I Why do we use the word interpretation?

[Jarvis323]

I've always struggled with the use of the word interpretation in the context of QM. Most so called interpretations do not seem like interpretations at all. Despite what might seem obvious, the conflation between the normal use of the word interpretation and the use of the word interpretation in QM seems to cause a great deal of confusion and ambiguity in the discussions.

Is this perception way off the mark? Why do people in the field of QM use this word, and what does it actually mean? Is it because of the historical conversations and careful/political wording to avoid appearing to challenge QM, a sort of loophole?

What word should we use to describe the so called QM interpretations?

 

[phyzguy]

QM has a well-defined mathematical structure that gives unambiguous answers to experimental questions, and the answers derived from this mathematical process agree with the experimental results. However, QM is silent on the physical meaning of the mathematical objects used in the calculation. As an example, there is no agreement on whether the wave function is a physical object like a magnetic field, or whether it is simply a mathematical tool to be used in the calculations. The assigning of physical meaning to the mathematical objects of QM is what we call an interpretation. To me it seems as good a word as any. Since different interpretations all give the same answer to experiments, there is no way to choose one interpretation over another, other than personal taste.

 

[Jarvis323]

Since different interpretations all give the same answer to experiments, there is no way to choose one interpretation over another, other than personal taste.

On this topic, I've found there are two points of view. The first is that what you say is a myth, and give as examples of interpretations which don't make the same predictions, spontaneous collapse models and Bohemian Mechanics. The second say that those interpretations aren't actually interpretations.

But for me, what are called different interpretations are really different beliefs, that often go beyond anything the math in QM says. I could almost agree with the second group that only the "interpretations" which don't change or add any predictions are really interpretations. But whether an assertion about the physical reality of QM mathematical objects is provable or not doesn't seem the right criteria to delineate what is an interpretation. To me, if you are saying something more than what the theory says, it's not what we should consider to be an interpretation, regardless if it is provable or not. For me, I am tempted to think that the only so called interpretation I know of which really is an interpretation, is the minimal statistical interpretation.

 

[Nugatory]

the conflation between the normal use of the word interpretation and the use of the word interpretation in QM seems to cause a great deal of confusion and ambiguity in the discussions.

No other English (or any other natural-language word, for that matter) seems any better. The underlying difficulty is that QM can only be stated without ambiguity and fuzziness using math, and the mathematical formulation of the theory does not map intuitively onto natural language. "Interpretation" is by no means the only problematic word: "particle", "wave", "observation", "observer" all mean something different in QM than in normal usage.

Is it because of the historical conversations and careful/political wording to avoid appearing to challenge QM, a sort of loophole?

History, yes. These problematic words were adopted by the physics community early in the 20th century before the modern formulation of QM, and once they entered into general usage there was no getting rid of them (sort of like how we still call islands in the Caribbean the "West Indies" even though we have since learned that they have about as much to do with India as they do with Siberia).

Political wording, no. There just isn't any better way of talking about these concepts.

 

[Nugatory]

and give as examples of interpretations which don't make the same predictions, spontaneous collapse models and Bohemian Mechanics. The second say that those interpretations aren't actually interpretations.

Both positions are reasonable as long as we're careful to state exactly what we mean by "interpretation"... but the debate is somewhat sterile as long as the different predictions are beyond our ability to distinguish experimentally.

(Do the Bohmians agree with you that that Bohmian Mechanics makes different predictions? If so, which ones?)

(I'm giving your post a like because I find the idea of Bohemian mechanics to be utterly delightful )

 

[phyzguy]

As @Nugatory points out, there are many words in physics where the meaning in physics is different from the meaning in everyday speech. Examples are 'force', energy', 'momentum', 'field', etc. You just need to learn what we mean when we talk about terms in physics, and you should not be surprised if different people use the same word in different ways. Such is the imprecision of natural language.

