I Anti-realist Interpretations of QM

[Lynch101]

Lynch101 is using a nonstandard definition of anti-realism, one that does not square with its use in literature. E.g. Asher Peres is a contemporary anti-realist and in his book "Quantum Theory: Methods and Concepts" he says

"The role of physics is to study relationships between these [objective experimental] records. Some people prefer to use the word intersubjectivity, which means all observers agree on the outcome of any particular experiment. Whether or not there exists an objective reality beyond the intersubjective reality may be an interesting philosophical problem, but this is not the business of quantum theory. Quantum theory in a strict sense is nothing more than the set of rules whereby physicists compute probabilities of the outcomes of macroscopic tests."

Anti-realism is the claim that , whether or not there is an objective reality beyond the intersubjective reality, the variables that manifest in quantum mechanics are not to be interpreted as elements of that objective reality.

What Lynch is describing is more akin to property nihilism than anti-realism.

I would interpret Peres's description (abvoe) as being a statement of instrumentalism or akin to SUAC. It doesn't seek to address the question of whether or not there exists an objective reality beyond the intersubjective reality, which is the question of the description of the quantum state prior to measurement.

As I see it, there are only two possible answers to that question:
1) Yes there is an objective reality
2) No there is no objective reality.

1) would be the realist position and would seem to imply that QM is not a complete description of nature, which I think was [at least part of] what Einstein was arguing. Would I be correct in saying that 1) would imply that there are "beables"?

2) is what I would interpret as the anti-realist position, as opposed to the instrumentalist position.

 

[Lynch101]

Please define what you mean by happen.

I'm using it in the broadest possible sense. Radioactive decay of a particle, triggering a geiger counter, which breaks a vile of poison to kill a cat, would be an example of something happening. A pilot wave directing a particle through an apparatus to interact with a measurement device would be another example. It's a place holder in lieu of a more complete description of the process unfolding in the box.

My definition is the system state vector evolves in time according to QM.

Would you say that the system state vector describes the process occurring prior to measurement, or does it ONLY give us information about the measurement outcome? If it only gives us information about the outcome then it doesn't describe the system prior to measurement. But you seem to be in agreement that the system does indeed exist prior to measurement. If it does exist prior to that, then there is something there to be described. At least that would be my thinking.

To try and give a different analogy. If we imagine looking at the outside wall of a building, with 5 windows and we set up a baseball pitching machine at the opposite side of the building to fire baseballs at the windows. If we have a mathematical formalism that ONLY tells us the probability of which window will get broken by a baseball then it doesn't tell us anything about the system prior the window breaking i.e. how the baseball got form the machine to the window.

I am inclined to think there is something to be described inside the building. It might never be possible to look inside the building and baseballs may not behave the way we think they do inside the building, but I think there is something to be described.

And hence the eternal problem I see with many such discussions. No macroscopic measurements device, no data. These arguments are like the sound of one hand clapping.

In the analogy above the windows would represent the macroscopic measurement device but there appears to be some information missing about what the baseball does from when it leaves the device to when it breaks the window. We don't have data for that, but that doesn't mean that there is nothing to be described inside the building.

System preparation, where to begin? For Schrodinger's cat experiment you've selected some cats to kill and put them in boxes and filled the poison vials. For a particle experiment you've turned on and tuned the accelerator and selected a target etc.

But how do you prepare a system to have absolutely no properties whatsoever? How does the particle get from the preparation device to the target, does it follow a specific trajectory through the apparatus? Can we describe the path that the particle takes through the apparatus before hitting the target? Is it reasonable to think that it must travel through the apparatus to get to the target?

Again, define what you mean by system property. I have no problem finding cats.

In the EPR paper, EPR tried to demonstrate that particles must have the very specific properties of location and momentum prior to measurement. I'm talking in a much more general sense, not about specific properties, but any properties whatsoever.

In QM properties involves operators and state vectors plus an implied ideal measurement device which is always macroscopic AFAICT.

The question I guess I'm trying to get at, in terms of the analogy of the baseball and the windows, is the question of what happens inside the building, prior to the window being smashed.

If our formalism only tells us the probability for any given window getting smashed, then it doesn't describe what happens inside the building. The question then becomes: is there something to be described inside the building?

 

[DrChinese]

I think we can ask a further question here: why is it as much as we can know? I think there are at least two answers to that question:
1) because there is a limit to our ability to probe nature.
2) because there is nothing more to know i.e. there is nothing inside the box.

