Anti-realist Interpretations of QM

[Paul Colby]

The question of how something without any properties can interact with a measuring device in the first instance, for example.

In my opinion one can't have such a discussion without being careful about the "something" one is actually discussing. The electromagnetic field has properties, photons being one. Knowing the state of the field one can make statistical statements about photon detections. However, a photon really isn't "a thing" in the same sense as the moon or a beachball.

 

[Lynch101]

1. ...The realist says they have well-defined values at all times, regardless of observer.

Is it possible to take a somewhat "weaker" realist position and remain agnostic on whether properties have well defined values at all times and simply say that there are properties?

 

[DrChinese]

I struggle with the anti-realist position myself ...

Just be aware that the alternative is one (such as Bohmian Mechanics) which contain/require FTL mechanisms. Nothing wrong with that at all, perfectly acceptable. But there is a good reason there are anti-realistic interpretations: a) they naturally explain the EPR paradox; b) locality may be retained in all respects, a la Bell.

 

[Lynch101]

In my opinion one can't have such a discussion without being careful about the "something" one is actually discussing. The electromagnetic field has properties, photons being one. Knowing the state of the field one can make statistical statements about photon detections. However, a photon really isn't "a thing" in the same sense as the moon or a beachball.

Sorry, I meant in the broadest possible sense. It might be better to ask how can anything without properties can interact with a measurement device?

 

[DrChinese]

Is it possible to take a somewhat "weaker" realist position and remain agnostic on whether properties have well defined values at all times and simply say that there are properties?

I'm sure. In fact several posters earlier mentioned that you might define "realism" as you see it. That may not really help much though. Once you address Bell, you see how difficult that is.

Many papers have been written attempting to provide alternative definitions of realism and locality such that Bell does not apply. That hasn't worked out so well. Quick, name the guy who got a Nobel for getting around Bell!

Those who go with the "weaker" definitions end up on a lonely road. And one that provides no more useful description or interpretation. You may as well end up with "shut up and calculate" and take no particular interpretation addressing Bell.

 

[Lynch101]

Just be aware that the alternative is one (such as Bohmian Mechanics) which contain/require FTL mechanisms. Nothing wrong with that at all, perfectly acceptable. But there is a good reason there are anti-realistic interpretations: a) they naturally explain the EPR paradox; b) locality may be retained in all respects, a la Bell.

I'm wondering if we are necessarily using the same interpretation of "anti-realism" here. I'm taking it to be the position that the system in question has absolutely no properties whatsoever, prior to measurement.

Are there other interpretations of non-locality that don't involve FTL mechanisms? I thought someone mentioned some alternatives in another thread - I'll see if I can find it.

 

[DrChinese]

However, a photon really isn't "a thing" in the same sense as the moon or a beachball.

It's a quantum object. And it really doesn't matter which quantum object we are referring to. The electron's non-commuting properties are just as simultaneously "real" or "not-real" as those of any photon.

 

[Paul Colby]

It might be better to ask how can anything without properties can interact with a measurement device?

Well, I don't think such things exist in exactly the way you are expressing it. One can show experimentally that a spin-1/2 system has no spin-value prior to measurement. That's not to say the spin-1/2 system doesn't exist or have spin values one can measure.

 

[Lynch101]

I'm sure. In fact several posters earlier mentioned that you might define "realism" as you see it. That may not really help much though. Once you address Bell, you see how difficult that is.

Many papers have been written attempting to provide alternative definitions of realism and locality such that Bell does not apply. That hasn't worked out so well. Quick, name the guy who got a Nobel for getting around Bell!

Those who go with the "weaker" definitions end up on a lonely road. And one that provides no more useful description or interpretation. You may as well end up with "shut up and calculate" and take no particular interpretation addressing Bell.

I think I have a bit more study to do before even being able to solve a question on Bell, not to mind anything else I'm mindful that I have mentioned in my previous post that we might be talking at cross purposes with regard to the term "anti-realism" and you may have replied to it before I post this, but just for clarity, taking the "weaker" position is more just for the purpose of juxtaposing it with the "anti-realist" position, as I understand it.

My understanding of the "anti-realist" position is that it states that the quantum system has no properties whatsoever prior to measurement. This to me would seem to be more problematic than an FTL mechanism which cannot be used for signalling.

