Anti-realist Interpretations of QM

[Lynch101]

TL;DR Summary

Interested in learning more about anti-realist interpretations of quantum mechanics and the consequences of such a position.

Having discussed interpretations of QM here and elsewhere, to only a basic level, I have encountered proponents of what is described as "anti-realism". This is not to be confused with "non-realism", which might be equated with simple instrumentalism or "shut up and calculate".

To try and clarify, from what I've read, anti-realism appears to go a step further than simple instrumentalism. Where instrumentalism says that the mathematics of QM are nothing more than a tool for calculating the probability of macro-level measurements, the anti-realist interpretation adds to this by saying that it doesn't make sense to talk about the system prior to measurement.

That statement - it doesn't make sense to talk about the system prior to measurement - in itself isn't anti-realist, but often when probed, some proponents seem to take an anti-realist position because the alternative i.e. talking about the system prior to measurement, would seem to necessitate the presence of hidden variables.

It is often argued that Neil's Bohr's position was anti-realist. I can't quite remember the quote closely enough to search for it, but there is a quote attributed to him which seems to suggest that reality doesn't exist until we measure it. Indeed, the views of Bohr prompted Einstein to say "I like to think that the moon is there, even when I'm not looking at it".

As Heisenberg put it, "the idea of an objective real world, whose smallest parts exist independently of whether or not we observe them, is impossible.

Bohr dismissed the idea of a quantum world altogether. "There is no quantum world", he said. "Isolated material particles are abstractions, their properties on the quantum theory being definable and observable only through their interaction with other syststems."

I have quite a few questions about the anti-realist interpretations of QM but to start:
1) What are some examples of anti-realist interpretations? I've heard that QFT can be interpreted as such. Are there others?
2) Do all the alternatives to anti-realist interpretations involve hidden variables?

 

[physika]

"reality doesn't exist until we measure it"

...and what exist before that
who/what exist before that...

 

[Paul Colby]

...and what exist before that

All the equipment and materials collected to actually take any real data?

 

[Halc]

It is often argued that Neil's Bohr's position was anti-realist. I can't quite remember the quote closely enough to search for it, but there is a quote attributed to him which seems to suggest that reality doesn't exist until we measure it. Indeed, the views of Bohr prompted Einstein to say "I like to think that the moon is there, even when I'm not looking at it".

This oft quoted quip is misleading. With one possible exception (Wigner), no interpretation suggests that the moon stops being there when nobody looks at it. It's been measured and can't be un-measured. It doesn't go away. A non-realist interpretation might assert there is no objective current state of the moon, only the state of a bit more than one second ago. The probability of the state one second from that measured state becoming "no moon" is zero to a wicked lot of decimal places.
From further away, the existence of the moon becomes more interpretation dependent.

I have quite a few questions about the anti-realist interpretations of QM but to start:
1) What are some examples of anti-realist interpretations? I've heard that QFT can be interpreted as such. Are there others?

Wiki lists a bunch of interpretations: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics
There's a nice colored chart about halfway down. If by realist you mean there is a state even when unmeasured, then 3 listed interpretations are realist (from 2nd to last column): Bohmian, Stochastic, and Transactional. That's quite a minority. They're all non-local interpretations as they must be.

2) Do all the alternatives to anti-realist interpretations involve hidden variables?

Apparently not. Of the 3 listed, Transactional doesn't claim hidden variables. I know nothing about that one. That's what the chart says.

I personally don't like RSF (relative state formulation) by Everett being lumped in with DeWitt's Many-Worlds. There are some very significant differences between the two.

 

[PeterDonis]

If by realist you mean

Note, though, that none of the columns in the chart are labeled "realist", and if we take "realist" at face value, it would seem like the closest thing to it is the second from the left column, "ontologically real wave function", which gives a different set of interpretations.

So probably the OP needs to define what he means by "realist" and "anti-realist" before we can answer further questions.

 

[PeterDonis]

probably the OP needs to define what he means by "realist" and "anti-realist"

In the OP, we have:

the anti-realist interpretation adds to this by saying that it doesn't make sense to talk about the system prior to measurement

To me, this means that "realist" could be either of the two columns in the Wikipedia chart that have been cited. So, @Lynch101, which is it? Does "anti-realist" mean the wave function is not ontologically real, or does it mean lack of counterfactual definiteness?

 

[Halc]

Note, though, that none of the columns in the chart are labeled "realist", and if we take "realist" at face value, it would seem like the closest thing to it is the second from the left column, "ontologically real wave function", which gives a different set of interpretations.

So probably the OP needs to define what he means by "realist" and "anti-realist" before we can answer further questions.

I had qualified my comment with a suggested definition of there being a state even when unmeasured, which seemed to align with the OP wording of : "saying that it doesn't make sense to talk about the system prior to measurement" which is almost exactly the counterfactual definiteness wording.

