What Defines the Standard and Realist Views in Quantum Mechanics?

[vanhees71]

If there's nothing "real" propagating between the source and the detector, why do experimentalists need all those fibre-optic links between them?

Of course, there's something real propagated, i.e., the electromagnetic field. According to QFT everything is described by quantized fields.

More on topic, I think everyone has an ontology. But the camps for me would be:

• A) those who suggest an ontology beyond the observations
  1. Those who believe the equations themselves are the "stuff" that exists beyond the observations
  2. Those who believe the equations are only a tool for performing calculations and do not necessarily correspond to real things in the world

Whatever it might be, what's "beyond the observations" it's not, what's studied within the natural sciences. With this you clearly go into other realms of human experience than objective observations of phenomena.

• B) Those who believe observations are all that exists and it's forbidden to talk about what may be there beyond observations

It's pretty likely that there is something beyond what we can currently observe with our contemporary means of technology, e.g., "dark matter".

I think you can't straddle more than one of these categories and stay consistent.

A good ontology may lead to testable predictions that ultimately could improve our theories and understanding of nature. A bad ontology can lead to stagnation if it doesn't give any ideas about where to look for new things or discourages further investigation. It's obvious how Camp B can lead to stagnation, but the pitfalls of A1 can't be ignored either.

Of course, it's always worth aiming at ever more sensitive detectors/measurement devices to discover "new stuff".

 

[WernerQH]

Of course, there's something real propagated, i.e., the electromagnetic field.

So you are finally outing yourself as a realist!

 

[WernerQH]

@vanhees71, how does that square with your previously expressed view that the Bell-type experiments force us to give up realism in favour of locality? If the quantum field is real and in a definite state at all times, a naive physicist would expect this to apply also to the polarization state of a photon. Shouldn't this be derivable from the state of the quantum field? But somehow the polarization becomes real (definite) only when photons encounter detectors, in a peculiar way that you insist on calling local.

 

[vanhees71]

But the polarization state for the entangled photons is clearly determined by the preparation procedure. I don't know, what you are looking for.

 

[WernerQH]

But the polarization state for the entangled photons is clearly determined by the preparation procedure.

It is not. Not if you think of the entangled photons as two photons, each with its own polarization state. I understand that you insist on describing the two photons as a single system, but I fail to see what you accept as "real". The two photons have indefinite polarization before the detection, but acquire a definite polarization state at the moment they cease to exist?

 

[vanhees71]

Of course the single photons are simply unpolarized and prepared in the corresponding mixed state. The state of a subsystem is given by the partial trace (i.e., tracing out the other subsystem). As I said the state of the full system as well as all of its subsystems is fully determined by the preparation of the photon pair in an entangled pair. The general quantum state is of course a mixed state.

What's real are the observable facts about the system, and these are the detector clicks for the photons.

 

[WernerQH]

Of course the single photons are simply unpolarized

Most physicists think of the polarization of a photon as real. It can be measured, after all. Talk of a property that a combination of these objects must have, but not the two objects individually until some intervention occurs, doesn't make sense to me. I know the formalism and that (after years of habituation!) it "suggests" the wording used in discussing the experiments. But I can't find it a reasonable use of language to talk of "photons" as objects. I know you dislike the word "photon" and prefer to speak of quanta (or $\gamma$'s), but it doesn't make the awkwardness go away.

 

[vanhees71]

Of course, polarization of photons is an observable and thus they are "real". As any observable within QT polarization (to be concrete take helicity as the observable) doesn't need to take a definite value. Whether that's the case depends on the state the photon is prepared in. I've never understood all the "hype" about what's "real" and what's "not real" in QT. It's not a very sharply defined notion anyway. For me, simply everything that can be observed and measured is real.

 

[lodbrok]

Whatever it might be, what's "beyond the observations" it's not, what's studied within the natural sciences. With this you clearly go into other realms of human experience than objective observations of phenomena.

Isn't the electromagnetic field between the source and the detector "beyond the observations"? It appears to be an appropriate subject of the natural sciences. Sure, you can place a detector there and detect it, but once you take that detector away, it appears you believe there's something there beyond the observations -- that's Camp A.