Answer 1) would imply that our model is not a complete description of nature but it also says that there is no more complete specification of the system. Am I correct in saying that those parts of the system which cannot be modeled would be referred to as "beables"? I have been referring to them as properties of the system prior to measurement, but beables might be the more accurate term. I have been thinking of beables as hidden-variables, but I might be conflating two terms there.

Answer 2) is what I have come to understand as the "anti-realist" position.

Is it possible to talk about unquantified properties of the system. I'm thinking of properties at a fundamental level without preassigned values. I'm not exactly sure how that would work, but to my mind it seems like a conclusion that can be drawn.

Answer 2) is mostly the standard view of physicists. That QM is complete as is. Although Bohmians might argue that a more complete specification of the system is possible. No quibble as interpretations each address this.

The unquantified properties of an ENTANGLED system can be discussed. They follow conservation rules. A+B=zero/constant/initial value.

 

[Morbert]

tl;dr: I might be missing something somewhere, but it reads to me that what you and others are describing as "anti-realism" is what I understand as instrumentalism or Shut up and Calculate (SUAC).

This is what I understand to be the instrumentalist position or the SUAC position. It says that the mathematics is just a predictive tool. If it is necessary to make a distinction, I would interpret this more as "non-realism" than anti-realism.

What I have described is anti-realism as it is used in literature: The claim that the properties in QM do not refer to real properties of the system.

It goes further than simply saying the position that the properties in our sample space do not correspond to actually existing properties of the system we are modelling, it says that they do not correspond to actually existing properties because there are no existing properties.

The bit in bold is not a prerequisite for anti-realism. It is only physical properties present in our models that an anti-realist rejects as real. Before we can discuss Einstein's concerns, we have to square away nomenclature as it is used in literature.

If we were to distinguish anti-realism from instrumentalism, we could say an anti-realist is agnostic towards the nature of the reality of quantum systems, and rejects physical properties in our models as real, while an instrumentalist is agnostic towards both the nature of reality of quantum systems and also the ontic status of the physical properties present in our models.

As I see it, there are only two possible answers to that question:
1) Yes there is an objective reality
2) No there is no objective reality.

1) would be the realist position and would seem to imply that QM is not a complete description of nature, which I think was [at least part of] what Einstein was arguing. Would I be correct in saying that 1) would imply that there are "beables"?

Beables are a feature of a particular category of physical models. If it is the case that there is an objective reality, but it cannot be modeled with beables or hidden variables, then beables would not be a good description of this objective reality. It may be the case, for example, that reality cannot be thoroughly accounted for with physical theories.

 

[Paul Colby]

Would you say that the system state vector describes the process occurring prior to measurement, or does it ONLY give us information about the measurement outcome?

Yes and yes.

How does the particle get from the preparation device to the target,

By the time development of the system state vector.

Can we describe the path that the particle takes through the apparatus before hitting the target?

Well, yes just not at all in the manner you describe. One can show experimentally for fields in $\alpha$-states that the photons in fact have no trajectory (in fact the entire question isn't meaningful). Closest one can get is the wave nature of the field. Again, asking a classically motivated question about quantum systems isn't at all meaningful. Similar arguments hold for electrons and nuclei. If this is the picture you hold in mind, it's counter-factual.

The question I guess I'm trying to get at, in terms of the analogy of the baseball and the windows, is the question of what happens inside the building, prior to the window being smashed.

So your classical biases are showing through. You clearly want to paint a nice neat real number on each property ignoring the fact it's been shown by experiment that this isn't possible. It's simply not the way the world is observed to work, ever. Classical macroscopic physics is an approximation superseded by QM, a better one. It's now the case that one must provide a theory of measurement justifying classical mechanics in terms of quantum measurements, not the reverse. Your philosophical machinations really aught to be inverted.

 

[Lord Jestocost]

Would you say that the system state vector describes the process occurring prior to measurement, or does it ONLY give us information about the measurement outcome? If it only gives us information about the outcome then it doesn't describe the system prior to measurement. But you seem to be in agreement that the system does indeed exist prior to measurement. If it does exist prior to that, then there is something there to be described. At least that would be my thinking.

Even if a quantum entity “exists” prior to a measurement, what would it mean that there is something to be described. At first, one has to state, how the quantum entity exists, i.e., what is the character of its “existence” (realism).^** That’s a requirement for a “pictorial representation” in relation to the quantum formalism.

