A Is quantum theory a microscopic theory?

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Demystifier

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If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?
Quantum theory is widely thought to be a theory of the fundamental microscopic constituents of matter. It is supposed to tell us something about how matter behaves at the fundamental microscopic level, from which the classical macroscopic behavior should somehow emerge as an approximation based on averaging over a large number of microscopic constituents. But is that really so? Does quantum theory really tell us something about the microscopic world? Or is it just a macroscopic theory which only tells us that macroscopic apparatuses sometimes behave differently than classical macroscopic apparatuses?

No doubt, some interpretations of quantum theory, such as the Bohmian interpretation, do explicitly tell us something about the microscopic world. But here I don't want to talk about such interpretations. I want to talk about the minimal instrumental view of quantum theory, which refrains from saying anything about quantum interpretations except that which is directly based on experimental evidence. So does such a minimal instrumental form of quantum theory tell as anything about the microscopic world?

Contrary to a widespread belief, I think it doesn't. The minimal instrumental form of quantum theory is nothing but a set of rules to predict the probabilities of measurement outcomes. And since all measurement outcomes are macroscopic events, the minimal instrumental quantum theory is not a theory of the microscopic world.

Or if you disagree, can you explain in what sense is quantum theory a microscopic theory? Can you really argue that quantum theory is a microscopic theory without assuming (either explicitly or implicitly) some interpretation that goes beyond the minimal instrumental view of quantum theory?

Sure, the minimal quantum formalism does contain objects, such as particle position operator or field operator, that are in a certain sense microscopic objects. But they are merely tools to compute the probabilities of macroscopic measurement outcomes. In this sense minimal quantum theory is not about local objects such as position or field operators. The minimal quantum theory is about macroscopic measurement outcomes, while the local objects above only make sense if they can be somehow used to predict the properties of macroscopic measurement outcomes. Hence the microscopic objects by themselves have no purpose, and hence no meaning at all, if the minimal instrumental view of quantum theory is adopted.
 
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A. Neumaier

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Summary: If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?
The premise is very questionable.

The minimal instrumental form of quantum theory is nothing but a set of rules to predict the probabilities of measurement outcomes. And since all measurement outcomes are macroscopic events, the minimal instrumental quantum theory is not a theory of the microscopic world.
Only in the same sense as that a medicine is only a theory for predicting the outcomes of diagnostic tests and not a theory of how the human body works.
 

Demystifier

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Only in the same sense as that a medicine is only a theory for predicting the outcomes of diagnostic tests and not a theory of how the human body works.
I used a similar analogy when someone who knows a lot about medicine but not so much about physics asked me what do I do as a physicist. I wanted to say that I do foundations of physics, so I explained that foundations of physics is to physics what medicine physiology is to medicine. The medicine physiology is not so much about methods of healing as it is about how the human body actually works. Likewise, foundations of physics is not so much about making measurable predictions as it is about how nature actually works. Indeed, from the point of view of a clinical medicine practitioner, some details in medicine physiology may look as a kind of almost useless "philosophy".
 
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martinbn

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There was a quote by Weinberg that said something like "Cosmology is the science of making predictions about images on photographic plates".
 

vanhees71

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Well, it's a theory about microscopic objects in the sense that it (also) deals with exactly such objects. An example are experiments with single photons, which are the "indivisible quanta" of the electromagnetic field in the sense that you detect one or nothing if you deal with a single-photon state (that's however only valid in the realm of linear quantum optics; in the non-linear regime you have up- and down-conversion processes).
 

martinbn

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Don't want to spam, but just remembered "The existence of dinosaurs is just an interpretation of our best theory of fossils."
 

Demystifier

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Well, it's a theory about microscopic objects in the sense that it (also) deals with exactly such objects. An example are experiments with single photons, which are the "indivisible quanta" of the electromagnetic field in the sense that you detect one or nothing if you deal with a single-photon state (that's however only valid in the realm of linear quantum optics; in the non-linear regime you have up- and down-conversion processes).
The bold words above should show why do I think that you are contradicting yourself. If the indivisible objects are defined through their detection, where detection is by definition a macroscopic event, then, by the very same definition, those indivisible objects are not microscopic.
 