 

[vanhees71]

Sure is there something, namely the em. field. We can observe it through its interaction with charged particles, which is the case within both classical and quantum electrodynamics. It seems to me that philosophers tend to overcomplicate things rather than clarifying them.

 

[lodbrok]

Sure is there something, namely the em. field. We can observe it through its interaction with charged particles, which is the case within both classical and quantum electrodynamics. It seems to me that philosophers tend to overcomplicate things rather than clarifying them.

On the contrary, Philosophy is important for Science: https://www.pnas.org/doi/10.1073/pnas.1900357116

A knowledge of the historic and philosophical background gives that kind of​

independence from prejudices of his generation from which most scientists​

are suffering. This independence created by philosophical insight is—in my​

opinion—the mark of distinction between a mere artisan or specialist and a​

real seeker after truth.​

— Albert Einstein, Letter to Robert Thornton, 1944​

Still very relevant to this generation.

 

[Demystifier]

So you are finally outing yourself as a realist!

@vanhees71 is sometimes realist and sometimes non-realist, but he is consistent because he is never both at the same time. His philosophy of QM is contextual, it depends on the question one asks him. Nevertheless his brain is classical, because it does not produce long-range correlations. For example, if you ask him a) Is it true that only measurable things are real?, and if in another post you ask him b) Is it true that electromagnetic field between two measurements is real?, there will be no correlation between the two answers.

 

[vanhees71]

I am a realist, if "realism" refers to objectively observable, quantitative facts about phenomena in Nature. I don't understand, why you think there's no correlation between the answers a) and b). The electromagnetic field is real, because it is measurable through its interactions with other fields, describing charged particles. For me on the most fundamental level physics is described by a spacetime model (the most comprehensive one is General Relativity or, most probably, some extension of it including torsion like Einstein-Cartan theory) and local relativistic QFT. Within this scheme, though incomplete, because the gravitational interaction with its strong correlation to spacetime structure is not yet fully understood within quantum (field?) theory, it's very clear what's "real", i.e., observable and what is not. The electromagnetic field is clearly observable and thus real (the electromagnetic potentials are not, because they are not gauge invariant).

 

[WernerQH]

I've never understood all the "hype" about what's "real" and what's "not real" in QT. It's not a very sharply defined notion anyway.

Ignoring a problem is an option. But being unable to perceive it can be a disadvantage.

 

[martinbn]

@vanhees71 is sometimes realist and sometimes non-realist, ...

This is your fault(QM foundations community). Because you use the word "real" with different meanings. He is a realist in the sense that objective reality exists, say the em field. He is also an anti-realist in the QM sense that observables have no values unless they are measured.

 

[vanhees71]

Ignoring a problem is an option. But being unable to perceive it can be a disadvantage.

A clear formulation of the question is more than half the way to the solution of the problem!

 

[WernerQH]

The electromagnetic field is real, because it is measurable through its interactions with other fields

It's an abstraction, and by no means dictated by observations, but dependent on your mindset, world view, or preferred theory. Decades before Faraday physicists grappled with electricity and magnetism and did not perceive the field concept as "obvious".

I'm not denying its usefulness and calling it real (far from it!), but it is a classical concept. In my view, just "quantizing" it has not sufficiently refined the concept to satisfy mathematicians. Not to mention baffled students of QED.

A clear formulation of the question is more than half the way to the solution of the problem!

Exactly. Einstein perceived the problems of electrodynamics more clearly than others. (He did not "shut up and calculate".)

 

[vanhees71]

This is your fault(QM foundations community). Because you use the word "real" with different meanings. He is a realist in the sense that objective reality exists, say the em field. He is also an anti-realist in the QM sense that observables have no values unless they are measured.

Observables take not determined values by being measured but by the preparation of the system in a corresponding state. If the system is determined in a state, where the outcome of the corresponding measurement is uncertain, the observable simply doesn't take a determined value and thus the state preparation only provides probabilities for each possible outcome of a measurement of this observable, and that's "real", at least according to all so far known observations.