As remarked by Jan Faye in “Copenhagen Interpretation of Quantum Mechanics” (https://plato.stanford.edu/entries/qm-copenhagen/):

“In these four statements Bohr mentions the absence of “pictorial representation” twice in relation to the quantum formalism. The term “pictorial representation” stands for a representation that helps us to visualize what it represents in contrast to “symbolic representation”. A pictorial representation is a formalism that has an isomorphic relation to the objects it represents such that the visualized structure of the representation corresponds to a similar structure in nature. Conversely, a symbolic representation does not stand for anything visualizable. It is an abstract tool whose function it is to calculate a result whenever this representation is applied to an experimental situation.”

^** Regarding claims concerning the character of the “existence” of a quantum entity, the problems were clearly expressed by J. Robert Oppenheimer in “Atom and Void: Essays on Science and Community”:

If we ask, for instance, whether the position of the electron remains the same, we must say "no"; if we ask whether the electron's position changes with time, we must say "no"; if we ask whether the electron is at rest, we must say "no"; if we ask whether it is in motion, we must say "no."

 

[Demystifier]

It's infuriating isn't it?! It's a bit like that quote about time " If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know."

Well, concepts the meaning of which is intuitive but cannot be defined precisely are called primitive. Similarly, claims the truth of which seems obvious but can be neither proved nor disproved are called axioms. Any system of thought must contain some primitive concepts and some axioms. Once one realizes that, it's no longer infuriating.

 

[Morbert]

As an aside, it's worth emphasising that, according to modern anti-realist accounts, what comes into existence upon measurement is the record of that measurement expressed by properties of the apparatus. The measured property of the quantum system remains fictitious even after measurement.

I suspect statements like "spin of a particle becomes real after measurement" are common because, in experimental contexts like the EPR experiment, we don't violate complementarity if we construct event algebras that only reference spins after the corresponding measurements have occurred. But these are not the only valid algebras consistent with complementarity that we can construct. We can construct algebras that reference spin before measurement has occurred that will still produce the same correct predictions.

 

[Lord Jestocost]

As an aside, it's worth emphasising that, according to modern anti-realist accounts, what comes into existence upon measurement is the record of that measurement expressed by properties of the apparatus. The measured property of the quantum system remains fictitious even after measurement.

Is this a modern anti-realist account?
Sir Arthur Stanley Eddington in “THE NATURE OF THE PHYSICAL WORLD” (Cambridge, At the University Press (1929)):

"But now we realize that science has nothing to say as to the intrinsic nature of the atom. The physical atom is, like everything else in physics, a schedule of pointer readings. The schedule is, we agree, attached to some unknown background."

 

[Morbert]

Is this a modern anti-realist account?
Sir Arthur Stanley Eddington in “THE NATURE OF THE PHYSICAL WORLD” (Cambridge, At the University Press (1929)):

"But now we realize that science has nothing to say as to the intrinsic nature of the atom. The physical atom is, like everything else in physics, a schedule of pointer readings. The schedule is, we agree, attached to some unknown background."

Yes. It would be in line with A. Peres R. Omnes etc

 

[DrChinese]

We can construct algebras that reference spin before measurement has occurred that will still produce the same correct predictions.

Unless I don't understand your comment, Bell precludes that.

There are no data sets or statistical averages for quantum spins independent of the measurement setting(s). Keeping in mind that there are a multitude of statistical requirements due to the multitude of possible settings (thinking of typical Bell tests here).

 

[Lynch101]

Answer 2) is mostly the standard view of physicists. That QM is complete as is.

That is the understanding I had gotten, although I wasn't exactly sure what the breakdown of proponents was.

I just think there is a serious issue with the idea that there is nothing at all "inside the box", because if there were something "inside the box" then to my mind there is something to be described. If there is nothing at all inside the box, then it raises the question of how nothing can interact with a measurement device to produce something i.e. a measurement outcome.

Although Bohmians might argue that a more complete specification of the system is possible. No quibble as interpretations each address this.

The Bohmian picture seems to be the most intuitive to me but I think there is a middle ground between the idea that a more complete specification of the system is possible and the idea that there is nothing in need of description. There might be something "inside the box" to be described, but it might not be possible to probe it in such a way that we can meaningfully describe it. We could still make certain limited deductions however.

The unquantified properties of an ENTANGLED system can be discussed. They follow conservation rules. A+B=zero/constant/initial value.

Is this more a statement about rules that they follow as opposed a description of the properties?

