A Jürg Fröhlich on the deeper meaning of Quantum Mechanics

DarMM

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Sorry I misunderstood the example you gave. The point is regardless of the example the state update rules are just those used in quantum tomography in practice.

In this case, if I have the example right, it's the usual measurements to determine the Stokes parameters, just reinterpreted. If I have your example wrong can you say what is the typical way this is done I can check.
 

A. Neumaier

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regardless of the example the state update rules are just those used in quantum tomography in practice.

In this case, if I have the example right, it's the usual measurements to determine the Stokes parameters, just reinterpreted. If I have your example wrong can you say what is the typical way this is done I can check.
This only shifts the problem. Given a prior for the Stokes vector, how is it updated when a new measurement comes in?
Quantum tomography does no updating. It estimates from scratch the expectations of three test operators, and that's it.
It does not tell you how to modify a subjective Stokes vector in a rational manner when one test result of an arbitrary test becomes known.
 

vanhees71

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Well I don't know if it's a "strange idea" simply because it mightn't be useful in modern quantum theory. However it is, since it's just an alternate motivation for statistical tools that you can use regardless of what you think of probability theory. Such an application is here:
The only thing I think about probability theory, successfully applied in statistics as well as theoretical physics for about 150+/- years, is that it works. That's all I need to justify the use of any specific mathematical concept in the natural sciences.
 

vanhees71

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Well, we are discussing here (in the whole thread) various interpretations of quantum mechanics, and some of them are based on subjective probability. I find it strange, too, but one cannot usually discuss other interpretations by casting them in ones own differing interpretation without losing important features - one must use the language in which they describe themselves.
Yes, and my very point is that it doesn't make sense to introduce more and more abstruse and esoterical "concepts" to clarify the meaning of Q(F)T.

This is a science forum (at least that's what I thought) and not about philosophy (even not about philosophy of science). However the QM-section more and more is deformed to a discussion forum about this off-topic subject, and I find this a pity. Particularly even threads where a student asks some scientific question about introductory QM it's soon turned to discussions about some quibbles with the standard minimal interpretation.

I still think, and I hope finally the mentors here agree, that one should split the QM section into a strictly scientific part, where standard QM is discussed and another philosophy-of-science part, where all these speculations about apparent problems, which are in fact pseudo-problems, are discussed without confusing people interested in science rather than cargo cult!
 

A. Neumaier

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Well, we are discussing here (in the whole thread) various interpretations of quantum mechanics, and some of them are based on subjective probability.
This is a science forum (at least that's what I thought) and not about philosophy (even not about philosophy of science).
Subjective probability is discussed even in theoretical books about probability theory, such as the one by Whittle (who primarily gives an exposition of the frequentist view), and is discussed at length in quite a number of books on Bayesian statistics, relevant for real data processing, even in physics.
Particularly even threads where a student asks some scientific question about introductory QM
This thread is explicitly about ''the deeper meaning of quantum mechanics'', so you shouldn't complain in this thread.
one should split the QM section into a strictly scientific part, where standard QM is discussed and another philosophy-of-science part
This split is as ill-defined as the Heisenberg cut - different people place it differently. Like everywhere in discussion forums, the controversial issues take the most space, but are for most readers and contributors also the most interesting ones.

In the last few years, my main motivation to discuss on PF (and alongside also contribute information to other topics of secondary interest to me) was that here one can sensibly discuss foundational questions. While some of the discussion repeats too often without presenting new aspects, I find those threads where I continue to contribute for the most part really informative. I simply stop watching and contributing to the ones that degenerate - you could easily do the same.