That wording doesn't seem to align at all with the "ontologically real wave function", so I didn't assume that column.
That column is closer to 'reality is real' than the counterfactual column. My take on that subject is that you don't have to explain the reality of whatever you consider to be real if you don't posit the reality of it. All the answers to the classic "why is there something instead of nothing" seem to commit some sort of fallacious reasoning.
I digress. This is a physics site, not a philosophy site, but QM interpretation is a philosophical topic after all.

 

[PeterDonis]

a suggested definition of there being a state even when unmeasured

And interpretations that say the wave function is ontologically real also make that claim, so they also say it does make sense to talk about the system prior to measurement. In other words, both columns--the "ontologically real wave function" column and the "counterfactually definite" column--qualify as "realist" according to the OP's definition, but they give a different list of interpretations. So the OP needs to clarify which list he means.

 

[physika]

All the equipment and materials

Maybe, but i mean who/what make real that stuf.

 

[Paul Colby]

Maybe, but i mean who/what make real that stuf.

The people that designed and built it. Signal generators or vacuum pumps are as real as it gets.

 

[Lynch101]

I'll try to outline my understanding of "anti-realism" in the context of QM by outlining [my interpretation] of certain statements I've encountered. The wiki article that @Halc posted might be helpful as a starting point.

A realist stance seeks the epistemic and the ontic, whereas an antirealist stance seeks epistemic but not the ontic. In the 20th century's first half, antirealism was mainly logical positivism, which sought to exclude unobservable aspects of reality from scientific theory.

Since the 1950s, antirealism is more modest, usually instrumentalism, permitting talk of unobservable aspects, but ultimately discarding the very question of realism and posing scientific theory as a tool to help humans make predictions, not to attain metaphysical understanding of the world. The instrumentalist view is carried by the famous quote of David Mermin, "Shut up and calculate", often misattributed to Richard Feynman.

This seems to describe the evolution of the "anti-realist" position from one that seeks to exclude unobservable aspects of reality* from scientific theory through to a purely instrumentalist, "shut up and calculate" (SUAC) one.

The purely instrumentalist, SUAC position isn't really an interpretation of QM, insofar as it remains agnostic on interpretational questions. For that reason I would distinguish it from an "anti-realist" interpretation, which would be one which excludes unobservable aspects.My understanding of the position has generally come from discussing it in the context of the question of completeness i.e. does QM give a complete description of the physical system? A question that usually arises in the context of this discussion is whether or not QM gives a complete description of the system prior to measurement. This is where I have encountered the statement above, that it is meaningless to talk about the system prior to measurement.

I have seen this generally interpreted in two ways:
a) Instrumentalism - it is meaningless because we cannot measure it.
b) Anti-realist - It is meaningless because it has no properties prior to measurement.Does that sound accurate, or have I misinterpreted something along the way there?*Am I right in saying that pertains to counterfactual definiteness or rather, the denial of it?

 

[Morbert]

I have seen this generally interpreted in two ways:
a) Instrumentalism - it is meaningless because we cannot measure it.
b) Anti-realist - It is meaningless because it has no properties prior to measurement.

"It is meaningless because we cannot measure it [a property of the system]" sounds like you are making an ontic commitment to a property existing even if we cannot know the property before measuring it. This would not be anti-realist, and so you're right to contrast it with b), but it would not be instrumentalist since an instrumentalist would not make an ontic commitment to the property existing.

Instead an instrumentalist understanding of "it is meaningless to talk about the system prior to measurement" might be "Our instruments cannot resolve alternative possibilities regarding a quantum system if those possibilities pertain to properties of the system prior to measurement".

 

[Demystifier]

1) What are some examples of anti-realist interpretations? I've heard that QFT can be interpreted as such. Are there others?

Anti-realism is not specific to QFT. All quantum theories, including non-relativistic QM, can be interpreted anti-realistically.

 

[Lynch101]

"It is meaningless because we cannot measure it [a property of the system]" sounds like you are making an ontic commitment to a property existing even if we cannot know the property before measuring it. This would not be anti-realist, and so you're right to contrast it with b), but it would not be instrumentalist since an instrumentalist would not make an ontic commitment to the property existing.

Instead an instrumentalist understanding of "it is meaningless to talk about the system prior to measurement" might be "Our instruments cannot resolve alternative possibilities regarding a quantum system if those possibilities pertain to properties of the system prior to measurement".

Thanks Morbert, this is much more precise.

This could then be contrasted with b) above, which says that the system does not have any properties prior to measurement. Is that accurate?

 

[Lynch101]

Anti-realism is not specific to QFT. All quantum theories, including non-relativistic QM, can be interpreted anti-realistically.

Surely not Bohmian mechanics though? Is that not specifically a realist interpretation?

 

[physika]

.