 

[Lynch101]

If we were to distinguish anti-realism from instrumentalism, we could say an anti-realist is agnostic towards the nature of the reality of quantum systems, and rejects physical properties in our models as real, while an instrumentalist is agnostic towards both the nature of reality of quantum systems and also the ontic status of the physical properties present in our models.

Ah thank you Morbert, I think I see the distinction you're making, now. Is the following accurate?

The anti-realist in QM says that the (for want of a better term) mathematical elements of QM don't have a 1-to-1 correspondence with the quantum system itself, or with reality we might say. It says that the model only enables us to make the probabilistic predictions we see in QM. It says that there might be something "inside the box" or there might not, either way the mathematical formalism doesn't describe what is "inside the box"?

On the other hand, instrumentalism says the model might have a direct correspondence to reality or it might not, who cares, just SUAC?

Beables are a feature of a particular category of physical models. If it is the case that there is an objective reality, but it cannot be modeled with beables or hidden variables, then beables would not be a good description of this objective reality. It may be the case, for example, that reality cannot be thoroughly accounted for with physical theories.

Ah, I see. Thanks for the clarification on that too.

 

[Lynch101]

I've re-ordered your post to reply to a specific point first. Hopefully this doesn't distort it in any way.

So your classical biases are showing through. You clearly want to paint a nice neat real number on each property ignoring the fact it's been shown by experiment that this isn't possible. It's simply not the way the world is observed to work, ever. Classical macroscopic physics is an approximation superseded by QM, a better one. It's now the case that one must provide a theory of measurement justifying classical mechanics in terms of quantum measurements, not the reverse. Your philosophical machinations really aught to be inverted.

Classical bias would be to say that the baseball follows a simple trajectory through the building with a definite location and momentum at every point en route to the window. Experiment shows that this isn't the case when we have say two slits, inside the building, through which the baseball can travel and both are open. If we open one slit at a time then it would appear as though we could infer our classical notions of trajectory. Of course, this wouldn't account for what happens when both slits are open, so we can't simply infer our classical notions of trajectory.

To account for what happens when both slits are open we might try to infer a different classical bias. We might say that the baseball spreads out like a wave and goes through both slits, interferes with itself and then strikes the one of the windows. We might say that the mathematical formalism is a description of what is happening inside the building and the process it describes is that of a baseball riding on top of a pilot wave and that it is the pilot wave that interferes with itself, carrying the baseball to one of the windows to smash it. All of these offer an explanation as to how the baseball gets from the machine to the window to smash it.

If, however, we reject all of these and say that our mathematical formalism only tells us the probability of which window will get broken, and nothing more, we are then left with the question of what is actually happening inside the building? How does the baseball get from the machine to the window to break it?

There are essentially two broad answers to this question:
1) There is something happening inside the building. For a complete model of nature we would need to describe the process occurring inside the building.
2) There is absolutely nothing happening inside the building and therefore nothing to describe.

Yes and yes.
By the time development of the system state vector.

Hopefully I'm phrasing this correctly but, do you attribute ontic status to these?

In my own terminology I would ask if you believe that these parts of the model correspond directly to reality?

Well, yes just not at all in the manner you describe. One can show experimentally for fields in $\alpha$-states that the photons in fact have no trajectory (in fact the entire question isn't meaningful). Closest one can get is the wave nature of the field. Again, asking a classically motivated question about quantum systems isn't at all meaningful. Similar arguments hold for electrons and nuclei. If this is the picture you hold in mind, it's counter-factual.

My apologies, I think my wording is a little to imprecise at times, or perhaps carries with it certain connotations that I don't intend. I try not to have any specific picture in mind.

Apologies again for coming back to this basic point, but it helps me to frame the issue in my mind; the question of the description of the system prior to measurement. If we say it is meaningless to talk about it or that the formalism only gives us predictions, then I interpret that to mean that it doesn't describe what happens "inside the building".

I may be misinterpreting what you are saying but in one sense you appear to be saying that the formalism does describe the system prior to measurement i.e. saying that it describes how the "particle" makes its way through the experimental set-up, prior to measurement.

 

[PeterDonis]

Experiment shows that this isn't the case when we have say two slits, inside the building, through which the baseball can travel and both are open.

Experiment shows no such thing. Nobody has done a double slit experiment showing interference with baseballs. Buckyballs, yes, but not baseballs. Buckyballs are many, many, many orders of magnitude smaller.

