The quantum state cannot be interpreted statistically?

[Fredrik]

Might it mean Einstein's original 2 views about the nature of the wave function? See p. 194-195 with direct Einstein quotes, in particular. All of Chapter 7 is pretty interesting. Maybe that is why this theory if accurate rules out Einstein's arguments ? I'm not sure.

http://www.tcm.phy.cam.ac.uk/~mdt26/local_papers/valentini.pdf

The question was rhetorical. I meant that PBR couldn't have meant anything else.

If the quote at the start of page 195 is what defines the two views mentioned on page 194, then I would say that this can't be the two views that PBR are comparing, not only because I have already made up my mind about what they are comparing, but also because Einstein's "view I" clearly contradicts QM (and experiments). So there's no need to compare those two views now.

 

[DevilsAvocado]

I haven’t read more than to #141, sorry if something is already dealt with.

Now ask yourself if temperature is a classical epistemic or ontic variable. Though it is the product of presumably ontic entities it is a variable that is not dependent on the state of any particular ontic entity nor singular state of those entities as a whole. It is an epistemic state variable, in spite of having a very real existence. In this sense I would say it qualifies as "epistemic ontic", since it is an epistemic variable in which it's existence is contingent upon on underlying ontic group state. Momentum is another epistemic variable, since the self referential momentum of any ontic entity (lacking internal dynamics) is precisely zero. That's the whole motivation behind relativity.

Thanks my_wan, interesting posts.

I enjoy the discussion, but sometimes I wonder if we are getting 'stuck in words'... I think Ken G’s comment is a quite striking (and entertaining):

The issue isn't local vs. nonlocal, it is in the whole idea of what a hidden variables theory is. It's an oxymoron-- if the variables are hidden, it's not a theory, and if they aren't hidden, well, then they aren't hidden! The whole language is basically a kind of pretense that the theory is trying to be something different from what it actually is.

If the variables are hidden, it's not a theory... Well, that’s it guys – problems solved!

From my perspective, the discussion what "hidden variables" are, and what properties they might posses, and how they commute these properties, is interesting but maybe 'premature', because I could claim that "hidden variables" are "Little Green Men with Flashlights" representing on/off, |0⟩ or |1⟩, and it would be quite hard to prove me wrong...

Therefore, this is clearly a question on realism. Is there "something" there when no one is watching?

Now, the PBR theorem has clearly a strong connection to the standard Bell framework, and therefore we cannot talk about realism without the other strongly related concept locality (despite Ken G’s 'aversion').

If we take your picture of the "temperature model" and implement it in an EPR-Bell context:

[PLAIN]http://upload.wikimedia.org/wikipedia/commons/6/6d/Translational_motion.gif[B]<---->[/B][PLAIN]http://upload.wikimedia.org/wikipedia/commons/6/6d/Translational_motion.gif

Now is this going to make our day?

I don’t think so... Even if we one day do find those "temperature particles" that 'triggers' the measured value – they can never be 'classical ontic particles', because if they are *real* they must also be *non-local*.

And this is something we can prove already today, it doesn’t matter which 'camp' you belong to. Very soon it will be an empirical fact that will never change, no matter what fancy theories comes along in the future; realism is 'doomed' to be non-local.

I think it interesting to examine the outcome of the PBR theorem:

[Pulled from Matt Leifer's blog]

epistemic state = state of knowledge
ontic state = state of reality

1. ψ-epistemic: Wavefunctions are epistemic and there is some underlying ontic state.
   
   
2. ψ-epistemic: Wavefunctions are epistemic, but there is no deeper underlying reality.
   
   
3. ψ-ontic: Wavefunctions are ontic.

Conclusions
The PBR theorem rules out psi-epistemic models within the standard Bell framework for ontological models. The remaining options are to adopt psi-ontology, remain psi-epistemic and abandon realism, or remain psi-epistemic and abandon the Bell framework. [...]​

And see what it means if we adopt it to the Bell framework:

1. ψ-epistemic: Obsolete, does not work anymore.
   
   
2. ψ-epistemic: local non-realism* / non-local non-realism.
   
   
3. ψ-ontic: non-local realism.

This is what we have to play with, and as said, it doesn’t matter if we are talking "Little Green Men" or something else.

Now, does it matter? Is it a breakthrough?

Well, I’m not the man to judge this... The 'feeling' I have is that non-realism feels very 'strange', and if I could choose, I go for non-locality instead...

*non-realism aka non-separable


P.S. I have to leave now, get back tomorrow.

 

[bohm2]

And see what it means if we adopt it to the Bell framework:

1. ψ-epistemic: Obsolete, does not work anymore.
   
   
2. ψ-epistemic: local non-realism* / non-local non-realism.
   
   
3. ψ-ontic: non-local realism.

I still don’t understand that local vs non-local non-realism. According to the anti-realist position, there should be no issue as to the locality/non-locality because there is no quantum world for quantum mechanics to localy or non-localy describe. This makes no sense to me? I'm thinking here Bohr's thoughts that "there is no quantum world".

but also because Einstein's "view I" clearly contradicts QM (and experiments). So there's no need to compare those two views now.

I didn’t think that Bell-inspired derivation attacked that part of Einstein’s arguments. Isn’t that what PBR is supposed to do? Bell’s establishes no local hidden variable theory can agree with QM's predictions but doesn’t address the arguments put forth by Einstein in 1927 that QM itself cannot be both complete and local. Isn't that the whole meaning of this quote by Matt Leifer:

Perhaps the best known contemporary advocate of option 1 is Rob Spekkens, but I also include myself and Terry Rudolph (one of the authors of the paper) in this camp. Rob gives a fairly convincing argument that option 1 characterizes Einstein’s views in this paper, which also gives a lot of technical background on the distinction between options 1 and 2.