Without these foundational discussions I'd have little incentive to spend time on PF, and would also not contribute to other quantum physics topics.
 

stevendaryl

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Yes, and my very point is that it doesn't make sense to introduce more and more abstruse and esoterical "concepts" to clarify the meaning of Q(F)T.
I disagree that anyone is being abstruse or esoterical. And I disagree with your labeling of discussions as "philosophical". I think that the discussions are physics, not philosophy.
 

vanhees71

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The discussion of "subjective" vs. "objective" probbilities IS esoterical. Probability is a clear defined mathematical concept with clear applications in terms of statistics. Completed by information theory and QT it provides objective assignments of probability distributions of real-world systems.
 

vanhees71

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Without these foundational discussions I'd have little incentive to spend time on PF, and would also not contribute to other quantum physics topics.
What's the problem discussing this simply in another subforum?
 

A. Neumaier

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Probability is a clear defined mathematical concept with clear applications in terms of statistics
The professional statisticians, even very applied ones, fall about half/half into objective (primarily frequentist) and subjective (primarily Bayesian) schools, using and recommending different analysis procedures based on the differences in the underlying understanding of probability.
What's the problem discussing this simply in another subforum?
I mainly discuss to learn, not to contribute. Probably I would look only very rarely at the quantum physics forum (as I do now with the other forums) and hence not contribute my knowledge there.

What's your problem with simply ignoring threads about foundations (i.e., what you label philosophy)?
 

A. Neumaier

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What's the problem discussing this simply in another subforum?
Another problem is that there is no clear demarcation line, and different contributors (and especially newcomers) will have quite varied views as to what belongs where. Who decides what belongs where? For each thread a majority vote among the mentors?
 
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The only thing I think about probability theory, successfully applied in statistics as well as theoretical physics for about 150+/- years, is that it works. That's all I need to justify the use of any specific mathematical concept in the natural sciences.
This is almost blatantly an assumption, with little to no foundational support, especially when speaking about QT.
where all these speculations about apparent problems, which are in fact pseudo-problems, are discussed without confusing people interested in science rather than cargo cult
The problems at the edge of knowledge are actually the only problems really worth worrying about. In this case they are distinctly physical problems, which can only become abundantly clear - i.e. to foundations skeptics such as yourself - once a new theory has actually been developed which manages to supersede the old one in some aspects. The production of any new theory which contains QM as a limit and is falsifiable is one of the major contemporary goals of theoretical physics.

The fact that such theories already exist makes the position that QT is unique untenable, regardless of how precise QT matches current experiment. If you don't believe this then you are explicitly overvaluing precision, possibly even in favor of accuracy. After the falsification of Ptolemean epicycle theory, we should all know that having a highly precise theory means exactly nothing if the theory is fundamentally conceptually inaccurate.

The only tried and true methodology to judge the accuracy of a precise theory is by comparing it with another theory; this essentially is the goal of statistical analysis. In other words, either you are assuming QT is the final theory of physics or you are assuming it isn't; if it isn't, then your stance is not scientifically justifiable beyond, being a mere plea for extending and upholding a temporary a facade of certainty about science where there should be genuine doubt and uncertainty.
Probability is a clear defined mathematical concept with clear applications in terms of statistics. Completed by information theory and QT it provides objective assignments of probability distributions of real-world systems.
This might surprise you, but actually no, it isn't as clear cut as it seems as pointed out by many experts. There are several conceptually different notions related to probability, which in the exact sciences are usually immediately interpreted as being probability as described by the standard theory and therefore reduced to standard probability; a few of these concepts are vagueness, uncertainty, likelyhood, possibility and propensity.

Standard probability theory, axiomatized or not (NB: having/making axiomatizations is almost completely irrelevant), is a theory about a particular form of probability which by historical accident we never created an additional monikor for (such as e.g. 'Pascalian probability theory'); despite its official name it is not THE theory of probability in general in exactly the same way that Newtonian mechanics - or QM if you prefer - is not THE theory of mechanics, but merely A theory of mechanics. Statistics itself is not (a) science either, it is a methodology based on making assumptions which may or may not be provable.