 

[Lynch101]

Even if a quantum entity “exists” prior to a measurement, what would it mean that there is something to be described. At first, one has to state, how the quantum entity exists, i.e., what is the character of its “existence” (realism).^** That’s a requirement for a “pictorial representation” in relation to the quantum formalism.

I guess its partly a question of what our physical models are supposed to represent. If they are meant to model reality then we might think of them as descriptions of what exists in nature or, more pointedely, a description of what nature comprises. If nature comprises quantum systems, then a complete model of nature would have to describe a quantum system. If nature comprises quantum systems prior to measurement then a complete model of nature would require a description of the quantum system prior to measurement.

If physical models aren't meant as descriptions of nature, rather computational tools to make predictions about nature, which may or may not represent reality, then that would carry a different set of requirements and consequences.

A pictorial representation is a formalism that has an isomorphic relation to the objects it represents such that the visualized structure of the representation corresponds to a similar structure in nature. Conversely, a symbolic representation does not stand for anything visualizable. It is an abstract tool whose function it is to calculate a result whenever this representation is applied to an experimental situation.”

The anti-realist position represents the symbolic representation and as such, appears to calculate the result of an experiment i.e. it gives us the probabilistic predictions of which measurement will occur. Measurements themselves are classical [level] phenomena, so if QM is only a tool to calcualte macro-level phenomena, to what extent does it describe the quantum world at all? As a computational tool does it just represent the amount of information that we can put into the calculation?

This still leaves us asking the question about the quantum system prior to measurement. Even if there is no quantum system, if it is classical all the way down and it is our lack of information that gives us probabilistic predictions, we still have the question of the system prior to measurement. What happens in the intervening time between switching on our device (which prepares the system) and seeing the exposure event on the Stern-Gerlach plate?

^** Regarding claims concerning the character of the “existence” of a quantum entity, the problems were clearly expressed by J. Robert Oppenheimer in “Atom and Void: Essays on Science and Community”:

If we ask, for instance, whether the position of the electron remains the same, we must say "no"; if we ask whether the electron's position changes with time, we must say "no"; if we ask whether the electron is at rest, we must say "no"; if we ask whether it is in motion, we must say "no."

I would be inclined to think that this points to an inability on our part to describe the quantum system as opposed to its lack of "existence", or to put it another way, it's absence from what nature comprises.

 

[Lynch101]

Experiment shows no such thing. Nobody has done a double slit experiment showing interference with baseballs. Buckyballs, yes, but not baseballs. Buckyballs are many, many, many orders of magnitude smaller.

Apologies, the baseball and the windows is meant to be an analogy highlighting where I perceive the gap in explanation. I mightn't have referenced it clearly to the previous post where I started the analogy.

 

[PeterDonis]

There are essentially two broad answers to this question:
1) There is something happening inside the building. For a complete model of nature we would need to describe the process occurring inside the building.
2) There is absolutely nothing happening inside the building and therefore nothing to describe.

You're leaving out a third possibility:

3) The "something" in between the source and the measurement results cannot be described in terms of "processes" or "something happening", or in terms of "nothing is happening" or "nothing to describe". None of those combinations of words are a good description of that something.

One should always beware of false dichotomies, but interpretations of QM are an area where one has to be particularly careful in this respect.

 

[Paul Colby]

1) There is something happening inside the building.

In classical physics without the tacit assumption of measurement you can't say what's happening inside the building either. You may construct a model but without ever looking it, it may or may not happen to follow your model. You simply don't know.

In regard to the ability to measure both classical and quantum both make sweeping tacit assumptions that certain types of measurements can be made.

If one looks at the Bohr Einstein debates one sees a series of thought experiments proposed challenging the quantum view. One by one Bohr provided the supplemental measurement device interactions showing that the quantum result would prevailed. Einstein was the clear loser in this debate. It's not an accident that Bohr used the physics of the measurements to make his argument.

Now, quantum mechanics says, go ahead and look in the building but by doing so you must introduce additional measurement devices which will effect what you find. That's just the way the world is structured. I suggest you get used to it.

 

[DrChinese]

I guess its partly a question of what our physical models are supposed to represent. If they are meant to model reality then we might think of them as descriptions of what exists in nature or, more pointedely, a description of what nature comprises. If nature comprises quantum systems, then a complete model of nature would have to describe a quantum system. If nature comprises quantum systems prior to measurement then a complete model of nature would require a description of the quantum system prior to measurement.

...