 

[WernerQH]

I think it's a warning sign that there are so many interpretations to choose from, and I can't believe that 22nd century physicists will still be debating these issues. Many physicists have tried to rationalize the situation, saying that every theory needs an interpretation. But most of them concede that Quantum Theory is special.

QM is just the back porch of an impressive edifice named QFT. The terms "particle" and "measurement" helped us to arrive at (or "derive") the equations. They are part of a scaffolding that was used to erect the building (QFT). But it is embarrasing if people insist on the scaffolding as an essential part of the edifice. Surely QFT can stand on its own, and already John Bell urged that there is no place for a concept like "measurement" in a fundamental theory.

It is often said that there were two revolutions in 20th century physics: Relativity (1905) and QuantumTheory (1925). But the dates signify quite different kind of events. The advent of Relativity signalled the completion of electrodynamics after a long gestation period (1864-1905) during which the equations were known, but their physical meaning under debate. I like to think that with respect to Quantum Theory we are in a similar gestation period (1925-2025?).

 

[Fra]

But for me, what are called different interpretations are really different beliefs, that often go beyond anything the math in QM says. I could almost agree with the second group that only the "interpretations" which don't change or add any predictions are really interpretations

I agree that "interpretations" have different levels of ambition. The ones you mention as the minimal ones, adds a minimum of explantoty value. These minimal ones typicaly focus on how to practically set things up, and how to map measured numbers to the model. They actively stay silent about other things.

But other "interpretations" has the ambition to add explanatory value, either in order to make it more graspable for humans, or to try to find an acceptable causal mechanism for things, or in order to make progress on open questions and this implicitly suggest a direction of modification of the theory at the border of it's domain of validity.

The absolute minimal interpretations, really does not raise much objections I think. The only objection you may have to that, is that it has no ambition to improve the theory. The typical argument is that as there is almost no experimentally accessible domains where the theory as it stands is limiting us, we do not need to care about this. This is a respectable position, from a pragmatic or engineering perspective. But from the perspective of intellectual inquiry I can not settle with that.

But here many theorist thing differently, because even so, there are obvious conceptual problems which begs for improvements. The goal is increased deeper understanding of the nature of interactios and their hierarchy, that can not be attained with just a patchwork of well corroborated theories. So so inclined will not get peace of mind until we have a coherent theory. So one can tell from peoples interpretations, in which direction they are looking for answers.

Edit: This brings us on the topic of philosophy of science again. The process of scientific progression is ugly and not straight. Any researcher obviously have some kind of "beleif" which is what the OT mentioned, wether it's admitted or not. I think that this belief is often indicated also in it's interpretations, so i agree with this association.

/Fredrik

 

[Demystifier]

Most so called interpretations do not seem like interpretations at all.

Are you saying that interpretations can only involve new talk about the theory, but not new equations? If not, then what exactly do you mean by interpretation?

 

[Demystifier]

What word should we use to describe the so called QM interpretations?

Theories? https://www.physicsforums.com/insights/against-interpretation/

 

[vanhees71]

QM has a well-defined mathematical structure that gives unambiguous answers to experimental questions, and the answers derived from this mathematical process agree with the experimental results. However, QM is silent on the physical meaning of the mathematical objects used in the calculation.

"QM" (or rather "QT") indeed tells you very precisely the meaning of the mathematical objects used in the calculation, namely the probabilities for the outcome of measurements given the preparation/state of the measured/observed system. The very fact that the "unambigous answers to experimental questions" of the "well-defined mathematical structure" is in excellent agreement with the observed facts shows that there are no interpretational problems with QT.

As an example, there is no agreement on whether the wave function is a physical object like a magnetic field, or whether it is simply a mathematical tool to be used in the calculations. The assigning of physical meaning to the mathematical objects of QM is what we call an interpretation. To me it seems as good a word as any. Since different interpretations all give the same answer to experiments, there is no way to choose one interpretation over another, other than personal taste.

That's, why I think the minimal interpretation is all there is in terms of physical theories. Why there seems to be a need of building some philosophical (or even religious?) world view around it, I never understood.