Hence since PBR shows that we can distinguish with certainty (complete), QM must be non-local? Well, unless, you're a anti-realist. Then it doesn't matter, I guess.

 

[Fredrik]

I didn’t think that Bell-inspired derivation attacked that part of Einstein’s arguments. Isn’t that what PBR is supposed to do?

I don't understand the question. Isn't what what PBR is supposed to do? Why would we want to attack Einstein's arguments, and what part are you talking about?

Bell’s establishes no local hidden variable theory but doesn’t address the arguments put forth by Einstein in 1927 that QM itself cannot be both complete and local.

I don't understand what you're saying. What do you mean by "address the arguments"? Do you mean prove them wrong?

Isn't that the whole meaning of this quote by Matt Leifer:
...
Hence since PBR shows that we can distinguish with certainty (complete), QM must be non-local?

The article he refers to (section 4, starting on p. 10...read at least until eq. (28)), says that Einstein's 1927 argument shows that an ontological model for QM can't be both (ψ-)complete and local. So we don't need PBR for that. PBR argue against ψ-epistemic ontological models.

 

[Fredrik]

My previous attempt to explain the argument wasn't quite successful, so let's try again.

Suppose that there's a ψ-epistemic ontological model for the quantum theory of a single qubit. (The terminology is defined in HS. See also ML). Denote the set of ontic states of that model by \Lambda. Then \Lambda\times\Lambda is the set of ontic states in an ontological model for the two-qubit quantum theory.

I'm going to simplify the presentation of the argument by pretending that \Lambda has a finite number of members. (I want to avoid technical details about probability measures). Denote that number by n, and denote the members of \Lambda by \lambda_1,\dots,\lambda_n.

Let \mathcal H be the Hilbert space of the quantum theory of a single qubit. Then \mathcal H\otimes\mathcal H is the Hilbert space of the quantum theory of two qubits. Let \{|0\rangle,|1\rangle\} be an orthonormal basis for \mathcal H. Define
$$\begin{align} |+\rangle &=\frac{1}{\sqrt{2}} \left(|0\rangle+|1\rangle\right)\\ |-\rangle &=\frac{1}{\sqrt{2}} \left(|0\rangle-|1\rangle\right). \end{align}$$ \{|+\rangle,|-\rangle\} is another orthonormal basis for \mathcal H.

For each |\psi\rangle\in\mathcal H and each \lambda\in\Lambda, let Q_\psi(\lambda) denote the probability that the qubit's ontic state is \lambda. The function Q_\psi:\Lambda\to[0,1] is called the epistemic state corresponding to |\psi\rangle. Similarly, for each |\psi\rangle\otimes|\psi&#039;\rangle\in\mathcal H\otimes\mathcal H and each (\lambda,\lambda&#039;)\in\Lambda\times\Lambda, let Q_{\psi\psi&#039;}(\lambda,\lambda&#039;) denote the probability that the two-qubit system is in ontic state (\lambda,\lambda&#039;). We assume that
$$Q_{\psi\psi'}(\lambda,\lambda') =Q_\psi(\lambda)Q_{\psi'}(\lambda')$$ for all values of the relevant variables.

Let X be a self-adjoint operator on \mathcal H\otimes\mathcal H with the eigenvectors
$$\begin{align} |\xi_1\rangle &=\frac{1}{\sqrt{2}} \left(|0\rangle\otimes|1\rangle +|1\rangle\otimes|0\rangle\right)\\ |\xi_2\rangle &=\frac{1}{\sqrt{2}} \left(|0\rangle\otimes|-\rangle +|1\rangle\otimes|+\rangle\right)\\ |\xi_3\rangle &=\frac{1}{\sqrt{2}} \left(|+\rangle\otimes|1\rangle +|-\rangle\otimes|0\rangle\right)\\ |\xi_4\rangle &=\frac{1}{\sqrt{2}} \left(|+\rangle\otimes|-\rangle +|-\rangle\otimes|+\rangle\right) \end{align}$$ Note that each of the state vectors
$$\begin{align} &|0\rangle\otimes|0\rangle\\ &|0\rangle\otimes|+\rangle\\ &|+\rangle\otimes|0\rangle\\ &|+\rangle\otimes|+\rangle \end{align}$$ is orthogonal to exactly one of the |\xi_k\rangle.

The result of an X measurement that corresponds to eigenvector |\xi_k\rangle will be denoted by k. For all k and all \psi,\psi&#039;\in\mathcal H, let P_{\psi\psi&#039;}(k|X) denote the probability assigned by the ontological model for the two-qubit quantum theory to measurement result k, given that we're measuring X, and that the epistemic state of the two-qubit system is Q_{\psi\psi&#039;}. For each k and each \lambda,\lambda&#039;\in\Lambda, let P(k|\lambda,\lambda&#039;,X) denote the probability assigned by the ontological model for the two-qubit quantum theory to the result k, given that we're measuring X and that the ontic state of the two-qubit system is (\lambda,\lambda&#039;).