 

[DrChinese]

I'm wondering if we are necessarily using the same interpretation of "anti-realism" here. I'm taking it to be the position that the system in question has absolutely no properties whatsoever, prior to measurement.

An entangled electron, for example, lacks well defined values for its entangled properties. You could say those properties are in a superposition of states. Whether it has "absolutely no properties" is just by definition at that point.

 

[Lynch101]

Well, I don't think such things exist in exactly the way you are expressing it. One can show experimentally that a spin-1/2 system has no spin-value prior to measurement. That's not to say the spin-1/2 system doesn't exist or have spin values one can measure.

This might be where my understanding is lacking. My understanding of the "anti-realist" position is that it says it is meaningless to talk about the properties of the system prior to measurement because it does not have any properties prior to measurement. I was inclined to think that allowing for properties of any kind would be to allow for hidden variables and necessitate a realistic interpretation.

 

[DrChinese]

My understanding of the "anti-realist" position is that it states that the quantum system has no properties whatsoever prior to measurement. This to me would seem to be more problematic than an FTL mechanism which cannot be used for signalling.

A perfectly reasonable position.

@Demystifier : You can thank me later for driving another one over to your side of the road.

 

[Paul Colby]

This might be where my understanding is lacking. My understanding of the "anti-realist" position is that it says it is meaningless to talk about the properties of the system prior to measurement because it does not have any properties prior to measurement. I was inclined to think that allowing for properties of any kind would be to allow for hidden variables and necessitate a realistic interpretation.

Perhaps I'm not using the same definition of property. A particle (AKA a quantum excitation of a quantum field) has a property called spin. What's being discussed is the specific value of spin prior to measurement not being determined.

 

[Lynch101]

An entangled electron, for example, lacks well defined values for its entangled properties. You could say those properties are in a superposition of states. Whether it has "absolutely no properties" is just by definition at that point.

Apologies, I'm not clear on what you mean by the sentence I emboldened above, as in how do you mean it is by definition?

In the anti-realist position does the wave function represent an ontic state i.e. does it correspond to the physical reality of the entangled photons, or does it just give the probabilities for measurement outcomes relating to the entangled photons? Or is there some other aspect of the mathematical formalism that governs that?

I'm not sure if I've used the relevant terms accurately enough there. I'm not sure if this will help, but I'm wondering if there is the mathematical formalism describing the superposition is ontic in a similar - but obviously very different - sense to how the wave function is ontic in Bohmian Mechanics?

 

[Lynch101]

Perhaps I'm not using the same definition of property. A particle (AKA a quantum excitation of a quantum field) has a property called spin. What's being discussed is the specific value of spin prior to measurement not being determined.

I do think we have slightly different ideas in mind. When we talk about things like spin, position, and momentum, we are talking about very specific properties. In this sense we might talk about whether or not those properties have specific or definite values prior to measurement.

I'm talking in the more general sense of any property whatsoever. To my understanding the "anti-realist" position says that the quantum system has no properties whatsoever, in the general sense. My interpretation is that the alternative to this is to say that the system does have some [unquantified] properties. My understanding is that to say the system has unquantified properties would be to say that it has hidden variables and is realistic.

 

[Paul Colby]

I'm talking in the more general sense of any property whatsoever. To my understanding the "anti-realist" position says that the quantum system has no properties whatsoever, in the general sense. My interpretation is that the alternative to this is to say that the system does have some [unquantified] properties. My understanding is that to say the system has unquantified properties would be to say that it has hidden variables and is realistic.

What anti-realist believe or say is beyond me. You're best listening to people more versed in what they actually say. They sound unhinged but that's only my opinion based on the snippets in this thread.

So, the axioms of QM actually provide a means of being clear about "properties." I read property as an observable. An observable is an operator which has a spectrum, a set of allowed values that may be observed. This spectrum is unchanging and I would call it a property.

Now, upon measurement only one of the spectrum set is obtained as a result. This result is the property-value.

 

[DrChinese]

1. Apologies, I'm not clear on what you mean by the sentence I emboldened above, as in how do you mean it is by definition?

