to require an additional postulate. — noAxioms

Agreed!

He was shunned by the physics community after his PHD and went into the defense industry instead, but was asked to present his work 5 years after the paper was published. Somewhere around that time DeWitt coined the MWI term from Everett's original "relative state formulation" which sounds an awful lot like RQM. — noAxioms

Interesting. BTW, I knew that Everett's original views are sometimes distinguished from DeWitt's et al interpretation of them. Anyway, the IEP article on 'Everettian interpretations' lists also some Relational interpretations. So, maybe Everett was really a 'relationalist' :wink:

That works given a postulate of such selection going on. My statement was an opinion, not an assertion. — noAxioms

I fully agree.

It seems that some objective collapse interpretations might fit the bill:

On the other hand, it is shown that dynamical collapse models, of the type originally proposed by Ghirardi-Rimini-Weber, can be re-interpreted as set selection criteria within a quantum histories framework, in which context they appear as candidate solutions to the set selection problem.
- Quantum Histories - Adrian Kent — Andrew M

Thanks, interesting. It makes sense.

Schrodinger himself took a 'wave-only' view during his life (he also took a 'non-representational' view for some years). I wonder if he did endorse that view (at least for some years). Maybe, however, he was more close to the 'usual' GRW-like approach.

Even eighteenth-century dictionaries still preserved echoes of this medieval usage, which rings so bizarrely in modern ears: “Hence a thing is said to exist OBJECTIVELY, objective, when it exists no otherwise than in being known; or in being an Object of the Mind.”" (Daston and Galison, Objectivity). — StreetlightX

I think we still use it that way. The 3rd person, objective voice is the world's voice. On analysis, we note that this has to be a construction of the mind.

The subjective view is not supposed to be mental. It's sensual. It has a mental or abstract component, but that's not obvious at first glance.

Putting this together with your earlier comment that you are not attached to locality, it sounds like you have an affinity to the 'non-local hidden variables' school, of which David Bohm's 'pilot wave' interpretation of QM is perhaps the best-known. In most other popular interpretations, the imprecision about location is not just epistemological. — andrewk

I believe that the de Broglie-Bohm (dBB) theory should be given more attention. I am not a dBB-supporter but I believe that it is a valid alternative. Interestingly, there are different views about the ontology of dBB. Some like Bohm himself in his original work consider the contribution of the wave-function to the motion of the particle in a similar way to an additional force, given by the Quantum Potential (the link is to Wikipedia article on it).
To my knowledge, most dBB-supporters however do not like this formulation and prefer a first-order formulation (i.e. without second order temporal derivatives, i.e. without accelerations and forces) - The SEP article on 'Bohmian mechanics' has a section that explains why the 'quantum potential' formulation is criticized. But even here there is no consensus about the ontology. Some take the wave-function as a physically real field. Others do not and prefer a 'nomological' approach (but even here in the 'nomological' camp there are different views: check this article https://arxiv.org/pdf/1406.1371.pdf. I commented on it and quoted an excerpt in my previous post).
A somewhat related discussion is found in this thread in physics forums: https://www.physicsforums.com/threads/the-reality-of-configuration-space.554543/ (note that it is not only about dBB, strictly speaking...)

As I said elsewhere, however, I find dBB somewhat 'bizzarre' in its ontology. I am inclined to believe that this shows that some radical 'paradigm change' is necessary. But I nevertheless believe that it is a very interesting theory.

I like Bohm. I have his book 'Quantum Theory' which is interesting because it was written before the modern Dirac notation for QM with 'bras' and 'kets' became standard. — andrewk

I like him too very much. He really had fascinating ideas during all his career. He really wanted to understand things in depth.

I unfortunately do not have this book (but I believe reviews are generally positive).

FWIW Bohm was quite a mystic, and had a famous series of public discussions with Krishnamurti about physics and spirituality. — andrewk

Yeah, that's correct. Unfortunately, this is a reason why people are averse of his (especially later) work. Personally, I disagree with them. Even if one is uninterested in spirituality, his later ideas are IMO intriguing.

What does the article mean by "created alternate realities"? I wasn't aware that humans had achieved the ability to create realities. What makes a reality "alternate"?

If there is no objective reality, then what is it that separates observers into individual units in order to have subjective (differing) experiences of the same thing?

Wouldn't the "same thing" be the object (objective reality) and our differing experiences of the "same thing" would be the outcome of our different positions in space-time and life-history (subjective, or unique representations of the "same thing"). I don't see how QM undermines objective reality if it admits that there are objects, or events, that remain the same independent of our observations. It is our observations that are different, not the thing. The fact that two different observers can experience the same thing differently isn't a new idea, or surprising. It is what is expected in an objective world with objects or things, like observers that are an amalgam of their life-history and location in space-time. The frequency at which our brains process information could be different (due to drugs, lack of sleep, age, etc.), which can give rise to different experiences of the same thing.