Now let \lambda be an ontic state of a single qubit that's assigned a non-zero probability by both Q_0 and Q_+. Define q=\min\{Q_0(\lambda),Q_+(\lambda)\}. Since |0\rangle\otimes|0\rangle is orthogonal to |\xi_1\rangle, we have
$$0=\left|\langle\xi_1| \left(|0\rangle\otimes| 0\rangle\right)\right|^2 =P_{00}(1|X)=\sum_{i=1}^n \sum_{j=1}^n Q_{00}(\lambda_i,\lambda_j) P(k|\lambda_i,\lambda_j,X)$$ Since every term is non-negative, this implies that all terms are 0. In particular, the term with \lambda_i=\lambda_j=\lambda is 0.
$$0=Q_{00}(\lambda,\lambda)P(1|\lambda,\lambda,X)$$
Since Q_{00}(\lambda,\lambda) =Q_{0}(\lambda)Q_{0}(\lambda)\geq q^2&gt;0, this implies that P(1|\lambda,\lambda,X)=0.

A very similar argument based on the fact that |0\rangle\otimes|+\rangle is orthogonal to |\xi_2\rangle implies that P(2|\lambda,\lambda,X)=0. A similar argument works for all four values of k, so we can prove that P(k|\lambda,\lambda,X)=0 for all k\in\{1,2,3,4\}. This implies that \sum_{k=1}^4 P(k|\lambda,\lambda,X)=0\neq 1. This implies that at least one of the assumptions that told us that we were dealing with a ψ-epistemic ontological model for the two-qubit quantum theory must be false.

 

[my_wan]

Definitions of ψ-ontic and ψ-epistemic from the HS article.

• ψ-ontic - Every complete physical or ontic state in the theory is consistent with only one pure quantum state.
• ψ-epistemic - There exist ontic states that are consistent with more than one pure quantum state.

Now since what we are dealing with experimentally is a supposed complete description, known or not, we call the complete description λ. So the two definitions correspond to:
Ontic if theory λ uniquely determines an outcome.
Epistemic if theory λ allows for multiple outcomes.

Now let's forget QM and ψ and simply see what kind of trouble we can get in. We have a theory λ of dice roll. It states that the probability of rolling any given number is 1 in 6. But any of those 6 outcomes is consistent with λ. This entails that our theory λ is epistemic in nature. Now we take a large number of n dice and dump them. Our epistemic theory λ now tells us that the number rolled is 3.5(n). Given some margin of error we see that the number rolled is indeed consistently 3.5(n). Since our theory λ now uniquely specifies the resulting state has our epistemic theory λ now been proven to be an ontic theory?

This exact same situation entails the same thing about classical thermodynamics, statistical mechanics and the associated state variables such as pressure, temperature, etc. The certainty with with we can uniquely determine a state variable tells us nothing about the nature of the variables used to arrive at that unique value. It can be said that Brownian motion has proved that classical thermodynamics is an ontic construct. Yet if QM is held to be a purely epistemic construct, no deeper underlying reality, and the classical ontic entities are a product of QM then isn't our classical ontic entities actually purely epistemic entities?

The fact is that if we partition a set of epistemic variables they can be treated for all intent and purposes as if ontic entities. Thus proving that some variable associated with some observable posses characteristics associated with ontic variables says nothing about the character of their constituents. The classical world is in effect a partitioned set of QM properties. We generally only see leaks in this partitioning at a very fine scale.

 

[Ken G]

My previous attempt to explain the argument wasn't quite successful, so let's try again.

OK, thanks for that distillation of their argument. It seems to me that the crux of it is that if there really is an ontological theory that underlies QM (in the sense that the ontological theory explains every prediction QM makes), and the ontological theory is the complete description of the situation (it involves the true properties of the system), then the state vectors of quantum mechanics must have a certain relationship with that complete ontological theory: they must be a subset of the same ontics. By that I mean, overlapping state vectors always imply overlapping properties, and complete overlap of the properties requires the same state vector (modulo the usual isometries that go into the state vector concept). That means that complete knowledge of the ontics of the ontological theory suffices to uniquely determine the state vector, I don't need to know anyone's knowledge of the system before I can say what they think the QM state vector will be. Another way to say this is "individual systems really have unique state vectors if a complete ontological theory underlies QM."

We might then say that "QM is a subset of the ontics of the ontological theory", and "QM must itself be an ontological theory." Here by "an ontological theory" I mean a theory that bears this relationship with the "true theory of the actual properties", that the properties determine the state (though not necessarily the other way around because that would require that QM itself be the true and complete ontological description).

Which brings be back to my initial objection-- the assumption that there exists such a true and exact ontological theory underlying quantum mechanics, the assumption that there are "properties." I'm really not surprised that if ontological properties exist, and if QM makes true predictions, then QM connects directly to those properties. What bothers me is the PBR attitude that "realism" is a "complete commitment to a belief in properties." To me, properties are clearly mental constructs of our theories, that get relaxed or become more sophisticated in some other theory, like how exact position is a construct of classical mechanics that is relaxed in quantum mechanics. Since when did being a "realist" depend on denying the demonstrably true character of every physics theory we've ever had? I think having the "existence of properties" as an assumption behind a proof casts the applicability of any such proof into serious doubt.

Instead, I would like to offer a different definition of "realism". We start from the stance that everything we can say about nature is going to be a mental construct that is not an actual truth of nature, but rather, is an effective or useful truth, involving the way we have chosen to characterize nature. Hence, a "property" is an "element of a theory", and does nothing to separate "ontic" theories from "epistemic" theories. Indeed, theories aren't either ontic or epistemic, they are just theories. What is ontic or epistemic is our philosophical choices about how we talk about a theory, and these choices are not testable, because the same theory can be either. I think the various interpretations of QM make that clear. Now, PBR says that we can't interpret QM as both realist and epistemic, but that's only because they already adopt too narrow of an interpretation of what a "realist" theory is that it leaves no room for epistemology. It was their philosophical choice to do that, it doesn't really tell us much of importance about quantum mechanics if we simply reject that choice.