Again, this is a very old position. Goes back to pre-1935 days. So you really must follow the EPR argument (which argued a more complete specification of the system was possible) and the Bell argument (that no such specification is possible if the quantum mechanical predictions are correct) and the Aspect argument (that the quantum mechanical predictions are correct). Conclusion: there is no more complete specification of the system.

If you fail to include Bell in the discussion, we will simply go 'round and 'round. You can say all day long that you believe there are things happening for which there is no evidence, and for which evidence exists that it cannot be. Even the Bohmian position is contextual (i.e. there is no well-defined value independent of the act of observation). That's because there are no data sets that match the quantum expectation values.

Quantum mechanics does not provide a "physical model" in the manner you describe. It is best considered a mathematical model. The interpretations attempt to supply some outline of a physical model, but all have issues of one sort or another.

 

[Paul Colby]

I may be misinterpreting what you are saying but in one sense you appear to be saying that the formalism does describe the system prior to measurement i.e. saying that it describes how the "particle" makes its way through the experimental set-up, prior to measurement.

Before measurement the isolated quantum system is described by a system state vector evolving in time. At measurement the quantum system interacts by exchanging some finite amount of energy with the macroscopic measuring device which then records the result. You just don't like this answer but it's what measurements actually are. Does this answer have ontic status? Don't know. Ontic Sounds like a fairly mushy meaningless term but I'm no expert.

 

[Lynch101]

You're leaving out a third possibility:

3) The "something" in between the source and the measurement results cannot be described in terms of "processes" or "something happening", or in terms of "nothing is happening" or "nothing to describe". None of those combinations of words are a good description of that something.

One should always beware of false dichotomies, but interpretations of QM are an area where one has to be particularly careful in this respect.

I'm inclined to agree with this and I think it demonstrates a fundamental limit on our ability to try and apply conceptual labels to reality, which is itself non-conceptual. There is a buddhist saying which says that "the finger pointing to the moon is not the moon" which can be interpreted as meaning that our conceptual descriptions of something are not the thing in itself and indeed, the thing in itself cannot be conceptualised.

So while we cannot capture the essence of the thing we wish to describe we can point to "something" that is there i.e. inside the box. If it is not possible to model whatever is there then it means that we can not have a complete model of nature. I think there are still certain conclusions that we can draw however.

I'm inclined to think that it points to the fact that there is information which we cannot acquire about a system, and this might explain why we end up with probabilistic predictions.

 

[Lynch101]

In classical physics without the tacit assumption of measurement you can't say what's happening inside the building either. You may construct a model but without ever looking it, it may or may not happen to follow your model. You simply don't know.

I think we are in agreement on this point. The point I am trying to get at however is that, to my mind, there must be, for want of a much better phrase, something happening inside the box. If we cannot model what is happening inside the box then our model does not represent a complete model of nature. There is information missing, which we may never be able to acquire. I'm inclined to think this lack of information is the reason for probabilistic interpretations but ultimately a realist explanation must be the case. That would be my reasoning.

If one looks at the Bohr Einstein debates one sees a series of thought experiments proposed challenging the quantum view. One by one Bohr provided the supplemental measurement device interactions showing that the quantum result would prevailed. Einstein was the clear loser in this debate. It's not an accident that Bohr used the physics of the measurements to make his argument.

Have you read the book What is real? by Adam Becker? I may be misremembering, but I think he paints a different picture of the Bohr-Einstein debates. IIRC he suggests that Bohr published a number of papers purporting to address Einstein's objections but didn't actually address them.

Was it Bell's work that ultimately demonstrated that the EPR argument was incorrect?

Now, quantum mechanics says, go ahead and look in the building but by doing so you must introduce additional measurement devices which will effect what you find. That's just the way the world is structured. I suggest you get used to it.

I completely accept that, but our inability to model what is inside the building doesn't mean that there is absolutely nothing inside the building. I think it is reasonable to say that there absolutely must be something inside the building and to have a complete model of nature we would need to model what is inside. If we cannot model it, then I don't think we can have a complete model of nature.

I believe the alternative is that there is absolutely nothing inside the building. This position however doesn't seem reasonable and has, what I believe are insurmountable issues, more so than even FTL [non-signalling[ communication.

@PeterDonis suggests that there is a third option but my reading of it is that it just demonstrates our inability to model what is happening inside the building. There is still either something or nothing in there we just could not model it either way. That is just my reading of it though, and there might be some nuance that I am not yet getting.

