I Interestingly Bohr Did Not Believe in Wavefunction Collapse

[vanhees71]

This makes sense to me, reactions to this was as if it was a joke, but I am not sure Peter meant it as a joke?

Rather than those that tries to suggest that "everything is quantum" (thus denying the observer), I find the copenhagen interpretation and Bohrs reasoning to be very honest and also "minimal". Namely that the formulation of quantum mechanics, requires the cut between quantum and macroscopic/"classical". And the latter is to most a very realist part. The "instrument" that makes QM possible is real; otherwise we would have trouble to define the quantum weirdness as well. So the duality here makes sense to me at least, os not sure if was a joke.

/Fredrik

The cut is not physical. There's no such thing. At least nobody could empirically prove that quantum phenomena cease to exist for sufficiently large systems. To the contrary with ever refined technique experimentalists find quantum phenomena in larger and larger systems, e.g., in the motion of the LIGO mirrors with masses in the few 10kg range!

 

[Lord Jestocost]

As the Wikipedia article on Bohr says, 'Bohr has been seen as an anti-realist, an instrumentalist, a phenomenological realist or some other kind of realist.'

Bohr was always difficult to understand.

Thanks
Bill

Catherine Chevalley has identified three reasons why Bohr’s writings seem obscure to certain people (C. Chevalley, “Why do we find Bohr obscure?,” in Epistemological and Experimental Perspectives on Quantum Physics, ed. D. Greenberger, W.L. Reiter, and A. Zeilinger (Kluwer, 1999), pp. 59–73):

“My argument will be in three parts. (a) I shall first argue that what makes Bohr difficult to read is the fact that his views were identified with the so-called 'Copenhagen Interpretation of Quantum Mechanics', while such a thing emerged as a frame for philosophical discussion only in the mid-1950's. (b) In the second place, I would like to emphasize that Bohr's ideas were not located in their proper background, either scientific or philosophical, until quite recently. (c) And finally, I shall suggest that Bohr's conception of what philosophical thinking ought to be was so different from the conception that prevailed in the literature after the war that it is no wonder if the connection turned up to be difficult to make.”

 

[vanhees71]

Indeed, Bohr and Heisenberg are the main culprits in obscuring QT, because they produced a lot of philosophical clouds around it. Fortunately, nearly 100 years later, there is a simple no-nonsense interpretation followed more of less conciously by most physicists not involved in high-brow "foundational debates", i.e., the minimal statistical interpretation. If you want to understand the physics without being distracted from philosophical pseudo-problems like "wave-particle duality", "complementarity", "collapse of the quantum state", "measurement problem", then just read physics books rather than philosophy books. For me Bohr is the most overrated physicist of the 20th century. In some way he has been treated more like a priest than a fellow scientist by many of his colleagues.

 

[Fra]

The cut is not physical.

It depends what you mean. I agree it's about as "non-physical" as an effective theory that depends on the observation perspective. Different cuts - different effective theories yes. Which is why I wish to understand the physics of this theory space better than the renormalization group idea.

There's no such thing. At least nobody could empirically prove that quantum phenomena cease to exist for sufficiently large systems.

Agreed, but it does not mean the cut plays no role, it just means its relative. But at some point we indees get beyond Bohrs meaning.

Bohrs "cut" as I understand has no scale limit per see. It is the cut between the DOMINANT classical/macroscopic context, and a smaller subsystem. But yes the subsystem can be "large", but never larger than the observing context as then the theory breaks

/Fredrik

 

[Lord Jestocost]

Indeed, Bohr and Heisenberg are the main culprits in obscuring QT, because they produced a lot of philosophical clouds around it. Fortunately, nearly 100 years later, there is a simple no-nonsense interpretation followed more of less conciously by most physicists not involved in high-brow "foundational debates", i.e., the minimal statistical interpretation. If you want to understand the physics without being distracted from philosophical pseudo-problems like "wave-particle duality", "complementarity", "collapse of the quantum state", "measurement problem", then just read physics books rather than philosophy books. For me Bohr is the most overrated physicist of the 20th century. In some way he has been treated more like a priest than a fellow scientist by many of his colleagues.