 

[Ken G]

The fact is that if we partition a set of epistemic variables they can be treated for all intent and purposes as if ontic entities.

Yes, this is what I have been saying also-- the key is in what is meant by "all intents and purposes." To PBR, the only intents and purposes they have in mind must be consistent with the idea that properties provide a complete physical description, such that everything that happens can be traced directly back to the properties. But QM has no step where you convert a preparation into a set of properties, you only connect the preparation to the outcomes, and along the way you might typically embrace the concept of properties (like quantum numbers) but you never need to attach any mechanistic connection between the properties and the outcomes. Any such connection amounts to belief in magic, in effect-- like those who believed that gravity was a force that appears (magically) due to the presence of a mass, or a curvature of spacetime that appears (magically) because of stress energy properties. PBR says that to be a realist, one must introduce this intermediate and unnecessary step of, in effect, believing in magic, but I say, being a realist means treating a physical theory like a physical theory. It is a realist attitude to treat ontological descriptions as a kind of intentional fantasy that we enter into because it is parsimonious to do so. And because that's exactly what we do, that is the realist stance-- ontology is epistemology. I say that to be a realist (not a naive realist), one merely needs to hold that there "actually is" a universe, but everything that we can say about that universe is epistemology, including the ontological claims we make on it for the purposes of advancing our conceptual understanding. I believe this is also what Bohr meant when he said that physics is not about nature, it is about what we can say about nature.

 

[my_wan]

Even if you move away characterizations of variables in terms of ontic, epistemic, and contextual, and move to actual physical constructs, the problem of such definitions do not go away. This can be illustrated by asking if a hurricane should be characterized as an ontic, epistemic, and contextual object. Its presumably ontic constituents are defined by the molecules and their behavior or phase space. But consider those properties.

1) No individual molecules have any property that is at all distinct from properties present in any other circumstance with no hurricane or even wind present.

2) If you attempt to define a distinct point like position moment of the hurricane there may in fact be no molecules, or anything else, at that position.

3) If you attempt to define the boundaries, within which the hurricane resides at a given moment, no such distinct boundaries exist.

In terms of constituent properties the hurricane, for all intent and purposes, does not "exist". Its existence is solely dependent on the contextual relations between the constituent properties of its parts, and not the constituent properties themselves. We can of course call these contextual properties a distinct higher order property. We can call this a purely contextual construct or entity, in which the ontic elements defining it may or may not be ontic. The constituent elements, presumed ontic, may or may not be contextual entities in themselves.

So can we delineate between ontic and epistemic or contextual variables simply on the properties they posses? Well our hurricane has a location, trajectory, and leaves a very distinct path of destruction in its wake. All the hallmarks of an ontic entity. Yet we cannot say when, where, or if ever, a reduction of parts will ever lead us to variables that represent actual ontic entities. Our knowledge is restricted to an epistemic regime. We can say that under circumstances in which epistemic variables can be partitioned, to some degree or the other, that we can treat those partitioned properties as if they were ontic.

The problem many realist have is not with contextual or epistemic variables in general, but the end game question: Is it really turtles all the way down? Of course the non-realist will say we have already hit bottom, and their is no deeper reality.

Now, wrt the PBR article, I think it makes some interesting points. Yet trying to delineate between ontic or epistemic characterizations based on possessing characteristics of partitioned properties shouldn't get very far. Our hurricane is, more or less, partitioned in space also.

Right now I'm going to eat some epistemic turkey.

 

[qsa]

all these epistemic-ontic arguments are well and good. but do they shed any light on the real problematic issue of wave-particle duality.

 

[Ken G]

The problem many realist have is not with contextual or epistemic variables in general, but the end game question: Is it really turtles all the way down? Of course the non-realist will say we have already hit bottom, and their is no deeper reality.

Exactly, and I'd like to offer the third choice: a realist is exactly the person who recognizes that the concept of "reality" as a whole is an effective notion, just like the way you described the reality of a hurricane. After all, is it not "realistic" to expect all of reality to have the same character as the elements we talk about as making up that reality?

Now, wrt the PBR article, I think it makes some interesting points. Yet trying to delineate between ontic or epistemic characterizations based on possessing characteristics of partitioned properties shouldn't get very far.

I completely agree.

Right now I'm going to eat some epistemic turkey.

Yes, and indeed, that jest alludes to a serious point often made by those who hold to a more naive version of realism-- that only the realist can account for why a rock hurts when it falls on your toe, and only a fool would deny the ontology of that rock. But that's just naivete talking-- logically, it is perfectly possible for me to form an ontological construct in my head that is consistent with my experiences, like rocks and pain and hurricanes, without that construct in my head having "true properties." There's just no connection there. I say the realist is the person who does not believe in magic, who does not believe that the rock has some innate propeties that "caused" it to hurt my toe. How does a rock have innate properties, and become such an action hero, anyway?

 

[Fredrik]

I believe I have found a flaw in the paper.

In short, they try to show that there is no lambda satisfying certain properties. The problem is that the CRUCIAL property they assume is not even stated as being one of the properties, probably because they thought that property was "obvious". And that "obvious" property is today known as non-contextuality.
...
They first talk about ONE system and try to prove that there is no adequate lambda for such a system. But to prove that, they actually consider the case of TWO such systems. Initially this is not a problem because initially the two systems are independent (see Fig. 1). But at the measurement, the two systems are brought together (Fig. 1), so the assumption of independence is no longer justified.

Can you explain where contextuality enters the picture in my version of their argument? (Post #155). I'm not saying that you're wrong. I just barely know what contextuality means, and I haven't really thought about whether you're right or wrong.