 

[Lynch101]

Again, this is a very old position. Goes back to pre-1935 days. So you really must follow the EPR argument (which argued a more complete specification of the system was possible) and the Bell argument (that no such specification is possible if the quantum mechanical predictions are correct) and the Aspect argument (that the quantum mechanical predictions are correct). Conclusion: there is no more complete specification of the system.

If you fail to include Bell in the discussion, we will simply go 'round and 'round. You can say all day long that you believe there are things happening for which there is no evidence, and for which evidence exists that it cannot be. Even the Bohmian position is contextual (i.e. there is no well-defined value independent of the act of observation). That's because there are no data sets that match the quantum expectation values.

To my mind, it sounds as though the baby gets thrown out with the bath water with Bell's theorem. I know that the EPR paper argued that particles do have definite postion and momentum even if we don't measure them. The idea was to measure the position of one pair of entangled particles and the momentum of another and by doing so we would know the position and momentum of the other (is that due to the conservation of momentum?)

I might be butchering this but did Bell show that, with the assumptions of local realism and statistical independence (and others?), if the particles did have these predefined values then they would obey an inequality pertaining to the measurement results? The experimental results violate the inequality however demonstrating that the system cannot have these predefined values*.

This is where it looks to me as though the baby is getting thrown out with the bath water. The system might not have predefined values for these specific properties but that doesn't mean that the system has no properties whatsoever, prior to measurement.

Quantum mechanics does not provide a "physical model" in the manner you describe. It is best considered a mathematical model. The interpretations attempt to supply some outline of a physical model, but all have issues of one sort or another.

My thinking is that, if there is a system prior to measurement then a complete model of nature would have to include some description of the system prior to measruement. This might not be possible, but that would mean that our model is not a complete model of nature.

 

[Lynch101]

Before measurement the isolated quantum system is described by a system state vector evolving in time. At measurement the quantum system interacts by exchanging some finite amount of energy with the macroscopic measuring device which then records the result. You just don't like this answer but it's what measurements actually are. Does this answer have ontic status? Don't know. Ontic Sounds like a fairly mushy meaningless term but I'm no expert.

I'm obviously no expert myself, I'm just reasoning on the basis of the information that I have encountered. I hadn't really encountered the term "ontic" outside the context of interpretations of QM, although I presumed it has the same root as the term "ontology".

With respect to certain realist hidden variable theories that attempt to explain the predictions of quantum mechanics, the theorem rules that pure quantum states must be "ontic" in the sense that they correspond directly to states of reality, rather than "epistemic" in the sense that they represent probabilistic or incomplete states of knowledge about reality.

PBR Theorem - wiki

I've come across the terms "psi-ontic" and "psi-epistemic" and, if I'm understanding correctly, "psi-ontic" refers to interpretations which treat the wave function as a physically real element of reality, while "psi-epistemic" treat the wave function as a representation of an experimenters [incomplete] knowledge of the system.In your statement above, does the system state vector correspond to an element of reality in the sense that the wave function does in Bohmian mechanics?

 

[Paul Colby]

In your statement above, does the system state vector correspond to an element of reality in the sense that the wave function does in Bohmian mechanics?

Reality as understood by humans is comprised of macroscopic objects. I would need to first understand macroscopic objects in terms of QM to answer this question. A quantum measurement is itself a macroscopic object comprised of an initially isolated quantum system evolving via hamiltonian, $H_{S}$ , which at the time of measurement, $t$, interacts via an interaction hamiltonian, $H_I(t)$, with a macroscopic detector, $D$. The question is what is $H_D$ and what are its internal micro states? How do the micro state effect $H_I$? When there are an astronomical sized set of possible states all recognized as the very same macroscopic object what is done with $|D\rangle$? Clearly a two level system, $|\text{alive}\rangle$ and $|\text{dead}\rangle$ is a comical over simplification.

 

[PeterDonis]

did Bell show that, with the assumptions of local realism and statistical independence (and others?), if the particles did have these predefined values then they would obey an inequality pertaining to the measurement results?

What you are calling "predefined values" are an example of what Bell called "hidden variables". Bell's theorem shows that any hidden variable model that satisfies his assumptions (which include what you are calling "local realism" and "statistical independence") must obey the Bell inequalities and therefore cannot account for the actual experimental data that violates them.

 

[DrChinese]

I might be butchering this but did Bell show that, with the assumptions of local realism and statistical independence (and others?), if the particles did have these predefined values then they would obey an inequality pertaining to the measurement results? The experimental results violate the inequality however demonstrating that the system cannot have these predefined values*.