It's just ironic to see people claim that they don't believe in an objective reality, yet they refer to theories and use language to communicate - as if everyone should experience the words as they experience them and understand what they write and a theory the way they understand it - as if theories and words can remain the same independent of some observers reading them and everyone that reads this theory or article will read the same words that they read and that the author wrote.

The physical properties of a photon are able to be measured in the same way as for any other particle. If you want to know a photon's position or speed, you set up an experiment and measure it. — Andrew M

What is measured is the effect of the photon. That's the difference between a machine observing and a human being observing, the standards of measurement are being applied to changes in the machine (effects of light energy), they are not being applied to the photon itself. Let's go back to my analogy of measuring water in the dish, as it evaporates in the sun. What is measured is the effect of the sunlight, not the sunlight itself. The point is that there is interpretive theory and logic which lies between the effects on the equipment, and the so-called "physical properties" of the photon. The so-called measurement of the photon is dependent on causal theory. But the equipment has a very focused observational capacity, and can't detect other factors, so it's just like measuring the water in the dish, and figuring that the loss of water is all caused by evaporation.

It occurs to me that the notion of QM undermining the notion of 'objective reality' only makes sense if one insists that only particles, not waves, can be objective. — andrewk

The problem with this is that a wave without a medium doesn't make sense. If we had a medium for that wave, then we could study the properties of the medium, the waves in the medium, and the whole field (pardon the pun) would be opened up to us. But without the medium the waves don't have objective existence, and the wave-function is just mathematics which predicts the probability of so-called "particles". So by the structure of the mathematical applications, the "particles" are what have real objective existence, and the wave-function uses probabilities to predict the existence of the particles. To switch objective reality, assigning it to the waves instead, would require principles for the existence of the waves, and developing a wave based mathematics, instead of a particle based mathematics.

This is why special relativity may not be the best theory here. It leads us astray by denying the possibility of a medium, when the empirical evidence indicates that the waves are real. So special relativity confines the activity of real physical existence to within the boundary of light, by denying that the waves have real physical existence. Light waves cannot be real. But then the activity of light cannot be understood with the normal descriptive terms that we use to describe physical activity because light activity needs to be described as waves, and a wave without a medium is nonsense. Now we can only understand the activity of light by means of how it affects physical things, and the wave-function is only grounded in this way, the effects of light on things, not by any real waves in a medium. So we do not have the principles to say that the wave-function itself represents real waves.

Thanks for your replies. Also for the example of the tuning fork. I understood the hunt to be for quarks which my comment was aimed at. Anyhoo, my subconscious preference for Bohm is no surprise. About 10 years ago I worked at ESA and Bohmian mechanics was popular with a lot of the younger physicists that raised a lot of discussions because it certainly wasn't the prevailing theory. As a lawyer I only understood half of it but something apparently stuck.

Hmmm. Let me think.

If different observers give different accounts of the same sequence of events, then each quantum mechanical description has to be understood as relative to a particular observer. Thus, a quantum mechanical description of a certain system (state and/or values of physical quantities) cannot be taken as an “absolute” (observer independent) description of reality, but rather as a formalization, or codification, of properties of a system relative to a given observer. Quantum mechanics can therefore be viewed as a theory about the states of systems and values of physical quantities relative to other systems. — StreetlightX

This can't be about observing several quantum events right because the observation would cause a collapse resulting in only one outcome? And since we can't do simultaneous measurements...? To me Wigner and his friend don't have conflicting views but different levels of knowledge. So what sort of sequence of events are we talking about?

I get the point about it being a relational thing and it would be inappropriate to not include the observer in a description of a quantum measurement.

The physical properties of a photon are able to be measured in the same way as for any other particle. If you want to know a photon's position or speed, you set up an experiment and measure it.
— Andrew M

What is measured is the effect of the photon. — Metaphysician Undercover

If so, then it would seem that the same principle should apply to an electron. One would be measuring the effect of the electron (on a measurement device), not a property of the electron itself.

Are you singling out the measurement of photons as unique here or claiming a general principle for the measurement of all particles and, by extension, all physical objects?

It's worth noting that superpositions have been created for objects with up to trillions of atoms (as in the case of the piezoelectric "tuning fork"). Probably most physicists would consider QM to be a universal physical theory (i.e., applicable to everything). Which is part of the point of the Schrodinger's Cat and Wigner's Friend thought experiments. — Andrew M

So you see it like - everything is a quantum system, just sometimes the corrections from quantum mechanics to macroscopic systems are sometimes negligible? Not that there is a particularly strong divide between macroscopic and quantum.

a wave without a medium doesn't make sense. — Metaphysician Undercover

That supposition was rejected more than a century ago given the results of the Michelson-Morley experiment. There is no medium in the model for electromagnetic waves.