The people that designed and built it.

what/who make real that people ?

how?
If:

"reality doesn't exist until we measure it"

.

 

[Paul Colby]

how?
If:

The time honored confusion between the reality we inhabit versus mathematical models/theories of the microscopic. Aspects of this modeling doesn't conform to our macroscopic intuition. Seems simple enough. I don't know why they should.

The moon is there whether you look at it or not whereas the spin of a photon observed, that is a click of a single photon in a detector, didn't have this value before it was measured and doesn't have that value after it is detected.

 

[Demystifier]

Surely not Bohmian mechanics though? Is that not specifically a realist interpretation?

Yes, but I think you are making a category mistake. https://plato.stanford.edu/entries/category-mistakes/

The category of quantum theories includes theories such as nonrelativistic QM, QFT, quantum gravity and string theory. The theories like Bohmian mechanics, GRW collapse theory, many worlds and Copenhagen interpretation belong to another category. So you may talk about Bohmian version of QFT or Copenhagen interpretation of string theory, but you cannot talk about quantum gravity version of nonrelativistic QM or Bohmian version of GRW collapse theory.

 

[Lynch101]

the spin of a photon observed, that is a click of a single photon in a detector, didn't have this value before it was measured and doesn't have that value after it is detected.

I think I'm correct in saying that this would fall into the category of instrumentalism, while an anti-realist interpretation would say that it is meaningless to talk about the photon prior to measurement because it has no properties prior to being measured..

Is that an accurate delineation?

 

[Lynch101]

Yes, but I think you are making a category mistake. https://plato.stanford.edu/entries/category-mistakes/

The category of quantum theories includes theories such as nonrelativistic QM, QFT, quantum gravity and string theory. The theories like Bohmian mechanics, GRW collapse theory, many worlds and Copenhagen interpretation belong to another category. So you may talk about Bohmian version of QFT or Copenhagen interpretation of string theory, but you cannot talk about quantum gravity version of nonrelativistic QM or Bohmian version of GRW collapse theory.

Ah, I see. Thank you Demystifier, I wasn't clear on that distinction.

With regard to the clarification of the difference between instrumentalism and anti-realist interpretations, would it be accurate to contrast Morbert's clarification of instrumentalism with what I set out above, that the anti-realist interpretation says that it is meaningless to talk about the quantum system prior to measurement because, prior to measurement, the quantum system does not have any properties?

 

[Demystifier]

I would say that instrumentalism is to anti-realism what agnosticism is to atheism. In other words, instrumentalism is a soft version of anti-realism. Anti-realism says that things prior to measurement don't exist and/or are meaningless to talk about. Instrumentalism allows that things prior to measurement may exist and does not forbid to talk about them, but it does not care too much about such things because they are of secondary importance.

 

[Lynch101]

I would say that instrumentalism is to anti-realism what agnosticism is to atheism. In other words, instrumentalism is a soft version of anti-realism. Anti-realism says that things prior to measurement don't exist and/or are meaningless to talk about. Instrumentalism allows that things prior to measurement may exist and does not forbid to talk about them, but it does not care too much about such things because they are of secondary importance.

Nice analogy.

I was thinking that any alternative to anti-realist interpretations would necessitate hidden variables, but I think it's been mentioned in this thread that that is not necessarily the case. My thinking was that anti-realist interpretations say that the quantum system has no properties prior to measurement, therefore, any alternative interpretation would say that it does have properties prior to measurement. I was thinking that these properties would constitute hidden variables. Is there an alternative to that?

 

[Demystifier]

The notion of "hidden variables" has several different meanings:
1) Any variables additional to the wave function.
2) Any variables that have values even when they are not measured.
3) Variables that determine the measurable outcomes, so that the outcomes are actually deterministic, rather than random.

So what notion of "hidden variables" do you have in mind?

 

[DrChinese]

I have quite a few questions about the anti-realist interpretations of QM but to start:
1) What are some examples of anti-realist interpretations? I've heard that QFT can be interpreted as such. Are there others?
2) Do all the alternatives to anti-realist interpretations involve hidden variables?

Interpretations in which the observer is a participant in creating (objective) reality (i.e. the results of a measurement chosen by the subjective observer) could be called "anti-realist". Another label often applied is "subjective realism" although of course every person has their own meanings for these terms which may or may not be considered consensus.

In my perspective, the following are anti-realist interpretations (and I don't expect consensus agreement on these):

- Many Worlds Interpretation (MWI)
- Time Symmetric (TS) and Transactional Interpretation (TIQM), which may not be anti-realist by all standards
- Relational Blockworld (RBW)

 

[Paul Colby]

Is that an accurate delineation?

Not an expert on the schools of thought or whatever this thread is about so I don't know. However, the electromagnetic field (or better our abstraction of it) does have a state or an ensemble of states prior to measurement. We can make statistical statements about said measurements. The electromagnetic field exists and can be discussed prior to measurement. So I don't really get the anti-realists position at all.