Assuming otherwise, as is constantly done in the practice of exact sciences such as physics and engineering doesn't change this any bit. In fact, in many other sciences and other disciplines this fact is widely acknowledged, where the theoretical/mathematical branches of said sciences sometimes use and develop more specialized mathematical and logical tools and methods which are more adequate for their specific purposes.

Incidentally, the justification of such novel tools often end up requiring a complete rewrite of many mathematical foundations, with the developed tool usually ending up being an alternative mathematical construct which better captures the properties of the relevant analysis than standard probability theory does. In fact, we all know the most famous example of such an alternative theory: the complex amplitude calculus from QT. This is only slightly different from standard probability theory, yet is a new mathematical area ripe for research in many directions.
 
That's all I need to justify the use of any specific mathematical concept in the natural sciences.
Why not, it can be heard and understood. However, others may legitimately want to go further for their intellectual satisfaction.

What is the link between Kolmogorov's axiomatics and the different deductive approaches of frequentist, Bayesian,... statistical inference which can be used to characterize a stochastic phenomenology?

Kolmogorov himself belatedly indicated that his axiomatic (and "nothing more") was only that of a theory of measurement. When we talk about probabilities, we are talking about something more.

It is easy to see that the notion of probability is practical, it is an application of certain mathematical theories (measurement theories), as physics or accounting are of other mathematical theories. An application is not limited to the mathematical tools it uses.

The Bayesian approach seems to be more an approach to cognition, i.e. an application to cognition of a theory of measurement. Hence probably the A. Neumaier skepticism about its use in interpreting of the QM.

/Patrick
 

Fra

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The problems at the edge of knowledge are actually the only problems really worth worrying about. In this case they are distinctly physical problems, which can only become abundantly clear - i.e. to foundations skeptics such as yourself - once a new theory has actually been developed which manages to supersede the old one in some aspects. The production of any new theory which contains QM as a limit and is falsifiable is one of the major contemporary goals of theoretical physics.
Fully agree with this. I think the main value of this forums is to see how other think and reason about open, difficult foundational problems.

If I have a basic question about something standard, that can be found in textbooks i would go consult the books, not ask people on internet to explain to this me and consider it an authorative answer. So for me, physicsforums is all about exchanging ideas and ways of reasoning and understanding foundations of physics and science.

/Fredrik
 
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Fully agree with this. I think the main value of this forums is to see how other think and reason about open, difficult foundational problems.
Indeed. What those who complain about threads like these don't seem to understand is that problems in the foundations of physics are always open problems! @vanhees71 such open problems are where contemporary physics simply has no clear answers and they are always with us - usually carefully tucked far away from more practical physics where higher levels of certainty often reign freely.

That is unless one wants to pretend that physics only contains closed problems i.e. pretend that physics is a completed science! The perceived subjectivity of the discussion of open problem is merely a reflection of the wide variety of attempts at answers that are being given; in the scientific process this usually is a necessary stage that must be passed through in order to reach a consensus.
If I have a basic question about something standard, that can be found in textbooks i would go consult the books, not ask people on internet to explain to this me and consider it an authorative answer. So for me, physicsforums is all about exchanging ideas and ways of reasoning and understanding foundations of physics and science.
I fully agree with this. The long form somewhat personal discussion format of physicsforums - IMO even moreso than on stackexchange and far more than Twitter - offers an open platform of free speech, to those interested at all levels and from all walks of life.

This reflects the intellectual world centres of the past, where thinkers spontaneously gathered and were free to speak without being judged prematurely, only now instead in a digital environment where we communicate with each other at a distance and where others can witness our debates since they are explicitly documented.

Any unwarranted calls for self-censorship because of some misguided sense of that a scientist always should have or pretend to have full knowledge and so be capable of offering certainty, especially when the matters are genuinely uncertain only hurts science.

