Ballentine's Ensemble Interpretation Of QM

[audioloop]

http://lanl.arxiv.org/abs/0904.2287 [Int. J. Mod. Phys. A25:1477-1505, 2010]

and

http://arxiv.org/pdf/1307.1714v1.pdf

Yes, one important service that the various interpretations could provide is guidance toward the next theory. Here we would have to let nature adjudicate what works, so there would be no purpose in "preferring" one interpretation over another, except for personal guidance in our own efforts to find the next theory if we were ambitious enough to try. A key role of theory is not just to explain what we've already seen, but also to help us know what we might want to look for next. I think that's the place where the interpretations have the most value, and who knows which one will be the most helpful there. Thus we see both the advantages and disadvantages of a "minimal" interpretation like the ensemble approach-- it keeps us from going too far out on a limb, but it also provides minimal opportunity to jump to the next branch.

indeed. soon i think.

 

[bhobba]

Guys - yes this discussion of CI is interesting.

However in most versions of Copenhagen, as far as I can tell anyway, while the state can be applied to a single system, it only represents a state of knowledge, simply a theoretical construct indicating a theorists 'confidence' in a particular observation outcome (when combined with an observable via the Born rule of course). Copenhagen also believes it gives a full and exhaustive description of that single system. It is of zero concern if such discontinuously changes, collapses or whatever terminology you want to use. Ballentine's arguments in Chapter 9 only apply if you think its in some sense real - and indeed interpretations where it is has issues with it. But if you don't think it real then there is no problem.

Thanks
Bill

 

[atyy]

Maybe take another look through Ballentine's chapters 8 and 9?

Does he introduce state reduction in Eq 9.28? Or is 9.28 derived, even in the case where R and S don't commute?

 

[Ken G]

Copenhagen also believes it gives a full and exhaustive description of that single system.

It is certainly true that there are "flavors" of CI, but I tend to let Bohr be its primary spokesperson, as opposed to, say, Heisenberg, and if we do that, then CI would probably rather say that the wave function gives an exhaustive description of the meeting of a single system and a macro instrument. In other words, "collapse" is not so much an effect on the quantum system carried out by the macro instrument (the quantum system has only a kind of inscrutable existence in CI in the first place), it is more like an effect on the macro instrument mediated by the quantum system. Collapse in CI is the only glimpse of the quantum world you ever get, and since it is seen through a glass darkly, we don't claim to have "really seen" the quantum world at all. Indeed, Bohr doesn't think we can even claim existence for that quantum world, we see it so dimly. So I don't think we can say that CI treats collapse as a real effect on the quantum world-- more like a real effect on a classically understood apparatus. You might even say collapse is a side effect of the means we choose to do science.

 

[kye]

Contemplating on all this, you guys didn't put emphasis on interference between the ensembles. Even though you view them as statistics, they have perfect interference.. in the one at a time electron or photon double slit experiment, each electron doesn't just appear in the screen randomly.. but they avoided the destructive interference zones.. so this gets us back to the position that particles are also wave. And force us back into the single experiment setup. The ensemble interpretation is easier to take if there is no interference, mathematically if what we see are all mixed states... but in pure states, there is interference, it's like throwing coins randomly at the floor and the ensembles form interference... so how does the individual coin knows where to avoid. This force us to see the coin as behaving as wave too. So is a coin particle or wave. In Heisenberg potentia. He conjectured that before measurement, the quantum state is literally a vector in Hilbert Space. I guess this is the essence of strong Copenhagen because if you still think superposition is due to the state b and c existing at the same time meaning the particle stays as particles during superposition, then it's really Many worlds in disguised. I got this arguments from Fredrik and it makes sense.

 

[strangerep]

Does [Ballentine] introduce state reduction in Eq 9.28?

Not sure what you're asking here. It's state preparation by filtering. I can't really say any more than what Ballentine has already said there.

IMHO, filtering is not measurement. Rather, a filter is an operator: you give it a state and it produces another state. That's what (9.28) represents. (The denominator is just a normalization factor.)

But imagine removing the final screen from a Stern-Gerlach setup, so the beams just continue on to infinity (or more likely the walls of a pipe, or whatever). Then nothing has been measured, except perhaps how many Watt-hours of electricity were wasted in operating the experiment.

Reinsert the detector screen, and you cannot do anything useful with the particles after they've been "detected" by interacting with the screen. See also Ballentine's commentary after eqn (9.29).