 

[Fredrik]

So the two definitions correspond to:
Ontic if theory λ uniquely determines an outcome.
Epistemic if theory λ allows for multiple outcomes.

These are reasonable definitions IMO, but they're not consistent with the ones used by HS. An ontological model for QM assigns a probability P(k|λ,M) to the result k, given an ontic state λ and a measurement procedure M. This probability isn't required to be 0 or 1. An ontological model for QM is ψ-ontic if an ontic state uniquely determines the state vector. Since a state vector doesn't uniquely determine an outcome, there's no reason to think that a λ from a ψ-ontic ontological model for QM determines a unique outcome.

 

[DevilsAvocado]

... Instead, I would like to offer a different definition of "realism". We start from the stance that everything we can say about nature is going to be a mental construct that is not an actual truth of nature, but rather, is an effective or useful truth, involving the way we have chosen to characterize nature. Hence, a "property" is an "element of a theory", and does nothing to separate "ontic" theories from "epistemic" theories. Indeed, theories aren't either ontic or epistemic, they are just theories.

Bewildering gibberish...

This could be quite confusing for the 'casual reader', since you are making up your own rules for what is what. Realism is never associated to scientific theories, in the meaning of truth, this is just nonsense, and I have no idea why you are making theses associations. It should be well known that any physical theory is always provisional, in the sense that it is only a hypothesis; you can never prove it.

Another very important factor of scientific theories is that they must be refutable, i.e. you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.

As anyone can see, it would be ridiculous to claim that realism, to be true, must be refutable and provisional. If (non-local) realism one day is found to be true, i.e. an empirical fact, then one of its "main features" is that it is not refutable and not provisional; i.e. it must be true forever, to qualify for an empirical fact!

What is ontic or epistemic is our philosophical choices about how we talk about a theory, and these choices are not testable,

More gibberish...

Could you please explain how we could ever test and validate our theories? If "these choices are not testable"??

It seems like you talk more about to philosophical realism in metaphysics, than realism in physics.

Local Realism as defined by physicists:

There is a world of pre-existing particles (objects) in the microscopic world, having pre-existing values for any possible measurement before the measurement is made (=realism), and these real particles is influenced directly only by its immediate surroundings, at speed ≤ c (=locality).​

As we can see, this definition of local realism will also make sense to "Joe the Plumber", if explained.

Now, is the "Joe the Plumber Realism” testable??

Of course it is!

Local Realism is tested and proven false by 99%, and all that remains is the Grand Funeral!

To say that we cannot test our theories, and via them, find out what is true or not true in nature, is just false.


Denial of facts and twisting of terms is something that never has appealed to me...

 

[Ken G]

An ontological model for QM assigns a probability P(k|λ,M) to the result k, given an ontic state λ and a measurement procedure M. This probability isn't required to be 0 or 1.

And that raises another important ontological issue: is a probability an ontic notion, or is it always fundamentally epistemic? In other words, is there "any such thing" as the probability of an outcome? If you deal me a card from a deck, is "the probability" 1/52 of the ace of hearts, or are all probabilities necessarily contingent on our information about that deck? This connects to my objection to the concept of a "property." I would argue that decks do not have properties that determine these probabilities-- what determines every probability that anyone ever used in connection with a deck of cards was their knowledge about that deck of cards, and no probability is ever worth anything more than that knowledge. I'm not just questioning the definition, because a definition is just a definition-- I'm questioning the ramifications of the definition, i.e., what can be assumed to come along with the definition. We can certainly define "ontic" to mean something that maps from a concept of a property to a concept of a probability, but both ends of that map are still concepts. So we cannot take that definition and say that an ontic model actually supports an underlying truth in which there are properties that determine probabilities. In other words, whether we have an ontic model or an epistemic model, however we define those terms, the only thing that ever determines a probability in any physics theory is always the knowledge of the physicist. This is so demonstrably true, that I marvel at what ends up getting called "realist."

 

[Ken G]

Bewildering gibberish...

I'm afraid you are falling into logical fallacy again. Here's the problem. You claim what I just said is gibberish. That means you didn't understand it (which is true, you didn't). Unfortunately, since you did not understand it, this means no one should pay any attention to your judgement of it (which they shouldn't).

Another very important factor of scientific theories is that they must be refutable, i.e. you can disprove a theory by finding even a single observation that disagrees with the predictions of the theory.

Hmm, that's certainly true, now what on Earth does that have to do with anything I said? I just can't pass judgement on the argument you are presenting, because I don't understand it at all. All that is clear to me is that you took not a single word of my intention correctly.

 

[Fredrik]

is a probability an ontic notion, or is it always fundamentally epistemic?

I don't know what this means. I understand the distinction between ψ-ontic and ψ-epistemic ontological models for QM, but you seem to be taking the terms "ontic" and "epistemic" outside of the framework of ontological models for QM. I'm not sure there's a meaningful distinction between the terms "ontic" and "epistemic" outside of that framework, but maybe that was your point.

By the way, something that assigns a probability P(k|λ,M) to the result k, given an ontic state λ and a measurement procedure M, is almost what I would call a "theory of physics". We just need to add some rules that associate preparation procedures with probability measures on \Lambda, and we're good to go.

I would argue that decks do not have properties that determine these probabilities

You don't think the order of the cards will influence the probabilities? (That's what it sounds like, but I assume that you meant something else).

 

[nucl34rgg]

If QM as we learned it is wrong, why does it work so well? Also, how can anyone know anything is "really there" when all you can actually know about "reality" is what you measure and observe? We can only scientifically make statements about measurements we take.