In the immortal words of Feynman:
Feynman said:
The scientist has a lot of experience with ignorance and doubt and uncertainty, and this experience is of very great importance, I think. When a scientist doesn’t know the answer to a problem, he is ignorant. When he has a hunch as to what the result is, he is uncertain. And when he is pretty darn sure of what the result is going to be, he is still in some doubt. We have found it of paramount importance that in order to progress we must recognize our ignorance and leave room for doubt. Scientific knowledge is a body of statements of varying degrees of certainty — some most unsure, some nearly sure, but none absolutely certain.

Now, we scientists are used to this, and we take it for granted that it is perfectly consistent to be unsure, that it is possible to live and not know. But I don’t know whether everyone realizes this is true. Our freedom to doubt was born out of a struggle against authority in the early days of science. It was a very deep and strong struggle: permit us to question — to doubt — to not be sure. I think that it is important that we do not forget this struggle and thus perhaps lose what we have gained.
 

ftr

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I agree with @vanhees71 in his view that maybe a subforum "quantum foundation" can be created, however, I think that might complicate many questions that could be borderline between standard and foundation.
 

Fra

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I agree with @vanhees71 in his view that maybe a subforum "quantum foundation" can be created, however, I think that might complicate many questions that could be borderline between standard and foundation.
In a way i feel that foundational QM questions (this is explicitly so in my personal perspective) often belongs to BTSM because some aspire not to be "interpretations" but rather proper reconstructions that impact the whole standardmodel.

I personally never quite understood the pure interpretation discussion which aim to make no difference to open issues. For me the choice of interpretation rather implicitly reveals how you think the real open problems should be analyzed for progress.

An example of this is some extremal versilns of the preditive inference take on foundations of p-theory and qm-theory. This is not a plain interpretation as i suggests a reconstruction starting from possible computatiomal models for predictive inference that will yield the consistency of standard ptheory as some average limiting attractor im theoryspace.

These things spin in my head every single time in these threads but such discussion really belong more in btsm than in qt section.

Like Neumaiersaid, i only look at qt subforum to sometimes read these discussions that imo are btsm-discussions.

/Fredrik
 

vanhees71

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What's your problem with simply ignoring threads about foundations (i.e., what you label philosophy)?
The problem is that even when I try this, also threads starting about "normal" quantum theory turns rapidly into a discussion about "interpretation" and philosophy. See, e.g., the thread about picture dependence, which has nothing to do whatsoever with interpretational (pseudo-)issues but rapidly was turned into a discussion about it and about non-scientific questions concerning subjective and objective meanings or probabilities!
 

A. Neumaier

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The problem is that even when I try this, also threads starting about "normal" quantum theory turns rapidly into a discussion about "interpretation" and philosophy. See, e.g., the thread about picture dependence, which has nothing to do whatsoever with interpretational (pseudo-)issues but rapidly was turned into a discussion about it and about non-scientific questions concerning subjective and objective meanings or probabilities!
I don't see where the thread 'Interpretation of QM in the Heisenberg Picture' degenerated in this way.
Or did you mean another thread?
 

ftr

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I personally never quite understood the pure interpretation discussion which aim to make no difference to open issues.
I agree. I myself have few times indicated such thoughts that a good theory should really be all encompassing. But you know the academic pressures moto, slowly does it, and its dangerous for them to go all out. Then on the other end of the spectrum you have people that seem highly missguided or mentally disturbed at worst, and whatever comes in between is lost and brushed off very quickly and not taken seriously to be examined carefully and further developed. That is just the way it is.
 
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My problem is to see the necessity for "completions", as long as there are no observations hinting at an incompleteness of QT. The problem I have with understanding, why some people are so obsessed with purely philosophical issues that they think the QT is somehow incomplete. The only incompleteness I'm aware of is the pressing issue of the missing quantum theory of gravity (and, in view common of the geometrical interpretation of GR, probably also spacetime).
Vanheez71. I've been thinking of the above statement of yours for 3 weeks. It is the gist of your aguments why you believed the minimal interpretation was enough. Hope you can put it as part of your profile to summarize your bottom line.