Or is 9.28 derived, even in the case where R and S don't commute?

There is no "S" in (9.28) in my copy of Ballentine.

The case of noncommuting R,S is discussed a little further on, over on p248. His equation (9.26) is only for commuting R,S.

BTW, the subtleties in all this are why he defers discussion of Cox's probability axiom #4 (see p30) until much later, here in ch 9.

 

[Demystifier]

and

http://arxiv.org/pdf/1307.1714v1.pdf

This is about relativity in BM, but not about quantum field theory. About relativity, see also
http://lanl.arxiv.org/abs/1309.0400

 

[kith]

Would you agree with Cthugha that in the ensemble interpretation this filtering can be called "state reduction"?

I would say this is mostly a question about semantics which doesn't address the main issue. Cthugha is of course right that you could call it so but I think this would be misleading. The filtering is not problematic. It simply means you split an ensemble in subensembles and pick out a certain one. The motivation to speak of state reduction comes from the macroscopic superposition state which occurs after a measurement without filtering. The CI says such superpositions are not possible and introduces state reduction to avoid them. But the ensemble interpretation has no problem with them.

Still, I see this as very CI-like, because to me the defining feature of CI is not collapse, it is the commitment to the idea that physics is represented by outcomes of classical devices.

There is no need for classical devices if the macroscopic superposition refers to an ensemble of systems and devices like it does in the ensemble interpretation. These interpretations may be not so different from an empiricist point of view but I think the conceptual differences are big enough to distinguish between them.

 

[vanhees71]

This is about relativity in BM, but not about quantum field theory. About relativity, see also
http://lanl.arxiv.org/abs/1309.0400

Thanks for pointing this references out. I'll have a closer look at it over the weekend. The most challenging task of a Bohmian in my opinion is twofold: (a) how to formulate a Einstein-causal non-local pilot wave theory that is at the same time Lorentz covariant and (b) how to implement the fact that relativistic interacting particles can be destroyed and created, i.e., how to describe the many-body aspects of relativistic QT, which is the very reason why modern formulations of relativistic QT (almost?) exclusively use the QFT formulation.

 

[Demystifier]

Thanks for pointing this references out. I'll have a closer look at it over the weekend. The most challenging task of a Bohmian in my opinion is twofold: (a) how to formulate a Einstein-causal non-local pilot wave theory that is at the same time Lorentz covariant and (b) how to implement the fact that relativistic interacting particles can be destroyed and created, i.e., how to describe the many-body aspects of relativistic QT, which is the very reason why modern formulations of relativistic QT (almost?) exclusively use the QFT formulation.

I definitely agree that these are the most challenging tasks for Bohmian mechanics. In addition to the suggested references above, see also
https://www.physicsforums.com/blog.php?b=2240

 

[atyy]

Not sure what you're asking here. It's state preparation by filtering. I can't really say any more than what Ballentine has already said there.

IMHO, filtering is not measurement. Rather, a filter is an operator: you give it a state and it produces another state. That's what (9.28) represents. (The denominator is just a normalization factor.)

But imagine removing the final screen from a Stern-Gerlach setup, so the beams just continue on to infinity (or more likely the walls of a pipe, or whatever). Then nothing has been measured, except perhaps how many Watt-hours of electricity were wasted in operating the experiment.

Reinsert the detector screen, and you cannot do anything useful with the particles after they've been "detected" by interacting with the screen. See also Ballentine's commentary after eqn (9.29).

My question is whether the filtering method of state preparation follows from the axioms in Chapters 2 (and 3), or whether it is an additional axiom.

There is no "S" in (9.28) in my copy of Ballentine.

The case of noncommuting R,S is discussed a little further on, over on p248. His equation (9.26) is only for commuting R,S.

BTW, the subtleties in all this are why he defers discussion of Cox's probability axiom #4 (see p30) until much later, here in ch 9.

I meant that 9.28 and 9.29 apply (I think) even in the case of non-commuting R, S which are discussed later, even though they were derived via 9.26 which only applies for commuting R,S. For commuting R,S it does look like he can derive 9.28 and 9,29. But for non-commuting R,S, are 9.28 and 9.29 then new axioms?

 

[Ken G]

There is no need for classical devices if the macroscopic superposition refers to an ensemble of systems and devices like it does in the ensemble interpretation. These interpretations may be not so different from an empiricist point of view but I think the conceptual differences are big enough to distinguish between them.