TeethWhitener

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The premise is very questionable.
I tend to agree. In particular, "macroscopic" and "microscopic" need precise definitions to answer the question effectively. @Demystifier, is there an answer that would satisfy you? I tend to think that "macroscopic" theories require observables to be ensemble averages of some kind, so in this sense quantum mechanics would be a microscopic theory.
 

Demystifier

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@Demystifier, is there an answer that would satisfy you?
My answer is that quantum theory makes sense as a theory of the micro world only if one goes beyond the minimal view and adopts some ontic interpretation.
 

TeethWhitener

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My answer is that quantum theory makes sense as a theory of the micro world only if one goes beyond the minimal view and adopts some ontic interpretation.
My point was that, without a precise definition of "microscopic," I don't think anyone can assert anything meaningful about the question. If you have a definition of microscopic that doesn't work with the instrumentalist interpretation, so be it. So far, it just seems tautological: in order to know anything about anything, some detection scheme needs to happen. If you consider detection to be necessarily macroscopic, then wouldn't all theories about which meaningful data can be gathered be macroscopic? In which case, "microscopic" would be equivalent to "metaphysical," or at the very least, "unobservable."
 

Demystifier

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My point was that, without a precise definition of "microscopic," I don't think anyone can assert anything meaningful about the question. If you have a definition of microscopic that doesn't work with the instrumentalist interpretation, so be it. So far, it just seems tautological: in order to know anything about anything, some detection scheme needs to happen. If you consider detection to be necessarily macroscopic, then wouldn't all theories about which meaningful data can be gathered be macroscopic? In which case, "microscopic" would be equivalent to "metaphysical," or at the very least, "unobservable."
My point is slightly different. All detections can indeed be considered macroscopic (despite the the fact that there is no sharp borderline between micro and macro), yet some theories can be microscopic. That's because theories are not only about the detections, but also about something undetectable. An excellent example @martinbn mentioned above is the example of dinosaurs, which clearly is not merely a theory of the detectable fossils, but a theory of undetectable living beings. So insisting that a scientific theory should only be a theory of the detectable is ridiculous.
 

Demystifier

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The point of this thread can also be viewed as a weaker variant of the thesis by Tim Maudlin in his recent book https://www.amazon.com/dp/069118352X/?tag=pfamazon01-20 . He argues that the standard "Copenhagen" quantum mechanics is not a physical theory at all, but merely a calculation recipe. (His examples of actual physical theories are Bohmian mechanics, GRW collapse theory and many-world theory). I argue that standard quantum mechanics is a physical theory, but not a microscopic physical theory.
 

TeethWhitener

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My point is slightly different. All detections can indeed be considered macroscopic (despite the the fact that there is no sharp borderline between micro and macro), yet some theories can be microscopic. That's because theories are not only about the detections, but also about something undetectable. An excellent example @martinbn mentioned above is the example of dinosaurs, which clearly is not merely a theory of the detectable fossils, but a theory of undetectable living beings. So insisting that a scientific theory should only be a theory of the detectable is ridiculous.
Again, this still lacks a definition of micro/macroscopic. Your answer seems to imply that only detections are macroscopic (macroscopic = detection). Are there other features in your definition of macroscopic-ness besides detection? Per your dinosaur example, is paleontology a microscopic theory because it posits unobservable living animals?
 

atyy

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Again, this still lacks a definition of micro/macroscopic. Your answer seems to imply that only detections are macroscopic (macroscopic = detection). Are there other features in your definition of macroscopic-ness besides detection? Per your dinosaur example, is paleontology a microscopic theory because it posits unobservable living animals?
You can just use the standard definitions in quantum theory. Macroscopic are the things on the same side of the classical/quantum cut as the outcomes that the Born rule refers to (eg. measurement apparatuses). Microscopic are the things in the quantum state (eg. electrons). Is the measurement apparatus made of electrons? That standard interpretation is silent on this issue. If one complains that the classical/quantum cut is not precise, that is a fault of quantum theory itself, as Bell explains in Against 'measurement'.
 

samalkhaiat

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Summary: If quantum theory is nothing but a set or rules to compute the probabilities of macroscopic measurement outcomes, then what is microscopic about it?
I am afraid there is no case to answer. Micor/macro worlds make sense provided we agree to use standard scales for length, time and mass. And we use well-defined experimental procedures to define those scales.
 