So you see it like - everything is a quantum system, just sometimes the corrections from quantum mechanics to macroscopic systems are sometimes negligible? — fdrake

Yes. The Schrödinger equation doesn't make a micro/macro distinction or specify a collapse of the wave function. Which is what gives rise to the measurement problem.

[

If so, then it would seem that the same principle should apply to an electron. One would be measuring the effect of the electron (on a measurement device), not a property of the electron itself.

Are you singling out the measurement of photons as unique here or claiming a general principle for the measurement of all particles and, by extension, all physical objects? — Andrew M

Yes, I think that is the case, electrons are measured as effects, and most forms of measurement are like this. But there is varying degrees of soundness in the theories involved. So for example, I think that the theory which makes an electron as part (property) of a molecule, and part of an atom, is quite sound. But a theory which has free, independent electrons is not as sound.

That supposition was rejected more than a century ago given the results of the Michelson-Morley experiment. There is no medium in the model for electromagnetic waves. — andrewk

I know, that's the point. The physics of a "wave" is such that a wave can only exist as a vibration in a substance. That's what a wave is. Since there is no such substance with electromagnetic activity, we cannot refer to this as "waves". So it is incorrect to assign objectivity to electromagnetic waves, because they are not waves. If people want to insist on the reality of these waves, then someone needs to do some more serious experimentation to determine the substance which they exist in. It's nonsense to say that there are real waves which do not exist in a substance. A wave can only propagate in a substance.

That's what a wave is — Metaphysician Undercover

Not in physics. In physics a wave is a phenomenon that behaves in accordance with the wave equation.

In physics a wave is a phenomenon — andrewk

Ah, but is it. A phenomenon is 'what appears'. And what appears are not waves, but patterns that look like waves. The pattern is predicted by an equation that describes a wave. But I don't know if the wave can be considered amongst phenomena. The Wheeler article that was discussed earlier in this thread says that 'no elementary phenomena is a phenomena until it is an observed phenomena'.

Fair enough. But we'd need to go on to acknowledge that even with ordinary old water waves and sound waves, the phenomenon is not the wave but the experiences it gives us - such as the sensation of our up and down movement in the water at the beach, or the sounds we hear, or the rippled patterns we see on a water surface.

I'll correct my statement to say that in physics a wave is a model that is used to predict phenomena, and the model does not require the assumption of any medium.

Yes, I think that is the case, electrons are measured as effects, and most forms of measurement are like this. — Metaphysician Undercover

I don't think physics provides any reason to doubt that the elementary particles (as described in the Standard Model) exist and have measurable physical properties just as everyday macroscopic objects do. Whatever issues QM raises apply to particles and macroscopic objects alike.

Bit of a sleight-of-hand there, really. There is the Berkeleyian point that ‘all we know are perceptions’, but that is not quite the point at issue. The big mystery with the wave function is precisely its ontological status, whereas the same can’t be said for water or sound waves, as they propagate through a medium; they are indeed ‘phenomena’.

(Have a glance at this thread I opened on Physics Forum about this point.)

I don't think physics provides any reason to doubt that the elementary particles (as described in the Standard Model) exist — Andrew M

Isn’t the ontological status of fundamental particles also precisely what is at issue in all this? The ‘Copenhagen interpretation’ does, after alll, say that ‘the particle doesn’t exist until it’s measured.’ I had the idea that this was the very conundrum that the ‘relative state formation’ was posed to avoid.

The big mystery with the wave function is precisely its ontological status, whereas the same can’t be said for water or sound waves, as they propagate through a medium; they are indeed ‘phenomena’. — Wayfarer

To complicate matters, the 'wave function' is, I believe, a misnomer. It's not a 'wave' in the way waves are understood either in physics or in everyday discussion. It is an element of a Hilbert space, and there's really no more user-friendly way to describe it than that. It has nothing to do with electromagnetic waves, gravity waves, sound waves or any other sort of wave, and it's not a solution to the wave equation.

Isn’t the ontological status of fundamental particles also precisely what is at issue in all this? The ‘Copenhagen interpretation’ does, after alll, say that ‘the particle doesn’t exist until it’s measured.’ — Wayfarer

That's one framing of the issue, sure. But, more impartially, what is at issue is the solution to the measurement problem. That is, what does it mean for a physical system to be in a superposition of states given that it will always be measured in a definite state?