I'm just saying that the interpretations seem to fall into three general classes based on their top priority. For CI and ensemble, the top priority is to hold firm to the idea that physics is a tool for predicting outcomes that we the physicists can perceive as outcomes, and theories organize the information we use to do that, but theories can mislead us if we take them too seriously. For BM, the top priority is that the classical models we use to understand our instruments are themselves fundamentally the reality, not just the way our minds work, and theories should always be interpreted so as to maintain that consistency. For MWI, the top priority is that the abstract elegance of the theory should be regarded as the fundamental truth, and observations test truth but can trap us in illusions of perception if we interpret them too literally. Against those basic schisms, I have a hard time seeing any other distinctions as being of equal importance, but certainly everyone can navigate the remaining nuances to tailor the details of their own picture.

Added: So what I'm saying is, if you want to see all the interpretations as similar, insofar as they are all quantum mechanics, a good approach is to start with the ensemble interpretation, and then make relatively minor tweaks to achieve results that function a lot like each of the other interpretations. But if you want to see the interpretations as very different, perhaps to use one to inspire some new theory, then it might be useful to expand on those three basic different priorities. As some of the other interpretations have already begun that process, the ensemble interpretation could be viewed as minimalist.

 

[bohm2]

So what I'm saying is, if you want to see all the interpretations as similar, insofar as they are all quantum mechanics, a good approach is to start with the ensemble interpretation, and then make relatively minor tweaks to achieve results that function a lot like each of the other interpretations. But if you want to see the interpretations as very different, perhaps to use one to inspire some new theory, then it might be useful to expand on those three basic different priorities. As some of the other interpretations have already begun that process, the ensemble interpretation could be viewed as minimalist.

That makes sense and I don't see how progress can occur without commitment to some form of scientific realism. Sure, all of our evidence will come to us in the form of macroscopically observable phenomena, but we need not restrict ourselves to these concepts when constructing scientific theories. That's where interpretation comes in. I mean, hasn't this always been the norm of science? I just find operationalism a dead end.

 

[Ken G]

What's more, I have never actually met anyone who "shuts up and calculates." Some claim they hold to that view, but they end up, just like the rest of us, wanting more from the equations than just testable outcomes. Let's face it, we do science because we want to understand.

 

[atyy]

What's more, I have never actually met anyone who "shuts up and calculates." Some claim they hold to that view, but they end up, just like the rest of us, wanting more from the equations than just testable outcomes. Let's face it, we do science because we want to understand.

The Bohmian interpretation is shut-up-and-calculate. Once we have Bohm we know all's well in principle (or at least not worse than classical statistical mechanics), so we can just shut-up-and-calculate

 

[Ken G]

That's really not what is meant by shut up and calculate though, as you can see if I ask you "why do I need a pilot wave". Does anyone really use the pilot wave to do the calculations, and if so, why doesn't everyone?

 

[atyy]

That's really not what is meant by shut up and calculate though, as you can see if I ask you "why do I need a pilot wave". Does anyone really use the pilot wave to do the calculations, and if so, why doesn't everyone?

Yes, I was joking. But there is a book "Applied Bohmian mechanics" http://arxiv.org/abs/1206.1084. Also, it doesn't mean that we have to use the full Bohmian apparatus everytime, even if one likes Bohmian ideas. It'd be like never using Newtonian mechanics, just because one had the standard model of particle physics.

 

[Ken G]

Sorry I missed the joke, but it's hard to tell, because people do have very different ideas-- some feel MWI is the simplest interpretation, a la Max Tegmark's "many worlds or many words", but I have a hard time imagining that 10⁵⁰⁰ or so other worlds is simple, just because you can write it in a simpler equation!

 

[Len M]

That makes sense and I don't see how progress can occur without commitment to some form of scientific realism. Sure, all of our evidence will come to us in the form of macroscopically observable phenomena, but we need not restrict ourselves to these concepts when constructing scientific theories. That's where interpretation comes in. I mean, hasn't this always been the norm of science? I just find operationalism a dead end.

Well I’m not sure about this. I know you have read some of d’Espagnat’s works, you may find this extract to be of interest - this is what he says about this particular criticism of operationalism that you seem to make.