With all due respect, I have the feeling that you are distracted from – as you call it – “philosophical pseudo-problems” like ‘wave-particle duality’, ‘collapse of the quantum state’, ‘measurement problem’.
As Carl Friedrich von Weizsäcker remarks in the preface of his book “The Structure of Physics” (the book is a newly arranged and revised English version of "Aufbau der Physik" by Carl Friedrich von Weizsäcker):

“The book reports on an attempt to understand the unity of physics. This unity began to manifest itself in rather unexpected form in this century. The most important step in that direction was the development of quantum theory; the emphasis of this book is therefore on the endeavor to understand quantum theory. Here, understand refers not merely to practical application of the theory – in that sense it has been understood for a long time. It means being able to say what one does when applying the theory.”

[Bold/Bold-Red by LJ]

 

[vanhees71]

Well, I don't comment on CFvW. I think that's really completely off-topic, particularly the mentioned book.

 

[AndreasC]

with vague gibberish like "complementarity" and a "divide in a classical and a quantum dynamics" ("Heisenberg cut").

It feels very rushed to say it was "vague gibberish". I haven't studied the original but the impression I'm getting from secondary sources (including scientists and philosophers who were previously of your point if view before engaging with it) is that that's a very unfair characterization that shouldn't have prevailed. Have you read his writings on the issue?

 

[Fra]

Indeed, Bohr and Heisenberg are the main culprits in obscuring QT, because they produced a lot of philosophical clouds around it.

Did they really? Perhaps Heisenberg a bit more but I have the impression that the cloud comes from confusion between humans and observers. If I remeber right Heisenberg was a bit more into the "observer" but Bohr was more into the "classicak side".. but they unified into the classical part beeing what the set of observers agree upon.

My view of Bohr is that he advocated honesty and simple clarity in how quantum theory does build upon a context. I always thought that this insight came naturally from someone that lived through the creation of QM from the perspective of classical physics. Unlike followers that has been brought up with beeing told about quantum wierdness in school, that grow up confused

/Fredrik

 

[AndreasC]

Well, I don't comment on CFvW. I think that's really completely off-topic, particularly the mentioned book.

Interestinly, it was Weizsacker who changed Feyerabend's opinion on Bohr, which was catalytic to his later development:

Von Weizsäcker showed how quantum mechanics arose from concrete research while I complained, on general methodological grounds, that important alternatives had been omitted. The arguments supporting my complaint were quite good… but it was suddenly clear to me that imposed without regard to circumstances they were a hindrance rather than a help: a person trying to solve a problem whether in science or elsewhere must be given complete freedom and cannot be restricted by any demands, norms, however plausible they may seem to the logician or the philosopher who has thought them out in the privacy of his study. Norms and demands must be checked by research, not by appeal to theories of rationality. In a lengthy article I explained how Bohr had used this philosophy and how it differs from more abstract procedures. Thus Professor von Weizsäcker has prime responsibility for my change to “anarchism”—though he was not at all pleased when I told him so in 1977. (SFS, p. 117).

So maybe @Lord Jestocost thought he was the right person to also change your mind!

 

[AndreasC]

Today appeared a paper claiming that Bohr was a realist.
https://arxiv.org/abs/2308.00814

That was I believe a big part of Fock's defense of Bohr and QM in the Soviet Union (where it was attacked for allegedly being idealist): https://www.sps.ch/en/artikel/gesch...defense-of-modern-theories-in-soviet-union-13

I am convinced that despite his slightly positivist language, Bohr believes as much as we do in the reality of phenomena of which he speaks, and then the difference between the views of Bohr and mine is more a difference of language than a difference of content