2. I'm not sure if I've used the relevant terms accurately enough...

1. At some point, everyone reverts to their own language to describe things. And similarly, it is impossible to say if your words are close in meaning to mine. If you believe the following, that a particle in a superposition of states lacks a well-defined value independent of a measurement basis chosen by an observer, then I would say you have adopted an "anti-realist" position. I would call it "non-realistic" rather than "anti-realist" but I can't see any difference.

"An entangled electron, for example, lacks well defined values for its entangled properties. You could say those properties are in a superposition of states."2. This is why there is an Interpretations sub forum. No one is...

 

[DrChinese]

What anti-realist believe or say is beyond me.

Bell's Theorem says: No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics.

Quantum Mechanics requires us to abandon at least one of "locality" and "realism" (hidden variables which may be unknowable). The anti-realist position would be that realism is abandoned, presumably so you can retain locality.

There are a number of interpretations that reject realism. In the time symmetric group, there is a mechanism of "handshake" (however that works) between the quantum object being observed and the observer. Therefore there is observer dependence and you have a subjective reality - retaining locality.

 

[Lynch101]

So, the axioms of QM actually provide a means of being clear about "properties." I read property as an observable. An observable is an operator which has a spectrum, a set of allowed values that may be observed. This spectrum is unchanging and I would call it a property.

Now, upon measurement only one of the spectrum set is obtained as a result. This result is the property-value.

Definining "properties" as "observables" seems to come back, somewhat, to the point about what we can say about the system prior to measurement.

My difficulty in defining properties as observables, but that might just be due to the fact that I don't have a good working knowledge of the various terms, so I can't articulate my point using the more precise terms. "Beable" might be the more suitable term - I just haven't used it enough yet. From @Demystifier's paper in his signature:

Beable: That word was coined by John Bell. It means the same as ontology: stuff which is there irrespective of observation. The concept of a beable is central to Bohmians, but not to instrumentalists.

Another difficulty I have with defining "properties" as "observables" is that "beables" are distinguished from observables but we might still talk about the properties of beables or perhaps say nothing more than the statemement, "beables have properties". Is there also a question of whether or not the observable can be said to be an inherent property of the system?

If we talk about the properties of a system prior to measurement, it also distinguishes a property from an observed property.It's in this broad sense of the term property that I tend to talk, to try to clarify my interpretation of "anti-realism". To try and use the term "beable": the "anti-realist" position seems to say that there are no beables prior to measurement.

In terms that seem more intuitive to me, I would say that the anti-realist position says that the system has no properties whatsoever prior to measurement. It is this which leads me to question how anything without properties can interact with a measurement device in the first place?

 

[Lynch101]

1. At some point, everyone reverts to their own language to describe things.

Thank you DrChinese, this is reassuring (if not comforting ).

And similarly, it is impossible to say if your words are close in meaning to mine. If you believe the following, that a particle in a superposition of states lacks a well-defined value independent of a measurement basis chosen by an observer, then I would say you have adopted an "anti-realist" position. I would call it "non-realistic" rather than "anti-realist" but I can't see any difference.

I would be inclined to distinguish between "non-realist" and "anti-realist", where non-realism and instrumentalism are effectively one and the same. My understanding is that it says that a particle in a superposition of states lacks a well-defined value independent of a measurement basis chosen by an observer. It does not make any ontological inferences from the mathematical formalism.

"Anti-realism" on the other hand goes further, it does make ontological inferences from the mathematical formalism. It seems to say that a particle in a superposition of states lacks a well-defined value independent of a measurement basis chosen by an observer, because a particle in a superposition does not have any properties prior to measurement.

"An entangled electron, for example, lacks well defined values for its entangled properties. You could say those properties are in a superposition of states."

This might be where my limited understanding of the mathematics is hindering me. My understanding is that, according to the "anti-realist" position, the mathematical formalism which describes the superposition only gives the probabilities for the measurement outcome of the entangled particles, meaning that it doesn't describe the ontic state of the particles prior to measurement.

If the entangled state defined by the mathematics is an ontic state that would make it a realistic interpretation, wouldn't it? Whereas, if the superposition described by the mathematical formalism is just a tool for predicting the outcome of experiments, then it doesn't describe the system prior to measurement. The instrumentalist/"non-realist" position stops there, but the "anti-realist" position would go further, as above.

That would be my understanding of it.