 

[Morbert]

If the system is determined in a state, where the outcome of the corresponding measurement is uncertain, the observable simply doesn't take a determined value

If by "an observable takes a determined value" you mean the outcome of a future measurement of that observable, if it is performed, is known with certainty, then that is uncontroversial.

If instead you mean the microscopic system has a real property corresponding to an eigenvalue of the observable, regardless of whether or not you measure it, then that is a realist position and is controversial.

 

[vanhees71]

Scripsi scripsi! If the state is such that the observable doesn't take a determined value, then it doesn't take a determined value. The meaning of the state is only that it provides the probability for finding any possible value of any observable (an eigenvalue of the representing self-adjoint operator of this observable) when it is (accurately enough) measured. The observable itself always exists, independent of the state, i.e., it can be measured with an appropriate measurement device indepenent of the state the system is prepared in.

 

[martinbn]

It's an abstraction, and by no means dictated by observations, but dependent on your mindset, world view, or preferred theory. Decades before Faraday physicists grappled with electricity and magnetism and did not perceive the field concept as "obvious".

But they were puzzled by some aspects of electricity and magnetism. For example current in a wire deflects a near by compass without any contact.

Also what is the alternative? If there is no em field, then what accounts for the observations? An action at a distance perhaps, but it is a strange one because it is at a distance and propagates with finite velocity!

 

[vanhees71]

The alternative is "action at a distance". Of course, it's indeed a re-conception to use "fields and the local-interaction principle" instead of the concept of "action at a distance". Interestingly already Newton had his quibbles with his action-at-a-distance description of the gravitational interaction. With Faraday and Maxwell in addition it started to become clear that the "em. field" can be interpreted as a dynamical quantity itself, but initially this was even for such progressive thinkers too far-fetched and thus there was, particularly in Maxwell's case, the idea of an aether, i.e., a "mechanical fluid-like medium" an essential idea to understand what we call nowadays the em. field.

Of course such arguments as about what's a "particle" or "field" "ontology or a "reality criterion" or in general anything concerning our epistemic views are theory driven, and of course one must discuss them within the theory we like to find a epistemic or even "ontological" interpretation for. For me, QT on the fundamental level must be discussed using relativistic QFT, when it comes to the question, what "locality" and "realism" means. For me locality makes only sense within a relativistic theory, and the only "realistic" (i.e., applicable to real-world observations) version of a relativistic QT is local relativistic QFT, is based on the "local-field-interaction concept" to make it consistent with the relativistic causality structure following from the relativistic spacetime model, and indeed there is no question that QFT is "local" in this sense, i.e., that there are no causal connections between space-like separated events, and that's implemented in basic foundation of the theory in terms of the "microcausality constraint" on the operators that represent local observables.

Realism is even less clearly defined in the philosophers' literature. It's not entirely clear to me, what they really mean. For me the most rational definition is that realism assumes as a given fundamental fact that there are "observables", i.e., quantifiable descriptions of phenomena and that these observables always take determined values. Both non-relativistic AM and relativistic QFT clearly contradict the reality criterion, i.e., observables take only determined values when the system under consideration is in a corresponding state. Otherwise there are only probabilities for the outcome of measurements on the system, and that's not a lack of information as in classical statistical mechanics ("subjective probabilities") but inherent in the very definition of "state" and "observable" within quantum theory ("objective/irreducible probabilities").

Since relativistic QFT is strictly "local" in the above sense but for sure not "realistic" and in accordance with all (objective quantitative) observations made so far, my conclusion is that Nature behaves according to the concept of locality (realized through local-field descriptions of interactions) but not according to the "realistic description" of classical physics (be it Newtonian point-particle or continuum mechanics or relativistic field theory).