This is where it looks to me as though the baby is getting thrown out with the bath water. The system might not have predefined values for these specific properties but that doesn't mean that the system has no properties whatsoever, prior to measurement.

You use the words "no properties whatsoever" and those have no connection to what is being assumed by EPR or Bell.

a) EPR says that if any value can be predicted in advance, it must be pre-existing (and therefore QM is incomplete). Entangled particle pairs demonstrate this feature as EPR believed in 1935. It is sometimes called "perfect correlations" as there is 100% agreement when appropriate measurement settings are chosen.

b) The question Bell asked was: If there are values prior to measurement, then are the values INDEPENDENT of measurement? I.e. are they objectively real? I.e. are they observer independent? I.e. are values for all possible measurement settings predetermined? Bell showed that this extension to a) was NOT possible.

In the language of EPR: the question was whether the values (for each of the many/infinite number of measurement basis choices) are simultaneously real. Bell precludes that, because there is no such set that reproduces the QM expectation values.

So you MUST consider both a) and b) when talking about this. It is easy to jump past one or the other. Your model must reproduce perfect correlations, and statistical percentages at other times. (Of course the perfect correlations are also a subset of the statistical percentages, where the percentage is either 0% or 100%.)

 

[Lynch101]

Reality as understood by humans is comprised of macroscopic objects. I would need to first understand macroscopic objects in terms of QM to answer this question. A quantum measurement is itself a macroscopic object comprised of an initially isolated quantum system evolving via hamiltonian, $H_{S}$ , which at the time of measurement, $t$, interacts via an interaction hamiltonian, $H_I(t)$, with a macroscopic detector, $D$. The question is what is $H_D$ and what are its internal micro states? How do the micro state effect $H_I$? When there are an astronomical sized set of possible states all recognized as the very same macroscopic object what is done with $|D\rangle$? Clearly a two level system, $|\text{alive}\rangle$ and $|\text{dead}\rangle$ is a comical over simplification.

Thanks for this breakdown Paul. I find posts like this very helpful.

I think, perhaps, thinking of reality in terms of macroscopic objects can be somewhat of a hindrance. It is entirely natural and almost impossible not to do, but the idea that I have in mind tries to get away from that. I'm trying to use very broad terms such as "anything" and "something" to try and circumvent this tendency.

The issue - on my part - with the above, is that I am taking it in the context of what other members have said also. @Morbert made a distinction that helped me to see an issue with how I was interpreting the anti-realist position. I was taking the anti-realist position to mean a denial of the existence of the quantum system prior to measurement, but he clarified:

anti-realist is agnostic towards the nature of the reality of quantum systems, and rejects physical properties in our models as real

In relation to the role of the mathematical formalism he says:

Quantum theory in a strict sense is nothing more than the set of rules whereby physicists compute probabilities of the outcomes of macroscopic tests.

This is in keeping with what I had generally understood and what you and others have reiterated.

If I interpret your post above, with regard to the Hamiltonians, in the context of the statement about computing the probabilities of outcomes, it appears to say that the mathematics only gives us the probabilities for the outcomes of experiments. In terms of the analogy I've used, it only tells us the probability of which window will get broken and tell us nothing at all about what happens inside the building.

I understand that the phrase "what happens inside the building" is not very precise but if we agree that there is a building and that there is a quantum system prior to measurement, then I would reason that, in order for our model of nature to be complete, it would have to model the system inside the building prior to measurement.

If it doesn't, then I'm inclined to think that our model of nature is incomplete. If it simply isn't possible to model inside the building then it means that we cannot have a complete model of nature. Either way, the conclusion would be that the model is not a complete model of nature. That doesn't make it any less effective in what it does, but that is the conclusion I find myself arriving at.

 

[Lynch101]

What you are calling "predefined values" are an example of what Bell called "hidden variables". Bell's theorem shows that any hidden variable model that satisfies his assumptions (which include what you are calling "local realism" and "statistical independence") must obey the Bell inequalities and therefore cannot account for the actual experimental data that violates them.

Thanks Peter. I follow the logic of Bell's theorem and how the violation of the inequality in experiments rules out local hidden variables. In my mind I am making a distinction between the system having properties with predefined values and the system simply having properties.

It's the idea I'm trying to get at by saying there must be something or anything inside the building or prior to measurement. That is what is leading me to the conclusion that any model which doesn't model the system prior to measurement must be incomplete.