2. This is why there is an Interpretations sub forum. No one is...

 

[Paul Colby]

Quantum Mechanics requires us to abandon at least one of "locality" and "realism" (hidden variables which may be unknowable). The anti-realist position would be that realism is abandoned, presumably so you can retain locality.

So, would it be accurate to say;

realism = systems have property values prior to measurement?

Since the realism view is born out of a classical macroscopic intuition we evolved with, I say that it's unsupported by the measurements and should be abandoned. There is no reason to expect it to hold for purely quantum objects.

 

[Paul Colby]

In terms that seem more intuitive to me, I would say that the anti-realist position says that the system has no properties whatsoever prior to measurement. It is this which leads me to question how anything without properties can interact with a measurement device in the first place?

I can only repeat what I've said. I believe you are conflating having a value with having a possible set of observable values. Things interact because they do and we have the formalism and data to prove it.

 

[Morbert]

In terms that seem more intuitive to me, I would say that the anti-realist position says that the system has no properties whatsoever prior to measurement. It is this which leads me to question how anything without properties can interact with a measurement device in the first place?

Anti-realism re/ QM pertains to the anti-realism of properties considered by QM (i.e. those represented by regions of Hilbert space), as opposed to all properties entirely.

 

[Demystifier]

Am I correct in saying that, according to the instrumentalist and anti-realist positions, the wave function doesn't describe the properties of the system prior to measurement?

Yes.

In this sense then, the properties of the system prior to measurement - if such properties exist - would be considered hidden variables according to 1) above?

Yes.

I had thought that hidden variables necessarily implied an underlying deterministic Universe, but would stochastic interpretations that treat the wave function realistically also involve hidden variables?

Yes, in the sense 1) and 2).

 

[Demystifier]

It is this which leads me to question how anything without properties can interact with a measurement device in the first place?

Exactly!

 

[Lord Jestocost]

...that there is no mind-independent reality?

You have to distinguish between the "empirical reality" and the "ultimate reality". As Bernard d'Espagnat points out:

What quantum mechanics tells us, I believe, is surprising to say the least. It tells us that the basic components of objects – the particles, electrons, quarks etc. – cannot be thought of as "self-existent". The reality that they, and hence all objects, are components of is merely "empirical reality".

This reality is something that, while not a purely mind-made construct as radical idealism would have it, can be but the picture our mind forces us to form of ... Of what ? The only answer I am able to provide is that underlying this empirical reality is a mysterious, non-conceptualisable "ultimate reality", not embedded in space and (presumably) not in time either.

From: https://www.theguardian.com/science/blog/2009/mar/17/templeton-quantum-entanglement

 

[Lynch101]

I can only repeat what I've said. I believe you are conflating having a value with having a possible set of observable values. Things interact because they do and we have the formalism and data to prove it.

Apologies, I don't quite follow this point because I'm not sure that I am conflating those two.

If we go back to the contention that "it is meaningless to talk about the quantum system prior to measurement", this begs the question as to why it is meaningless. This usually stems from the instrumentalist notion that the mathematical formalism only gives probabilistic predictions for the outcomes of measurements. If the mathematics only describes the outcome of experiments i.e. only gives the probability that a particular measurement will be observed, then it doesn't appear to describe the properties of the system prior to measurement.

The istrumentalist position doesn't go any further than to say the mathematics only allows us to make probabilistic predictions. It is essentially the shut up and calculate position and isn't an interpretation of the mathematical formalism.

The anti-realist interpretation does go further and makes ontological deductions from the instrumentalist position. It says that properties don't have well defined values prior to measurement because there are no properties prior to measurement.

So, would it be accurate to say;

realism = systems have property values prior to measurement?

I would be more inclined to say that realism, at its most fundamental, is simply the position that systems have properties prior to measurement, not necessarily property values.

Since the realism view is born out of a classical macroscopic intuition we evolved with, I say that it's unsupported by the measurements and should be abandoned. There is no reason to expect it to hold for purely quantum objects.

If we take realism to be as outlined above, then anti-realism has, what I would think, is an insurmountable issue, namely, how something without any properties can interact with a measurement device in the first place.

 

[Lynch101]

Anti-realism re/ QM pertains to the anti-realism of properties considered by QM (i.e. those represented by regions of Hilbert space), as opposed to all properties entirely.