On the ground that operationalism limits the aims of science to synthesizing communicable human experience its opponents often claim that taking it up often prevents invention and speculation from playing their parts in research. This view has only a small amount of truth in it. True, the theorist working in a field governed by – say – the quantum laws will not strive to invent naïve mechanistic theories since he or she knows beforehand that such constructs, grounded on concepts of “near realism” would automatically be doomed to failure. But the said theorist – operationalist, might we say, of necessity - still has many opportunities to imagine and speculate. Two possibilities along these lines are available and were indeed made use of -often simultaneously - by the most creative twentieth century physicists. One of them consists in letting oneself be guided – or better to say, inspired - by naive notions and pictures whilst being fully aware they are naive. One example is the astounding notion Dirac put forward of a “sea” of negative energy electrons; and another that electrons may pop out from it, leaving “holes” that “are” positrons. Let us remember that the two basic notions of antimatter and pair creation just come from this.

On the other hand, it must be realized that the fruitful concept of “sea” and “holes” were inspired by a most precise but also - until the relevant experimental checks were made - quite conjectural mathematical formalism. This brings to light the other manner the theorist has of putting his or her imaginative powers into action. It consists of in reflecting on the mathematical formulas, in wondering how they might be made more general and/or more beautiful (the two often go hand in hand), in discovering possible symmetries hidden in their bosom, and in striving to extend them. All this being, of course, subject to a posteriori experimental monitoring. In short, we recognise here the Pythagorean standpoint and observe once more that, after all, it is not conditioned by realism. Even though the theorists who make use of these methods incline in most cases (sometimes unconsciously) towards some kind of Einsteinian realism, such a realism hardly serves them in their work and is not therefore what guides them in practice.

I would suggest the scientific realism you refer to is intended by proponents as being more than naive pictures used by Dirac as described in the extract, rather I think it refers to a genuinely held (philosophical) view of what conceivably exists outside of phenomena. Operationalism would dismiss such notions of realism of course, but according to d’Espagnat, such realism is hardly necessary in which to invent new models.

I know that physicists could hardly work without pictures such as (for example) particles and trajectories, but there is quite a difference I would have thought between needing these kind of pictures as a means of creating new and imaginative predictive models and that of wishing to use them as realist interpretations. The former can certainly be an aid to understanding phenomena acting upon phenomena in terms of the verified mathematical model, the latter however offers little in the way of objective understanding of that which exists outside of phenomena.

So perhaps operationalism is not such a "dead end" area of physics, I tend to think it is exactly what physics proper is (in that I hold great store by the predictive mathematical model) whilst allowing free reign in using any form of enquiry in which to expand existing and create new mathematical predictive models - forms of enquiry that do not have to rely on realist models.

 

[Ken G]

Perhaps, by "operationalism", bohm2 simply meant the attitude that "if the formula works, I don't need to use it to inspire any naive pictures." So in that sense, Dirac's thinking about a sea of negative particles, with "holes" for positrons, would not be classified by bohm2 as operationalism, as Dirac is inspired by a picture rather than just the way the equations work. D'Espagnat seems to classify that as operationalism any time Dirac does not actually hold that the negative sea actually exists. So perhaps we can identify three levels of realism, not two: naive realism, where we take our pictures literally, inspirational realism, where we look for organizational pictures that may or may not be literally real (think, virtual particles as an example), and operationalism, where we just solve equations and "go through the motions." That doesn't seem to be the way D'Espagnat uses the terms, but it does seem like what bohm2 means, unless I'm mistaken.

 

[strangerep]

My question is whether the filtering method of state preparation follows from the axioms in Chapters 2 (and 3), or whether it is an additional axiom.

I wouldn't call it an axiom, but rather an application of projection operators, hence a special case.

But for non-commuting R,S, are 9.28 and 9.29 then new axioms?

By my reading, no. Instead, he has reinterpreted the 4th Cox axiom for probability (which is the same equation as (9.22)), i.e.,
$$
\def\Pr{\text{Prob}}
\Pr(A\&B|C) ~=~ \Pr(A|C) \; \Pr(B|A\&C)
$$ into
$$\Pr(A\&B|C) ~:=~ \Pr(A|C) \; \Pr(B|A\&C) ~.$$
The only difference here is that I've changed "$=$" into "$:=$". In classical probability, both sides of the original equation make sense, but not so in QM for noncommuting quantities. Here, Ballentine generalizes the concept, though still restricted (iiuc) to the case of filtering-type operations.

 

[kith]

My question is whether the filtering method of state preparation follows from the axioms in Chapters 2 (and 3), or whether it is an additional axiom.

I would say that it's neither of these possibilities but merely a choice of the observer which allows him to redefine the system.