vanhees71

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There is no Micro-macro cut. Quantum theory is a description of everything objectively observable in nature (nothing more but also nothing less) except gravity. In this sense it's incomplete as is any theory so far. If you want something beyond, like religion, phisophy etc. it's no more science. Scientific theories are by definition restricted to empirically objectively testable phenomena but not about personal views on ontology. That's the same time its strength, providing among the pure joy of knowledge also a lot of practical advantages for mankind like the invention of all kinds of gadgets like this laptop I'm writing this posting on. That this is, of course, not a comprehensive collection of all human knowledge is also clear, but it is important to keep the subjects separated and not to mix them in ways which lead to (sometimes quite dangerous) errors.
 

Lord Jestocost

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The point of this thread can also be viewed as a weaker variant of the thesis by Tim Maudlin in his recent book https://www.amazon.com/dp/069118352X/?tag=pfamazon01-20 . He argues that the standard "Copenhagen" quantum mechanics is not a physical theory at all, but merely a calculation recipe. (His examples of actual physical theories are Bohmian mechanics, GRW collapse theory and many-world theory). I argue that standard quantum mechanics is a physical theory, but not a microscopic physical theory.
In the description of Maudlin’s book one reads:

“Maudlin argues that the very term “quantum theory” is a misnomer. A proper physical theory should clearly describe what is there and what it does…….”

Now, David Bohm's understanding of "theory" (in “Wholeness and the Implicate Order”):

“The relationship between thought and reality that this thought is about is in fact far more complex than that of a mere correspondence. Thus, in scientific research, a great deal of our thinking is in terms of theories. The word ‘theory’ derives from the Greek ‘theoria’, which has the same root as ‘theatre’, in a word meaning ‘to view’ or ‘to make a spectacle’. Thus, it might be said that a theory is primarily a form of insight, i.e. a way of looking at the world, and not a form of knowledge of how the world is.” [italics in original, LJ]
 

vanhees71

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Well, I don't understand Maudlin's statement since quantum theory does right this: It describes what is there and what it does. It doesn't mean that you can derive from the general formalism of QT "what is there", but that has to be put in from observation, and that's how we got to the Standard Model. I strongly recommend to study the history of elementary particle physics. It's one paradigmatic example for how the scientific method works and which models/theories are finally successful: It are these models that stay close to the observations but at the same time seek for fundamental principles that can summarize the empirical findings as well as the theoretical methodology: In the case of the Standard Model (as of all 20th century physics) it were the symmetry principles that lead to a comprehensive description what "is there".

Of course, it's only a description of what is clearly observed, which is according to our presen understanding only about 4% of what there is in total). But also this discovery is just based on observations analyzed with help of the theories yet developed. It's one of the hints to future even better theories.

Nevertheless QT fufills Maudlin's demand to describe a large piece of "what there is" and of course "how it behaves".
 

TeethWhitener

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You can just use the standard definitions in quantum theory. Macroscopic are the things on the same side of the classical/quantum cut as the outcomes that the Born rule refers to (eg. measurement apparatuses). Microscopic are the things in the quantum state (eg. electrons). Is the measurement apparatus made of electrons? That standard interpretation is silent on this issue. If one complains that the classical/quantum cut is not precise, that is a fault of quantum theory itself, as Bell explains in Against 'measurement'.
Ok, I freely admit I’m not well-versed in interpretations of quantum mechanics, but this definition seems to imply that only quantum theories can have this microscopic/macroscopic distinction. So for example, Boltzmann’s kinetic theory of gases is a macroscopic theory as long as the particles in question are treated as classical particles. Which is fine, if that’s the definition everyone agrees on. But I have to admit, it sounds a little weird.
 

vanhees71

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There is no classical/quantum cut to be well defined within quantum theory. It's more a matter of ability to isolate a macroscopic system well enough from "the environment" to prevent decoherence to observe "quantum effects" than anything else.

There are plenty of macroscopic quantum effects known, and it came with the ability to cool down matter to very low temperatures. One of the first such effects is superconductivity, then there came superfluidity, the "anomalous behavior" of macroscopic properties like heat capacity and what not.