 

[DrChinese]

Observables take "not determined" values by being measured but by the preparation of the system in a corresponding state. If the system is determined in a state, where the outcome of the corresponding measurement is uncertain, the observable simply doesn't take a determined value and thus the state preparation only provides probabilities for each possible outcome of a measurement of this observable, and that's "real", at least according to all so far known observations. ... If the state is such that the observable doesn't take a determined value, then it doesn't take a determined value. The meaning of the state is only that it provides the probability for finding any possible value of any observable (an eigenvalue of the representing self-adjoint operator of this observable) when it is (accurately enough) measured. The observable itself always exists, independent of the state, i.e., it can be measured with an appropriate measurement device independent of the state the system is prepared in.

This frightens me... I actually agree with all of the above. I better understand your terminology now, as you use the concept of Observable to include both cases: when the outcome is certain, and when it is not.

Now: if we could agree the Observable goes from having a "not determined" (completely uncertain) value to having a specific value for that Observable, well... that sounds discontinuous to me. (I call that "Collapse".) Which then begs the question of "when" and "where" that happens, and whether it is the local measurement that causes it or if it is "something else".

QM itself is silent on these points. Presumably the "Collapse" occurs between the Preparation (not determined) and the Measurement (value measured/recorded). But even that is not a requirement of QM (due to existence of delayed choice experiments that blur the usual ordering). And presumably, the "collapse" occurs near (i.e. local to) the Observable. And again, that is not a requirement of QM (due to quantum nonlocality, which blurs where the "cause" originates and the "effect" appears).

 

[WernerQH]

But they were puzzled by some aspects of electricity and magnetism. For example current in a wire deflects a near by compass without any contact.

Also what is the alternative? If there is no em field, then what accounts for the observations? An action at a distance perhaps, but it is a strange one because it is at a distance and propagates with finite velocity!

Ampere developed an accurate theory for the force of one current element on a distant one. At the time he was dealing with what we now call magnetostatics, and all of the experiments seemed to indicate that the action is instantaneous.

 

[WernerQH]

... observables take only determined values when the system under consideration is in a corresponding state. Otherwise there are only probabilities for the outcome of measurements on the system ...

This only makes sense when you talk about the experiment as a whole, i.e. including "state preparation" and "measurement" (none of which happen in an instant). But with the word "state" most physicists associate something that refers to a specific instant in time (and often changes with time). It is unfortunate that you use the word "state" as synonymous with "object". The rules of QM (including the Born rule, of course) give us the frequencies with which certain patterns of events can be expected to occur in a small region of spacetime. Unfortunately many have the desire to think of a "system" and how its "state" evolves with time. I think Consistent Histories was introduced to counter this misrepresentation of QM.

"System", as well as "measurement", was among the terms that John Bell was hoping to ban from the foundations of QM.

 

[Peter Morgan]

As you may have noticed, I am obsessed with understanding the difference between two views of quantum mechanics, one of which can be called the "standard" view, and the other the "realist" view. The difference, of course, is very complicated, but I believe that the essence and origin of the difference must be simple. In this thread I am trying to explain the difference in terms of two simple schemes, corresponding to two approaches to theoretical physics as a science.

The standard approach:
1. Write down the equations.
2. Make the measurable predictions implied by the equations. That's the most important part to do if you aspire to be a scientist, rather than just a mathematician.
3. If you can, try to say what does it all tell us about what the world is made of. But if you are not sure, stay silent about it.

The realist approach:
1. Say what is the world made of.
2. Write down the corresponding equations. That's the most important part to do if you aspire to be a scientist, rather than just a philosopher.
3. If you can, try to make the measurable predictions implied by the equations. But if you are not sure, stay silent about it.

I suggest the standard approach can be twisted around:

1. Construct and perform experiments, as a result of which we have some number of Gigabytes or Exabytes of (noisy) data (it seems to me notable that in modern times we often generate that data at ~MHz rates, far faster than any real-time intervention by people.) We take that actually recorded experimental data to be "real" (I suppose that's Bohr's view when he says, in 1949, "It is decisive to recognize that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.") Certainly, if we don't have credible data available in computer memory, a journal editor will not publish an article about the experiment so that other physicists will take the experiment really seriously.
2. Find ways to systematize that noisy experimental data. Statistics (and probability as an idealization) seem to be more-or-less essential because of the noise. It turns out that Fourier analysis and Hilbert spaces are rather good mathematical tools (the "equations"), but they go beyond the tools we have in ordinary Classical Physics (I suppose that's what Dirac suggests when he says, in 1949, "My own opinion is that we ought to search for a way of making fundamental changes not only in our present Quantum Mechanics, but actually in Classical Mechanics as well.")
3. Use our various systematizations to predict statistics for the results of new experiments. Adjust as necessary.
4. Understand how the ways in which Fourier analysis and Hilbert spaces go beyond ordinary Classical Physics are classically natural, without twisting one's classical intuition very much.