The moon really basically isn't there unless something interacts with it to confirm that it is there. Otherwise there is no reason to believe it is actually there at all. The measurement doesn't just disturb the system in QM. It seems to define it.

 

[Ken G]

I don't know what this means. I understand the distinction between ψ-ontic and ψ-epistemic ontological models for QM, but you seem to be taking the terms "ontic" and "epistemic" outside of the framework of ontological models for QM.

I would argue it is not I who is doing that-- the PBR proof does that. It asserts, as a central part of the logic of the theorem, that we must imagine there are properties that determine the outcomes, independently from the system preparation. The crucial picture, associated with "realism", is that the preparation influences the properties, which in turn generate the outcomes. But if the preparation influences the properties, how are the properties not themselves just outcomes? What if a given preparation has a probability of creating a certain property, and another probability of creating a different property? They assume a very particular (and unlikely) relationship between the preparation and the properties, and then investigate two possible relationships between the preparation and the properties. Thus, if I adopt the stance that "there are no properties, there is only preparations and outcomes", or equivalently, that whay they call properties is what I call outcomes, then their entire argument is about nothing-- yet I still retain all of quantum mechanics, every scrap.

I'm not sure there's a meaningful distinction between the terms "ontic" and "epistemic" outside of that framework, but maybe that was your point.

Yes, the distinction is artificial. They assume a distinction exists, then prove certain constraints on the distinction, but there is nothing in quantum mechanics that suggests or requires that distinction exists. That's clear enough, but I'm saying that adding "realism" to quantum mechanics actually does nothing to alter that situation-- unless one takes the narrow (and dubious) stance that realism should be identified with belief that hidden properties determine everything. I call that belief in magic-- all we have in physics are our theories, and the next theory will have different variables than the last one, but that doesn't make any of them any "less hidden" than the ones before.

By the way, something that assigns a probability P(k|λ,M) to the result k, given an ontic state λ and a measurement procedure M, is almost what I would call a "theory of physics". We just need to add some rules that associate preparation procedures with probability measures on \Lambda, and we're good to go.

Yes, I completely agree. So you can see why I object to a claim that the mere existence of theories of physics requires that we must regard them as fundamentally ontological! That's the circularity I object to, most of what this theorem "proves" is embedded in its assumptions, all that's left to prove is a minor issue that is not fundamental to what any physics theory actually is.

You don't think the order of the cards will influence the probabilities? (That's what it sounds like, but I assume that you meant something else).

The "order of the cards" is not a property that determines observables, it is an observable. If I deal the cards out one by one, and you say "see, the order of the cards determined the order of the cards", I accuse you of tautology.

 

[Ken G]

The measurement doesn't just disturb the system in QM. It seems to define it.

Yes I agree, and indeed, this is nothing new in quantum mechanics-- it was always true in physics. QM is simply the place where we are forced to confront the issue, we always got away with lazy (yet highly parsimonious) ways of describing the situation in classical physics. Realism should not be regarded in the naive belief that the distinction does not exist, instead it should be associated with not making the distinction when it is not necessary to do so.

 

[qsa]

So what is the relation between the nature of psi, its interpretation and the wave-particle daulity. would a choice for one affect the others. or is that too much to ask.

 

[billschnieder]

There is a lot of confusion going around on this thread making it difficult to actually discuss what the authors of this article are talking about. Let me attempt to through some light to this:

Ontological - means "what is real", "what exists", independent of whether we know it or not.
Epistemological - means "what we know"

Words like "ontic" and "epistemic" are simply variants of those words, so phrases such as "ontic epistemic" or "ontic ontological" are just unnecessary confusion.

Let's use the example of a die to make these concepts more clear.

Reality (Ontology): Our die is a cube with six sides, with dimensions x,y,z, and physical properties i, j, k, ... etc. In other words, the ontology of the die is the complete specification of all existing physical properties of the system. Note that no two dice can have the same physical properties if these properties have been completely specified.

Epistemology (What we know): We could simply know the physical properties of a given die. But note a few very important points:
* Just because the die has a given physical property does not mean we can know it -- reality by definition exists whether we know it or not. Therefore we can never be sure we know all the possible physical properties of any system.
* Without absolute knowledge, we can never be certain that we know the exact value of any given knowable physical property. We could however have a value together with a confidence interval or a margin of error within which the real value lies.
* Our knowledge of the system may not even be represented in terms of the physical properties exactly but some other properties which are derived from some combinations of the hidden physical properties of the system.
* In the case in which a die is thrown and we have to predict the outcome, even if you know the complete physical properties of the with certainty, you will not know the outcome with certainty unless you also know the complete physical properties/conditions of the experiment. Probabilities arise ONLY due to uncertainty. The presence of probabilities in ANY theory implies lack of information or INCOMPLETE knowledge.
* You can only know something that is true or exists. It makes no sense to know nothing. In other words, our knowledge itself can not be the thing we are knowing. Therefore the idea that you can have a epistemic theory floating in the aether with nothing as it's object is not even wrong. Once you seriously look at what it is the theory is trying to know, the veil begins to lift just a bit.

The real question here is: does |ψ|^2 apply to an individual system or to a class of systems. Note that the quantum particle may be completely specified but if the preparation of of the experiment is not completely specified, the outcome will not be uniquely determined by ψ.