Thanks Morbert, this offers a different insight that I haven't come across before. I need a bit of help parsing what it means though.

I keep coming back to the shared position of instrumentalism and anti-realism, which says that the mathematical formalism is only a predictive tool which gives us the probability that a certain measurement outcome will be observed.

If the mathematical formalism only describes the probability that a certain measurement outcome will be observed, then it wouldn't seem to describe the properties of the system prior to measurement. The statement about the meaninglessness of talking about the quantum system prior to measurement seems to stem from this idea.

The instrumentalist position remains agnostic on the question of the properties of the system prior to measurement, while the anti-realist position appears to say that the mathematical formalism doesn't describe an ontic state and only gives us probabilistic measurement outcomes because there is no ontic state prior to measurement i.e. there are no properties whatsoever.If we take the interpretation you propose above that the anti-realist position of QM refers to anti-realism of properties considered by QM, does that mean that there are other properties which QM does not consider? Would these then qualify as hidden variables?

 

[Lynch101]

You have to distinguish between the "empirical reality" and the "ultimate reality". As Bernard d'Espagnat points out:

What quantum mechanics tells us, I believe, is surprising to say the least. It tells us that the basic components of objects – the particles, electrons, quarks etc. – cannot be thought of as "self-existent". The reality that they, and hence all objects, are components of is merely "empirical reality".

This reality is something that, while not a purely mind-made construct as radical idealism would have it, can be but the picture our mind forces us to form of ... Of what ? The only answer I am able to provide is that underlying this empirical reality is a mysterious, non-conceptualisable "ultimate reality", not embedded in space and (presumably) not in time either.

From: https://www.theguardian.com/science/blog/2009/mar/17/templeton-quantum-entanglement

Thanks Lord Jestocost, I would be inclined to ideas like that. I have only recently been introduced to d'Espagnat but I find myself nodding along in agreement with much of what I have read so far. I must read some more of his work.

This position would be a denial of the anti-realist position, wouldn't it?

 

[Lord Jestocost]

This position would be a denial of the anti-realist position, wouldn't it?

I don't think so. David J. Chalmers writes in "Ontological Anti-Realism":

"The basic question of ontology is “What exists?”. The basic question of metaontology is: are there objective answers to the basic question of ontology? Here ontological realists say yes, and ontological anti-realists say no."

 

[DrChinese]

So, would it be accurate to say;

realism = systems have property values prior to measurement?

Since the realism view is born out of a classical macroscopic intuition we evolved with, I say that it's unsupported by the measurements and should be abandoned. There is no reason to expect it to hold for purely quantum objects.

My thoughts exactly.

Einstein (1935, 1948) thought systems DID have property values prior to measurement. He defined the essential debate early on.

"According to [QM], the particle possesses neither a sharply defined momentum nor a sharply defined position. In which sense shall I imagine that this representation describes a real, individual state of affairs? Two possible points of view seem to me possible and obvious and we will weigh one against the other:

(a) The (free) particle really has a definite position and a definite momentum, even if they cannot both be ascertained by measurement in the same individual case. According to this point of view, the ψ-function represents an incomplete description of the real state of affairs. ...

(b) In reality the particle has neither a definite momentum nor a definite position; the description by ψ-function is in principle a complete description. The sharply-defined position of the particle, obtained by measuring the position, cannot be interpreted as the position of the particle prior to the measurement. ..."

Einstein believed (a). Bohr and most all other believed (b). Bell and Aspect had not yet come along to settle matters. So formulated as above, (a)=realist; (b)=anti-realist.

If you extend the "anti-realist" concept to a quantum interpretation, then you would reject Bohmian Mechanics and certain other interpretations. Those assert (a) explicitly. Of course, you can accept (b) - which is essentially a rejection of "local realism" - without picking a particular interpretation. But even then, you would essentially be adopting your perspective anyway, which translates to this:

anti-realism = systems don't have [well-defined] property values prior to measurement

 

[Paul Colby]

If we take realism to be as outlined above, then anti-realism has, what I would think, is an insurmountable issue, namely, how something without any properties can interact with a measurement device in the first place.

You keep repeating this and people here keep reinforcing this. It's a non-sensical statement, IMO.