In a Stern-Gerlach experiment, the composite system of the particle and the Stern-Gerlach apparatus evolves unitarily from an initital product state ρ to an entangled macroscopic superposition state ρ' = p_upρ_up + p_downρ_down. If we block particles with spin down, only ρ_up is a state were particles leave the apparatus. This means only ρ_up is relevant for future experiments.

Nothing in the formalism picks out ρ_up. The full unitary time evolution gives us the state of the composite system at all times. But ρ_down is superflous for future measurements on particles which leave the apparatus. Considering only ρ_up leads to the same predictions but the calculations are a lot easier because it is a product state, so we can separate the particle from the apparatus again. In the other case we have to drag all the previous history of the particle with us.

So I would say the picking of a subensemble in filtering is not a fundamental process but a convenient choice to simplify calculations.

 

[kith]

I'm just saying that the interpretations seem to fall into three general classes based on their top priority. For CI and ensemble, the top priority is to hold firm to the idea that physics is a tool for predicting outcomes that we the physicists can perceive as outcomes, and theories organize the information we use to do that, but theories can mislead us if we take them too seriously.

Yes, I agree with your post almost completely. Still I want to highlight the conceptual differences between the CI and the ensemble interpretation because this whole talk about classical apparatuses leads people to think that classical mechanics is necessary for QM. What is necessary to talk about experiments in QM is an observer, not some special device which cannot be described by QM.

 

[atyy]

I wouldn't call it an axiom, but rather an application of projection operators, hence a special case.

There doesn't seem to be an axiom which says that one is allowed to act on the state with a projection operator.

The axioms in chapters 2 & 3 seem to be
(1) observables correspond to operators
(2) given a state, we can calculate the expectation value of an observable
(3) the time evolution of the state is governed by the Hamiltonian

Am I missing some axioms, or can the action of a projection operator be derived from these?

By my reading, no. Instead, he has reinterpreted the 4th Cox axiom for probability (which is the same equation as (9.22)), i.e.,
$$
\def\Pr{\text{Prob}}
\Pr(A\&B|C) ~=~ \Pr(A|C) \; \Pr(B|A\&C)
$$ into
$$\Pr(A\&B|C) ~:=~ \Pr(A|C) \; \Pr(B|A\&C) ~.$$
The only difference here is that I've changed "$=$" into "$:=$". In classical probability, both sides of the original equation make sense, but not so in QM for noncommuting quantities. Here, Ballentine generalizes the concept, though still restricted (iiuc) to the case of filtering-type operations.

OK, that's my understanding too.

I would say that it's neither of these possibilities but merely a choice of the observer which allows him to redefine the system.

In a Stern-Gerlach experiment, the composite system of the particle and the Stern-Gerlach apparatus evolves unitarily from an initital product state ρ to an entangled macroscopic superposition state ρ' = p_upρ_up + p_downρ_down. If we block particles with spin down, only ρ_up is a state were particles leave the apparatus. This means only ρ_up is relevant for future experiments.

Nothing in the formalism picks out ρ_up. The full unitary time evolution gives us the state of the composite system at all times. But ρ_down is superflous for future measurements on particles which leave the apparatus. Considering only ρ_up leads to the same predictions but the calculations are a lot easier because it is a product state, so we can separate the particle from the apparatus again. In the other case we have to drag all the previous history of the particle with us.

So I would say the picking of a subensemble in filtering is not a fundamental process but a convenient choice to simplify calculations.

Actually, back to the successive measurements we discussed a few posts back. If we start in a pure state, after a measurement doesn't the state generally become a mixed state? This would be non-unitary evolution, so isn't this state reduction?

Yes, I agree with your post almost completely. Still I want to highlight the conceptual differences between the CI and the ensemble interpretation because this whole talk about classical apparatuses leads people to think that classical mechanics is necessary for QM. What is necessary to talk about experiments in QM is an observer, not some special device which cannot be described by QM.

If the observer is not an ensemble, and QM only applies to ensembles, then the observer is not governed by QM.

 

[strangerep]

There doesn't seem to be an axiom which says that one is allowed to act on the state with a projection operator.
[...]

A pure state operator is a projection operator. More generally, a projection operator may correspond to a dynamical variable that takes on only the values 0 and 1. See section 2.2 (just after eq(2.2)), and the discussion in section 2.3 about general states and pure states.