 

[DrChinese]

The electromagnetic field exists and can be discussed prior to measurement. So I don't really get the anti-realists position at all.

The anti-realist says that a specific outcome only materializes once a final choice of measurement basis occurs. Therefore the outcome is observer dependent. And it is meaningless to assert that there was a well-defined value to the measured property without that.

 

[Paul Colby]

Thanks, that's generally what I thought. It's still seems like a stupid position.

The anti-realist says that a specific outcome only materializes once a final choice of measurement basis occurs.

One could say this of Lotto numbers. They aren't picked till they're picked.

Quantum mechanics only provides statistical statements about measurements. Measurements aren't measurements till the equipment is chosen set up and run. Yes, the results depend on how the experiment is structured. Now if one forgets all that and starts to try to visualize unmeasured quanta as little independent pre-existing bbs, things get mysterious and counter intuitive. So what?

 

[DrChinese]

1. It's still seems like a stupid position.

2. One could say this of Lotto numbers. They aren't picked till they're picked.

1. Not really. If you take the HUP literally, then momentum and position don't simultaneously have well-defined values. That is an anti-realist position. The realist says they have well-defined values at all times, regardless of observer.

2. I think most physicists would deny that classical stochastic behavior (represented by lotto balls) is an analog to quantum indeterminacy.

-----------------------------------

The reason for lining up behind the anti-realist position is simple: Bell leads us to be anti-realist if we reject FTL influences. Rejecting FTL influences is a very reasonable position. Of course, "reasonableness" is in the eye of the beholder.

 

[Paul Colby]

2. I think most physicists would deny that classical stochastic behavior (represented by lotto balls) is an analog to quantum indeterminacy.

You are certainly correct, nor did I imply such. The numbers aren't picked till they are picked. This is true even classically.

 

[DrChinese]

You are certainly correct, nor did I imply such. The numbers aren't picked till they are picked. This is true even classically.

I guess your definition of "imply" is different than mine, because you imply it again in the above. They aren't they same at all.

One is from lack of knowledge of something that is, in principle, knowable. (A high speed camera could show us the number to be picked in advance.) On the other hand: Non-commuting quantum properties do not seem to possesses that (except of course in realist interpretations of QM). There is not the slightest evidence - even in such interpretations - that non-commuting quantum properties are knowable in principle prior to measurement.

 

[Paul Colby]

A high speed camera could show us the number to be picked in advance.

You just redefined the measurement apparatus. The camera shows the pick. Big Deal.

Now, what you say about quantum measurement is certainly true. That wasn't what I was commenting on.

 

[Lynch101]

The notion of "hidden variables" has several different meanings:
1) Any variables additional to the wave function.
2) Any variables that have values even when they are not measured.
3) Variables that determine the measurable outcomes, so that the outcomes are actually deterministic, rather than random.

So what notion of "hidden variables" do you have in mind?

I would be inclined towards 1) and 3).

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? 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?

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?

 

[Lynch101]

Interpretations in which the observer is a participant in creating (objective) reality (i.e. the results of a measurement chosen by the subjective observer) could be called "anti-realist". Another label often applied is "subjective realism" although of course every person has their own meanings for these terms which may or may not be considered consensus.

What role does the wave function play in such interpretations? Am I wrong in thinking that it treats the wave function instrumentally also i.e. as a predictive tool?

Are such interpretations idealisitic in the sense that they say that there is no mind-independent reality?

 

[Lynch101]

Not an expert on the schools of thought or whatever this thread is about so I don't know. However, the electromagnetic field (or better our abstraction of it) does have a state or an ensemble of states prior to measurement. We can make statistical statements about said measurements. The electromagnetic field exists and can be discussed prior to measurement. So I don't really get the anti-realists position at all.

I struggle with the anti-realist position myself because [based on my possibly incorrect interpretation of it] I think there are some insurmountable hurdles to it. The question of how something without any properties can interact with a measuring device in the first instance, for example. Also, in what sense can something without properties even be said to be a part of the Universe?

As I say, that is based on my current interpretation of it but I have been known to be inaccurate in my understanding

 

[Lynch101]

The anti-realist says that a specific outcome only materializes once a final choice of measurement basis occurs. Therefore the outcome is observer dependent. And it is meaningless to assert that there was a well-defined value to the measured property without that.

Would that be closer to the instrumentalist position?

My understanding is that the anti-realist position has to go further than simply saying it is meaningless to assert that there was a well-defined value to the measured property, prior to the final choice of measurement basis. I think it has to deny that there is any property whatsoever prior to the measurement. To allow that there is some unquantified property of the system prior to measurement would, I was thinking, be a realistic, hidden variables interpretation?