Neglecting the point I was trying to make, let's take the DrChines's response to my Lotto number choice for example. His point was that classically he could take whatever picking device I chose and replace it with a prior device that would preselect the very same choice. This of course is nonsensical even from a classical perspective. Errors and noise would rapidly accumulate the further back in time one pushes the next choosing device. At some point, the residual quantum noise due to macroscopic objects really being composed of quantum ones would prevail. So even in the macroscopic case extrapolating a system to its infinite past is nonsense.

Now that said, Bell's theorem is a statement about the statistics of measurement. One uses measurements and statistics of those measurements showing that predefined values for a system can't exist. Okay, that not the same as the system doesn't exist or the system can't interact or the system isn't real.

 

[DrChinese]

Now that said, Bell's theorem is a statement about the statistics of measurement. One uses measurements and statistics of those measurements showing that predefined values for a system can't exist. Okay, that not the same as the system doesn't exist or the system can't interact or the system isn't real.

Agreeing with your essential comment: The quantum object is real and exists. Properties (of that object) don't seem to appear until a final measurement. The measurement requiring an interaction with a measuring device.

Obviously, my perspective gives weight to the concept of a "final measurement" - with all the problems associated with that. So again, that's where one's interpretation comes to the rescue (or not).

 

[Morbert]

The instrumentalist position remains agnostic on the question of the properties of the system prior to measurement, while the anti-realist position appears to say that the mathematical formalism doesn't describe an ontic state and only gives us probabilistic measurement outcomes because there is no ontic state prior to measurement i.e. there are no properties whatsoever.

If we take the interpretation you propose above that the anti-realist position of QM refers to anti-realism of properties considered by QM, does that mean that there are other properties which QM does not consider? Would these then qualify as hidden variables?

We should distinguish between a system having no ontic state, and the ontology of the system.

When we we say there is no ontic state, what we mean is we do not model the physics of the system terms of an ontic state or ontic state space. But we can still discuss the ontology/metaphysics of the system apart from our physical models. E.g. We can say the system exists. We can say the system interacts with our measurement apparatus in a predictable way. We can discuss the system as perhaps possessing some lawmaker property that's responsible for its dynamics etc.

 

[Lynch101]

I don't think so. David J. Chalmers writes in "Ontological Anti-Realism":

"The basic question of ontology is “What exists?”. The basic question of metaontology is: are there objective answers to the basic question of ontology? Here ontological realists say yes, and ontological anti-realists say no."

I'm mindful of the possibility of deviating too far from a discussion of QM interpretations on this point but the above would seem to turn on the meaning of "objective answers".

There are probably more than two ways to interpret the idea of "objective answers", but two examples I can think of that relate to QM would be taking it to mean:
a) measurables or observables
b) objective reality, independent from the mind of each person.

If the ontological anti-realist says that we cannot measure/observe ultimate reality, then this would be more a statement of realism with the implication being that there is a fundamental limit on how deep we can probe the world around us. But if that is the anti-realist position with respect to QM, to my mind, it would be tantamount to saying that there are hidden-variables and, again, would essentially be a realist position. It might be contrasted with an alternative realist position that says it will be possible to measure fundamental reality in the future, when we further refine our instruments.

In short, both ontological realism and anti-realism would, in this sense of "objective answers", point to an incomplete theory and the existence of hidden-variables (or "beables").The interpretations of anti-realism in QM, which I have encountered, don't seem to point to hidden-variables rather they deny the presence of them.Apologies for continually returning to this point, but I find it the easiest to digest and use as the basis for explanation and inquiry; if we go back to the question of the quantum system prior to measurement and talk about the mathematical formalism:

If we say that the mathematical formalism only gives us probabilistic predictions for the outcomes of experiments then it would seem to imply that the mathematics doesn't describe the quantum system prior to measurement. Am I correct in saying that this is what lead to the EPR paper and the charge of incompleteness?

If the mathematical formalism doesn't describe the quantum system prior to measurement we might then ask the question, is there a quantum system prior to measurement? If there is a quantum system prior to measurement then I think it is reasonable to state that it must have some/any properties, even if we cannot ascribe definite values to these properties. It might be down to my misinterpretation, but this to me sounds like a statement that there are hidden-variables.

The alternative to this, as far as I have reasoned thus far, would be the position that either there is no quantum system prior to measurement or that it has absolutely no properties whatsoever.