If we start in a pure state, after a measurement doesn't the state generally become a mixed state? This would be non-unitary evolution, so isn't this state reduction?

It's unitary if one analyzes the experiment properly by modelling the apparatus as well. See Ballentine section 9.2, where he explains how and why one does not need the usual principle of "collapse to an eigenvector". (That part of the discussion begins around halfway down p233.)

 

[Ken G]

Yes, I agree with your post almost completely. Still I want to highlight the conceptual differences between the CI and the ensemble interpretation because this whole talk about classical apparatuses leads people to think that classical mechanics is necessary for QM. What is necessary to talk about experiments in QM is an observer, not some special device which cannot be described by QM.

I agree, it's not so much classical mechanics one needs, it is decoherence. We manipulate and test mixed states,not superposition states. Physics must simplify to be effective, and our best simplification tool is reductionism, so we must separate the quantum system from the apparatus and the apparatus from the observer. Reality doesn't do that, but quantum mechanics does (the way we do it in practice, and the way we verify that it works), and decoherence is the reason we don't notice the disconnect, a disconnect that is there even in the ensemble interpretation. So it's not quantum vs. classical that is the "Heisenberg divide", it is the full system vs. the projected subsystem, but it's more or less the same in the end (so CI could easily be translated to fit these alternative words). In any of the interpretations, we simply don't test the formal theory we write down, we test something else, and that is the source of the "measurement problem." That's also why none of the interpretations do away with that problem, but the ensemble approach pushes it the farthest out of sight, while other approaches (like BM and MWI) make it an organic structural element. CI, on the other hand, pays homage to it, but doesn't really know what to do with it, and that's actually why I like CI-- I think we should not know what to do with the measurement problem as long as we insist on reductionism.

 

[kith]

Actually, back to the successive measurements we discussed a few posts back. If we start in a pure state, after a measurement doesn't the state generally become a mixed state?

My post is about successive measurements and it answers your question.

If the observer is not an ensemble, and QM only applies to ensembles, then the observer is not governed by QM.

The observer can be considered to be part of an ensemble and you can include him in the unitary time evolution. You simply can't test hypotheses about such a system without an additional observer who is again excluded from the description.

 

[atyy]

A pure state operator is a projection operator. More generally, a projection operator may correspond to a dynamical variable that takes on only the values 0 and 1. See section 2.2 (just after eq(2.2)), and the discussion in section 2.3 about general states and pure states.

But that just says that pure states exist. It doesn't tell you how they are prepared. I can understand there is no collapse, if there is an axiom that says that one can prepare pure states by filtering, ie. one moves collapse from measurement to preparation. Is that what is being done?

It's unitary if one analyzes the experiment properly by modelling the apparatus as well. See Ballentine section 9.2, where he explains how and why one does not need the usual principle of "collapse to an eigenvector". (That part of the discussion begins around halfway down p233.)

It's the same in interpretations with collapse. But then there cannot be successive measurements, ie. a measurement is by definition the final step in the analysis.

 

[bohm2]

Perhaps, by "operationalism", bohm2 simply meant the attitude that "if the formula works, I don't need to use it to inspire any naive pictures."... So perhaps we can identify three levels of realism, not two: naive realism, where we take our pictures literally, inspirational realism, where we look for organizational pictures that may or may not be literally real (think, virtual particles as an example), and operationalism, where we just solve equations and "go through the motions." That doesn't seem to be the way D'Espagnat uses the terms, but it does seem like what bohm2 means, unless I'm mistaken.

You are correct. I would not classify guidance for progress via symmetry, beauty, elegance and the Pythagorean standpoint (e.g. Tegmark) as operationalism. I think Tegmark goes too far to argue that there is nothing but math as I think it's clear that this is false. For how do mathematical entities lead to phenomenology? It's the latter "shut up and calculate" approach that I find too restrictive and dead end. With respect to the ensemble and CI interpretation, there are some versions that can be considered to take this approach. As Lucien Hardy and Robert Spekkens write:

But operationalism is not enough. Explanations do not bottom out with detectors going ‘click’. Rather, the existence of detectors that click is the sort of thing that we can and should look to science to explain.

 

[Len M]

You are correct. I would not classify guidance for progress via symmetry, beauty, elegance and the Pythagorean standpoint (e.g. Tegmark) as operationalism.