How (4) works is too much to rehearse here. I gave a talk two weeks ago to the Lisbon Philosophy of Physics Seminar, "A Field & Signal Analysis Approach to Quantum Measurement",
(which points to articles in Physica Scripta 2019, Annals of Physics 2020, and Journal of Physics A 2022) that I hope gives some indication of how I think this can be made to play out. The PDF for the talk, which is on Dropbox, https://www.dropbox.com/s/nh4504m6tjrejaa/Lisbon 2023 (as given).pdf?dl=0, has DOIs for those papers and can be skipped through more quickly than listening to me talking round it. I hope a few people here might find it stimulating even though of course nobody will agree with all of it.

 

[Peter Morgan]

This only makes sense when you talk about the experiment as a whole, i.e. including "state preparation" and "measurement" (none of which happen in an instant). But with the word "state" most physicists associate something that refers to a specific instant in time (and often changes with time). It is unfortunate that you use the word "state" as synonymous with "object". The rules of QM (including the Born rule, of course) give us the frequencies with which certain patterns of events can be expected to occur in a small region of spacetime. Unfortunately many have the desire to think of a "system" and how its "state" evolves with time. I think Consistent Histories was introduced to counter this misrepresentation of QM.

"System", as well as "measurement", was among the terms that John Bell was hoping to ban from the foundations of QM.

That seems to me remarkably close to the view I express in the video and papers I refer to in the comment I just posted.

 

[Demystifier]

If there is no em field, then what accounts for the observations?

If there is em field (even when we don't measure it), what is its mathematical representation?
a) A real valued function of ${\bf x},t$.
b) A self-adjoint operator valued function of ${\bf x},t$.
c) A vector in the Hilbert space on which the operator in b) acts.
d) Something else - what?
e) Something as yet unknown.
f) It doesn't have a mathematical representation at all.
g) Prefer not to say.

 

[martinbn]

If there is em field (even when we don't measure it), what is its mathematical representation?
a) A real valued function of ${\bf x},t$.
b) A self-adjoint operator valued function of ${\bf x},t$.
c) A vector in the Hilbert space on which the operator in b) acts.
d) Something else - what?
e) Something as yet unknown.
f) It doesn't have a mathematical representation at all.
g) Prefer not to say.

In the classical theory a) in the quantum b) (may be a distribution rather than a function, and may be not necessarily self-adjoint). But why do you ask me! You tell me what the physical theories say.

 

[Demystifier]

in the quantum b) (may be a distribution rather than a function, and may be not necessarily self-adjoint). But why do you ask me! You tell me what the physical theories say.

I ask you because I want to know what you mean by that.

My problem with your answer in the quantum case is that it doesn't explain how and why real-valued values (i.e. values $\in\mathbb{R}$) appear when we measure the field. It looks as if, in addition to the field operator, there is also something which exists only when it is measured, but the answer b) doesn't explain why. It doesn't mean that your answer is wrong, but it suggests that something in your answer (which is pretty much standard quantum theory) is still missing. In other words, there is the measurement problem and the answer b) doesn't help to solve it.

If you ask me to explain the standard quantum theory, I don't agree that it says that the field operator exists in the physical sense. It is just a tool for computing the probabilities of measurement outcomes. The standard quantum theory presented correctly, in my view, is completely silent about whether anything exists or not in the absence of measurement. Anyone who is talking something about physical existence in the absence of measurement is either saying something beyond standard quantum theory, or saying nonsense (or both).