I can agree with you if indeed there were no observations hinting at an incompleteness of QT, no need to be almost obsessed about it. In fact, if there were none. I wouldn't have anything to do with the quantum and wouldn't have spent the last 20 years thinking about it.

But because I spent half of my life (50 years) on something that may point to indicate an incompleteness. I got obsessed figuring out the quantum or beyond QG, not knowing which domain is my interest falling under.

Please enlighten over something (since I don't want to spent the remaining 50 years (or 30 years) of my life chasing for air (if that's what it is)t. What kind of "observations" do you expect or should be expected that showed there were something hinting at an incompleteness of QT? Does Supersymmetry or dark matter fall under it? If not, then what for example?

How about this. We only have access to observable via the hermitean operators. So did you mean observations hinting at an incompletely of QT must be something that is not observable? Please give some examples of what standards you hold before you can consider something as hinting towards an incompleteness of QT. Thanks very much
 

vanhees71

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Your problem seems precisely to be that there's no problem. I can't say, which phenomena would indicate an incompleteness of QT since there are none known today.

Concerning the incompleteness of QT with regard of describing the gravitational interaction (or spacetime geometry) the problem is that the only hint are the singularities of GR (black holes, big bang/crunch etc.), where obviously something is wrong, but whether this is cured by some future quantum formulation or not I can't say. As far as I know there are no hints about phenomena related to quantum features of gravity either.

Concerning supersymmetry, it's a purely theoretical construct which in some sense look mathematically natural and predicts some interesting features like the coupling constants of the SM merging in a closer region (maybe 1 point) hinting at some unification of all known forces. It also may provide dark-matter candidates. The trouble here is that it's with today technology impossible to look for the entire SUSY-extension landscape of the SM. Only the minimal extensions are under test at the LHC and so far it hints at more and more regions of the landscape of the minimal extension are ruled out.
 
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Your problem seems precisely to be that there's no problem. I can't say, which phenomena would indicate an incompleteness of QT since there are none known today.
I want to show you there are strong indications of incompleteness in QT. If I go to Germany. Will you meet me (perhaps at the university coffee cafe) so I can show you? You don't have private messages on so let me ask here.


Concerning the incompleteness of QT with regard of describing the gravitational interaction (or spacetime geometry) the problem is that the only hint are the singularities of GR (black holes, big bang/crunch etc.), where obviously something is wrong, but whether this is cured by some future quantum formulation or not I can't say. As far as I know there are no hints about phenomena related to quantum features of gravity either.

Concerning supersymmetry, it's a purely theoretical construct which in some sense look mathematically natural and predicts some interesting features like the coupling constants of the SM merging in a closer region (maybe 1 point) hinting at some unification of all known forces. It also may provide dark-matter candidates. The trouble here is that it's with today technology impossible to look for the entire SUSY-extension landscape of the SM. Only the minimal extensions are under test at the LHC and so far it hints at more and more regions of the landscape of the minimal extension are ruled out.
 

zonde

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This is a science forum (at least that's what I thought) and not about philosophy (even not about philosophy of science). However the QM-section more and more is deformed to a discussion forum about this off-topic subject, and I find this a pity. Particularly even threads where a student asks some scientific question about introductory QM it's soon turned to discussions about some quibbles with the standard minimal interpretation.

I still think, and I hope finally the mentors here agree, that one should split the QM section into a strictly scientific part, where standard QM is discussed and another philosophy-of-science part, where all these speculations about apparent problems, which are in fact pseudo-problems, are discussed without confusing people interested in science rather than cargo cult!
It seems that you do not shy away from philosophical statements when you want to heighten importance of QFT.
Ironically for photons you come very far with just thinking about them not as classical particles (which is far from their true nature which is only describable in QED) but as classical electromagnetic waves.
 

vanhees71

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It's not a philosophical statement to say that QFT provides the only correct description of what a "photon" is. It's a scientific statement based on observational facts.
 

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