 

[Jarvis323]

Are you saying that interpretations can only involve new talk about the theory, but not new equations? If not, then what exactly do you mean by interpretation?

This is an interesting way to think about it.

I agree with @vanhees7 that the mathematical objects already have precise meaning. So their interpretation is a matter of only squaring that mathematical meaning with any underlying mathematical framework, and exploring what it directly tells us in that framework.

So I guess I would think the opposite, if you are involving new talk, or making new assertions, which aren't connected through the underlying mathematical framework, then it's not interpretation.

I also believe it's important to ponder about physical meaning. But QM doesn't say exactly what the physical meaning is of these objects, so trying to figure out what that physical meaning is by interpreting QM alone may be fruitless, because there are gaps which are not cross-able through interpretation. So I don't think that people are just interpreting QM when they do this. By defining this goal as interpretation, it would seem to imply or connotate that QM IS complete and fundamental and not subject to change. This seems like an unnecessary obligation.

 

[Demystifier]

So their interpretation is a matter of only squaring that mathematical meaning with any underlying mathematical framework, and exploring what it directly tells us in that framework.

I am trying to understand what exactly do you mean by "squaring" the mathematical meaning. For example, would formulating QM more rigorously in terms of functional analysis count as "squaring" of the mathematical meaning? Would that be an example of interpretation?

 

[A. Neumaier]

What word should we use to describe the so called QM interpretations?

The natural and time-honored word for it is 'interpretation of QM'.

That's, why I think the minimal interpretation is all there is in terms of physical theories. Why there seems to be a need of building some philosophical (or even religious?) world view around it, I never understood.

Because everyone wants to have an underlying common sense picture. Since 'common sense' differs between physicists, interpretations differ. Your view is not less religious than that of anyone with a deep conviction.

For me, maximal common sense is obtained in the picture painted in my new paper on QM via quantum tomography!

 

[WernerQH]

I also believe it's important to ponder about physical meaning. But QM doesn't say exactly what the physical meaning is of these objects, so trying to figure out what that physical meaning is by interpreting QM alone may be fruitless, because there are gaps which are not cross-able through interpretation. So I don't think that people are just interpreting QM when they do this. By defining this goal as interpretation, it would seem to imply or connotate that QM IS complete and fundamental and not subject to change. This seems like an unnecessary obligation.

I don't quite understand what kind of "non-crossable gaps" you have in mind. The mathematics is rather well defined and physicists apply it routinely without giving much thought to the fine points of "interpretation". The only gap that I perceive is between the ontology (the mathematical objects) of the theory and what people think that physics (QM) should be about. The theory is about wave functions, and people think it's about particles. As you said, QM is vague about the physical meaning of these "objects".

The central problem, in my view, is that we are strongly biased to think of the world around us as made up of "objects" (subject to "laws of nature"). QM is vague about the properties of these objects; what's left of electrons and photons in QED are just propagators, i.e. correlation functions. The "interpretation problem" of QED disappears when we learn to think of it not as a theory of particles, but as a machinery for computing correlations between events.

 

[Jarvis323]

I am trying to understand what exactly do you mean by "squaring" the mathematical meaning. For example, would formulating QM more rigorously in terms of functional analysis count as "squaring" of the mathematical meaning? Would that be an example of interpretation?

I think that sounds like what I imagine interpretatation to be. Or if QM isn't forumulated precisely in terms of its axioms/assumotions, then before interpretation you might need to establish a new formulation.

It's just my take. Whenever I say Many-Worlds Interpretation, for example, my brain signals me that what I've said doesn't make sense in my mental model.

The actual problem I think isn't really how interpretation is used by specific people, it's more that it doesn't have clear meaning, and people have arguments using the word, each with different takes, in a context that demands extreme precision.

 

[vanhees71]

The natural and time-honored word for it is 'interpretation of QM'.

Because everyone wants to have an underlying common sense picture. Since 'common sense' differs between physicists, interpretations differ. Your view is not less religious than that of anyone with a deep conviction.