 

[my_wan]

Yes, this is what I have been saying also-- the key is in what is meant by "all intents and purposes." To PBR, the only intents and purposes they have in mind must be consistent with the idea that properties provide a complete physical description, such that everything that happens can be traced directly back to the properties. But QM has no step where you convert a preparation into a set of properties, you only connect the preparation to the outcomes, and along the way you might typically embrace the concept of properties (like quantum numbers) but you never need to attach any mechanistic connection between the properties and the outcomes.

Yes, and I'm still interested in this article because it does appear to establish some theoretical constraints. Even though I'm a lot less convinced that the characterization of those constraints as outlined really hold in general.

Any such connection amounts to belief in magic, in effect-- like those who believed that gravity was a force that appears (magically) due to the presence of a mass, or a curvature of spacetime that appears (magically) because of stress energy properties.

When I read this I got a bad impression. However, I looked back at your previous post #101, where the last paragraph clears it up for me and I would concur. For me this "magic" you speak of has a lot to do with how Bell's theorem gets interpreted, where properties are used as proxies for physical states while effectively denying a distinction, i.e., assuming the proxies are the real thing. Classical physics itself is replete with this particular kind of "magic" thinking and, from my perspective, almost certainly in need systematic revisions.

This reminds me of reading about the electron a a pre-teen. I was thinking: Yeah right, so you have a point entity lacking internal dynamics. Yet because of a "property" it possesses it has these dynamics. Meanwhile other point entities have different "properties". My take was BS, if this is how things are then what's to stop my toy car from being endowed with the "properties" of my parents car.. Without getting into detail this lead me to EPR type paradoxes, if such logic held, before I had ever heard of EPR.

PBR says that to be a realist, one must introduce this intermediate and unnecessary step of, in effect, believing in magic, but I say, being a realist means treating a physical theory like a physical theory. It is a realist attitude to treat ontological descriptions as a kind of intentional fantasy that we enter into because it is parsimonious to do so. And because that's exactly what we do, that is the realist stance-- ontology is epistemology.

I find it interesting simply on the grounds that it seems to indicate that a property set should, at least in principle, be able to accomplish this. Something QM really makes no provisions for and previous "property" modeling attempts have left a lot to be desired. I think that if the theorem holds, and a property set can in principle accomplish what this paper seems to indicate, it is an important result. However, I am with you in that this property set is still just an epistemological rendering representing only a valid symmetry between the model and the actual state. Valid and true are not synonymous. So I can appreciate the fact of the symmetries the article seems to indicate should hold without applying an undue reality to how things really are.

I say that to be a realist (not a naive realist), one merely needs to hold that there "actually is" a universe, but everything that we can say about that universe is epistemology, including the ontological claims we make on it for the purposes of advancing our conceptual understanding. I believe this is also what Bohr meant when he said that physics is not about nature, it is about what we can say about nature.

I have my own take on this. I am a realist, and concur that everything we can know about the Universe is epistemology. At a young age I came to this conclusion for reason quiet similar to some of the things our Dr Chinese said in "Hume’s Determinism Refuted". My thinking was that rinsing the "magic" off of the notion of ontic elements left you with entities that did not posses any properties in the usual "magic" sense you spoke of. Since properties is what defines measurements this made any ontic elements that might underpin the universe unobservables in any direct way, like the independent variables Dr Chinese spoke of. This, however, does not mean that you can't theorize about them and build a hierarchy of epistemological sets of observables from them. It does mean that any emergent observables have to be built out of relational data, where all empirically accessible variables are more akin to verbs than nouns, including the constants.

So unlike Dr Chinese I don't hold the position that the inability to empirical access ontic state properties, or independent variables, as a death blow to their existence or potential theoretical usefulness. However, it is true that claiming existence of X in any absolute real sense remains a non-starter. Fundamentally no different from a theoretical field specification except the properties are emergent rather than innate. Modeling attempts of this type are not even conceivable when either realism is interpreted as a property set sprinkled on a set of ontic entities or realism is rejected outright claiming no deeper ontic underpinning exist, only properties. Anyway, that's my take on it, not that it's likely to get me anywhere, and in terms of what we know it still doesn't remove the fundamental fact that our knowledge is limited to the epistemological regardless of what a successful theoretical construct is predicated on.

 

[Ken G]

* In the case in which a die is thrown and we have to predict the outcome, even if you know the complete physical properties of the with certainty, you will not know the outcome with certainty unless you also know the complete physical properties/conditions of the experiment. Probabilities arise ONLY due to uncertainty. The presence of probabilities in ANY theory implies lack of information or INCOMPLETE knowledge.

Would that it were so simple! But your stance involves making all kinds of assumptions about how reality works, assumptions that no theory in the history of physics has ever required, and no analysis of reality has ever supported. The fact is, no physics theory requires that there be any such thing as "complete physical properties", that is a complete fantasy in my view. Also, no physical theory requires that it be true that probability must appear solely due to a lack of information. Information is something you can have, it never refers to anything that we cannot have. Thus, we can say that probabilities are affected and altered by our information, but we certainly have no idea "where probability comes from." Imagining that we did leads to all kinds of absurd claims even in classical physics-- like the claim that butterflies "change the weather." The situation is even worse in quantum mechanics, where pretending that we understand what "causes probability" leads to all kinds of misconceptions about how the theory of quantum mechanics works, let alone how reality works.

Therefore the idea that you can have a epistemic theory floating in the aether with nothing as it's object is not even wrong.

Well, I'd say it's just obviously what a physics theory is, and quite demonstrably so. A little history is really all that is needed to establish this.

Note that the quantum particle may be completely specified but if the preparation of of the experiment is not completely specified, the outcome will not be uniquely determined by ψ.