Also the quantum behavior like entanglement of macroscopic (collective) modes has been observed like vibrations (phonons) of diamonds.

Last but not least quite large molecules like buckyballs, cooled sufficiently down to avoid thermal radiation of photons, which leads to decoherence, can be used to demonstrate "wave-like properties" like interference patterns in the well-known double-slit experiment.

What's "microscopic" is in my opinion rather a matter of scale. E.g., in celestial mechanics with very good approximation we can treat stars, planets, moons, etc. as "mass points" since compared to the distances relevant for their motion their extension can be neglected. In a sense a "mass point" is a microscopic object in classical mechanics. The same holds true in the realm of what's usually considered really "microscopic". Also there you have several layers of scales. E.g., there's the scale of molecules, which can be treated as elementary objects, e.g., in describing gases. The molecules themselves consist of atoms, which are made of a nucleus (on the scale of atomic physics a "point particle", this time in the sense of quantum mechanics) and the surrounding electrons. The atomic nuclei themselves are built of (on the "nuclear scale" elementary, pointlike) protons and neutrons. Looking even further, the nucleons are also found to be extended objects consisting of quasiparticles called constituent quarks and (even better resolved) fluctuating color fields (gluons and sea quarks).
 

atyy

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Ok, I freely admit I’m not well-versed in interpretations of quantum mechanics, but this definition seems to imply that only quantum theories can have this microscopic/macroscopic distinction. So for example, Boltzmann’s kinetic theory of gases is a macroscopic theory as long as the particles in question are treated as classical particles. Which is fine, if that’s the definition everyone agrees on. But I have to admit, it sounds a little weird.
Many people do think it is weird. The standard Copenhagen-style interpretation of quantum mechanics has the famous measurement problem, which other interpretations try to solve. Even Landau and Lifshitz comment on this unusual aspect of quantum mechanics in their famous textbook.
 
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I want to talk about the minimal instrumental view of quantum theory, which refrains from saying anything about quantum interpretations except that which is directly based on experimental evidence. So does such a minimal instrumental form of quantum theory tell as anything about the microscopic world?
Has our knowledge of chemistry improved as a result of our understanding of quantum mechanics?

Do you consider atoms, molecules, atomic bonds, etc. to be microscopic?

If the answer to both of those is yes, then quantum theory has told us about features of the microscopic world.

In addition to chemistry, I'd add solid state physics.

If transistors were in fact discovered and improved to the state of the art we have today based on predictions of quantum mechanics, then the answer to your question is "yes."
 

TeethWhitener

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Many people do think it is weird. The standard Copenhagen-style interpretation of quantum mechanics has the famous measurement problem, which other interpretations try to solve. Even Landau and Lifshitz comment on this unusual aspect of quantum mechanics in their famous textbook.
The weirdness I was referring to had nothing to do with the measurement problem or even any interpretation of quantum mechanics. I meant that I found it weird to use the word “microscopic” to refer only to objects on the quantum side of the Heisenberg cut—hence my example of classical statistical mechanics. It seemed like a really good example of a theory with a microscopic/macroscopic divide: ensemble averages are emergent from collective dynamics of constituent particles (and moreover, the motions of the constituent particles are largely unobservable). But if everything classical is automatically macroscopic, then classical statistical mechanics is purely macroscopic.
 
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My answer is that quantum theory makes sense as a theory of the micro world only if one goes beyond the minimal view and adopts some ontic interpretation.
I believe once understood properly it makes perfect sense. I have never had any problem, except when I first learnt it. Each year many students pass QM courses with no problems suggesting they do as well. What is much more difficult is figuring out what it means. We know the 'why' of it ie it allows continuous and differentiable transformations between pure states. Mathematically this is very convenient - and we do not understand why mature is so obliging. But this is nothing new - until we had QM at our disposal we didn't understand the reason for Lagrangian's in classical mechanics. That too is very convenient because it means we can apply Noether. I am hopeful something similar will happen in QM - but who knows when. When that happens perhaps what it means will take a different turn and we will want to know what that new knowledge means. Maybe this is a moving frontier and we will always have questions - turtles all the way down perhaps :DD:DD:DD:DD:DD:DD:DD:DD:DD

Those that have read my posts over the years will noticed my views have changed a bit over time.

Thanks
Bill
 

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