But the formal definition of scientific realism (you referred to the desirability of adopting a form of scientific realism in your post #293) states that there is a reality totally independent of phenomena along with the hypothesis that we do have access to the said reality in that we can say something "true" concerning it. From this standpoint we then proceed to build up a representation of that independent reality, the nature of that representation being dependent on the particular flavour of realism chosen. You seem (I think) to put forward the view that such a scientific realism is needed in order to prevent physics from becoming a dead end - that state of affairs being encompassed within the stance of operationalism. D’Espagnat is saying that no such realism is required to take physics forward.

D’Espagnat’s remarks concerning the mathematics does not refer to “Tegmark like” approaches to physics, rather he is making clear that the representative element associated with mathematical realism need not be invoked at all by operationalists – the representative element that Tegmark invokes at a later stage serves no purpose for them, rather it is the manipulation of the mathematics itself that leads to new and imaginative models. Operationalists wouldn't claim that the model represents independent reality as per Tegmark, they would simply want to show that it works as a predictive mathematical model of phenomena.

Perhaps I’m unclear over what you mean by scientific realism. If by this term you just mean pictures having no pretensions of being representational elements of independent reality then I think we are just describing this scientific process (involving these pictures) by different names, my description being operationalism. If however you are referring to scientific realism in the formal manner as being indicative of what may exist within independent reality then it would seem to suggest that you are saying that physics needs this potential “reality” in which to grow. I don’t think that physics needs such a potential “reality” in which to proceed because for me that potential is as far from what may actually exist within independent reality as anyone can imagine, simply because a miss is as good as a mile. How close or how far a model may be from independent reality is a meaningless question to me because we are never going to know, but as d’Espagnat has illustrated, it is possible for models to be created on the basis that the pictures and mathematics used are not taken to represent independent reality at all, as in the case of his example of Dirac and his sea of holes.

For me and I would say d’Espagnat, operationalism is not simply shut up and calculate, that surely is a just the application of the model to technology. Rather operationalism is physics with no regard for what may be a true picture (or not) outside of phenomena. Instead, the picture is a means to an end in creating the verifiable mathematical predictive model. That of course doesn't exclude meaning being given to pictures of phenomena acting upon phenomena, it's simply that operationalism is entirely unconcerned with what the phenomena may or may not be within independent reality (or, as per radical idealism, may not even be concerned over there needing to be an independent reality)

 

[kye]

I agree, it's not so much classical mechanics one needs, it is decoherence. We manipulate and test mixed states,not superposition states. Physics must

Ken, isn't mixed states also in superposition? you just treat certain portion of the pure states which makes it mixed states, but it should still be in superposition but there is no longer interfering terms in the density matrix. Do you equate mixed states with born rule applied and the state taking on classical eigenvalues?

simplify to be effective, and our best simplification tool is reductionism, so we must separate the quantum system from the apparatus and the apparatus from the observer. Reality doesn't do that, but quantum mechanics does (the way we do it in practice, and the way we verify that it works), and decoherence is the reason we don't notice the disconnect, a disconnect that is there even in the ensemble interpretation. So it's not quantum vs. classical that is the "Heisenberg divide", it is the full system vs. the projected subsystem, but it's more or less the same in the end (so CI could easily be translated to fit these alternative words). In any of the interpretations, we simply don't test the formal theory we write down, we test something else, and that is the source of the "measurement problem." That's also why none of the interpretations do away with that problem, but the ensemble approach pushes it the farthest out of sight, while other approaches (like BM and MWI) make it an organic structural element. CI, on the other hand, pays homage to it, but doesn't really know what to do with it, and that's actually why I like CI-- I think we should not know what to do with the measurement problem as long as we insist on reductionism.

 

[Ken G]

Ken, isn't mixed states also in superposition?

Only if you embed them in a larger system than the physicist actually deals with when recording outcomes. The mixed state is the projection of the superposition, but that's all we deal with in physics experiments. The superposition is purely conceptual, all we test is the projection onto our experience, which is a mixed state before we look, and a "collapsed" state after we look. Whether we look or not is not so important, because we know all about the difference between mixed states and collapsed states from classical physics (and from playing poker), it is that tricky superposition that is in our mathematics, and we need it to explain what we get, but the superposition is not the outcome of the experiment.

Do you equate mixed states with born rule applied and the state taking on classical eigenvalues?

Yes, it is the decohered projection onto what we experience when we do measurements. It can be explained using the formal theory, but most of that formal theory is never actually tested, all we test is that is makes sense of what we see.

 

[Dale]

This "interpretations" thread has run its course.