For me, maximal common sense is obtained in the picture painted in my new paper on QM via quantum tomography!

"Common sense" can't work in situations, we never directly experience, i.e., it doesn't work in the "microscopic realm", and that's it's not so surprising that there other "rules"/"natural laws" apply than in everyday "macroscopic low-energy situations". The same holds for the "very large" (cosmology/astronomy) or the "very fast" (special relativity), where relativistic laws rule rather than every-day "common sense" Newtonian (or even Aristotle!) ones.

 

[Sunil]

What word should we use to describe the so called QM interpretations?

I think one has to understand the reason why there are so many interpretations. Why does one start to create a new interpretation of QM? To make sense of this strange theory. And the problem is not the measurement problem and so on. There would be no problem if this would be a particular theory, say, in biology of ants or so, where it is clear that human observers and the objects under study, ants, are different, and that we cannot observe what is inside the minds of the ants, but can only observe their behavior.

But how to make sense of QM? The Copenhagen interpretation would be fine for that theory of ants. We would know when to apply it, we would not try to apply it to humans, would not tend much to assign ants human abilities. But with the Copenhagen interpretation of QM all this is far from clear.

We have a boundary in reality between macroscopic and microscopic, which is quite objectively defined by decoherence. But this boundary does not tell much, it does not define a boundary of what could possibly be described in the quantum part and what has to be defined in the classical part. And there is the pretense that it has to be the theory of everything, thus, quantum mechanics should be sufficient to describe even macroscopic things.

But the macroscopic domain contains the trajectories we see in reality. And Schroedinger's cat shows that QM does not define them. So all this does not seem to make sense. And therefore on tries to make sense of it.

 

[A. Neumaier]

"Common sense" can't work in situations, we never directly experience,

Please show me a proof of this claim. It seems to me logically unfounded, and flatly contradicts my recent paper Quantum mechanics via quantum tomography. Everything there is common sense without misleading the reader.

 

[WernerQH]

Common sense can be very misleading. But you could call, for example, probability theory a refined form of common sense. And quantum theory even more so. :-)

 

[vanhees71]

Please show me a proof of this claim. It seems to me logically unfounded, and flatly contradicts my recent paper Quantum mechanics via quantum tomography. Everything there is common sense without misleading the reader.

If you call POVMs "common sense", we have very different notions of "common sense".

 

[A. Neumaier]

If you call POVMs "common sense", we have very different notions of "common sense".

Unlike Peres or Nielsen/Chuang, I do not assume POVMs but derive (a simple version of) them from simple common sense assumptions.

 

[Demystifier]

Unlike Peres or Nielsen/Chuang, I do not assume POVMs but derive (a simple version of) them from simple common sense assumptions.

Your paper is written too mathematically to perceive it as common sense. Do you perhaps have a simplified presentation of the main ideas?

 

[A. Neumaier]

Your paper is written too mathematically to perceive it as common sense. Do you perhaps have a simplified presentation of the main ideas?

All mathematics used is mastered by undergraduates after their first year. Thus it should be accessible to you, too. If you repeat your request on the page where I posted the link to my paper I'll try to give there a simplified version for dummies.

 

[vanhees71]

Your paper is written too mathematically to perceive it as common sense. Do you perhaps have a simplified presentation of the main ideas?

That's, however "not a bug but a feature". "Common sense" is not an argument for or against anything in the natural sciences. Even the 1st Newtonian postulate is not "common sense" in the most direct experience about the mechanics of everyday bodies. There Aristotle is much closer to "common sense".

 

[Demystifier]

"Common sense" is not an argument for or against anything in the natural sciences.

Yes, but to find motivation to start reading a long technical paper, it's nice to first have a simple intuitive picture of what this paper is about and why it could be interesting.

 

[vanhees71]

Sure, but I'd like to read no-nonsense physics without philosophical confusion with not clearly defined words like "ontology", "realism", etc.