Note that one one has the slightest idea if "the quantum particle may be completely specified", and there is certainly plenty of evidence that this is not the case. Even the very concept of "a quantum particle", when interpreted in the narrow way you interpret ontology, is quite a dubious notion. Your position is really just a bunch of sweeping generalizations, and even though they are very common, there really is a lot deeper that we can dig into these kinds of fairly superficial assumptions.

 

[my_wan]

Instead, I would like to offer a different definition of "realism". We start from the stance that everything we can say about nature is going to be a mental construct that is not an actual truth of nature, but rather, is an effective or useful truth, involving the way we have chosen to characterize nature. Hence, a "property" is an "element of a theory", and does nothing to separate "ontic" theories from "epistemic" theories. Indeed, theories aren't either ontic or epistemic, they are just theories.

Bewildering gibberish...

Actually it's a standard part of logic 101. The same logic that states that validity and truth are very different things. Theoretical constructs are predicated on validity, not truth. That's why they remain theories no matter how solidly the predicted consequences have been proven factual. Ken merely contextualized this logical fact in an unusual way.

The point to take from this is that we can theorize, opine, and ponder about how nature really is all we want, but at the end of the day all we have, that we can know, is the validity (not truth) of the matter as it has been empirically demonstrated. Too many people people, inside and outside of science, place too much truth value in the validity condition. The validity of a claim does not make it true.

 

[Maui]

Actually it's a standard part of logic 101. The same logic that states that validity and truth are very different things. Theoretical constructs are predicated on validity, not truth. That's why they remain theories no matter how solidly the predicted consequences have been proven factual. Ken merely contextualized this logical fact in an unusual way.

The point to take from this is that we can theorize, opine, and ponder about how nature really is all we want, but at the end of the day all we have, that we can know, is the validity (not truth) of the matter as it has been empirically demonstrated. Too many people people, inside and outside of science, place too much truth value in the validity condition. The validity of a claim does not make it true.

What you just said is not only valid but quite true . Very pleasant thread to read and follow so far; love the depth of analysis and self-critique

 

[qsa]

what is the criteria for TRUTH then.

 

[Maui]

what is the criteria for TRUTH then.

This is slightly offtopic so i'll be very brief - Death.

Only death is absolutely certain(in the sense of cessastion of existence as we know it - billions of years of history, trillions of life forms, not a single exception). Please ask similar questions in the philosophy forum to keep this thread on topic. Thank you

 

[DevilsAvocado]

Can you explain where contextuality enters the picture in my version of their argument? (Post #155). I'm not saying that you're wrong. I just barely know what contextuality means, and I haven't really thought about whether you're right or wrong.

It’s absolutely safest if DM answers this question, but if you want to be 'prepared' I can give you a little something to 'chew on' in the meantime. It’s about HVT:

Value definiteness (VD) – All observables defined for a QM system have definite values at all times.​

And a second assumption of:

Non-contextuality (NC) – If a QM system possesses a property, then it does so independently of any measurement context (i.e. independently of how that value is eventually measured).​

The Kochen–Specker (KS) theorem establishes a contradiction between VD + NC and QM. Therefore, QM logically forces us to give up either VD or NC.

According to KS, it’s NC that has to be excluded in any HVT compatible with QM.

And I make the assumption that ψ-epistemic with an underlying ontic state is forced to 'deal' with a contextual HVT...


P.S. This paper could maybe be useful:

Hidden Variables, Non Contextuality and Einstein-Locality in Quantum Mechanics
http://arxiv.org/abs/quant-ph/0507182

 

[Fredrik]

I don't know what this means. I understand the distinction between ψ-ontic and ψ-epistemic ontological models for QM, but you seem to be taking the terms "ontic" and "epistemic" outside of the framework of ontological models for QM.

I would argue it is not I who is doing that-- the PBR proof does that. It asserts, as a central part of the logic of the theorem, that we must imagine there are properties that determine the outcomes, independently from the system preparation.

I really can't tell what you're thinking here. You seem to be saying that PBR are taking the terms "ontic" and "epistemic" outside of the framework of ontological models for QM, by talking about ontological models for QM. I'm sure you see the problem with that claim. They are certainly not going outside of the framework of ontological models for QM in the theorem or the proof.

We don't have to "imagine there are properties". We don't have to assume anything about what an ontic variable in an ontological model for QM really is. It's convenient to say that they represent all the properties of the system, but this doesn't actually mean anything. It's just a suggestion about how to think about it.

If we define "theory of physics" as I did in my previous post, the theorem says that state vectors in QM do not correspond bijectively to epistemic states in any theory of physics such that a) it makes the same predictions as QM, and b) some of the probability distributions are overlapping.

The crucial picture, associated with "realism", is that the preparation influences the properties, which in turn generate the outcomes. But if the preparation influences the properties, how are the properties not themselves just outcomes?

Because an outcome is something you can read off a measuring device.

What if a given preparation has a probability of creating a certain property, and another probability of creating a different property? They assume a very particular (and unlikely) relationship between the preparation and the properties, and then investigate two possible relationships between the preparation and the properties.

I don't understand why you think there's something weird here. Later in this post, you agreed that a theory of physics needs a rule that identifies preparations with probability measures on the set whose members determine the probabilities of measurement results. Now you seem to be dismissing that very thing, and it's very hard to tell why.

Thus, if I adopt the stance that "there are no properties, there is only preparations and outcomes", or equivalently, that whay they call properties is what I call outcomes, then their entire argument is about nothing-- yet I still retain all of quantum mechanics, every scrap.

I have really tried to make sense of this. Their argument is clearly not about nothing, and why would anyone want to call equivalence classes of preparations "outcomes" instead of "states"?