Interestingly Bohr Did Not Believe in Wavefunction Collapse

In summary: I don't think it's fair to say that Bohr didn't introduce a collapse. He talked about it a lot, and his work is often cited as an early example of it.
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I was reading an article 'What Einstein Really Thought about Quantum Mechanics' in Scientific American. There they mentioned something I didn't know - Bohr did not believe in Wave Function Collapse as an issue (I don't either - but that's just my opinion, so means nothing). I found this surprising because just about everyone cites it as an issue eg Sabine Hossenfelder. So I dug a bit deeper and found a paper on Bohr's views:

https://arxiv.org/pdf/1603.00353

People might find it interesting.

BTW aside from Bohr's view on QM the Scientific American article didn't contain anything that has not been discussed here before. For those that may not have seen discussions on this, the usual remarks such as God does not play dice misses the mark on his real issue(s). If it is not behind a paywall here is the link:

https://www.scientificamerican.com/article/what-einstein-really-thought-about-quantum-mechanics/

Thanks
Bill
 
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R. Tumulka, in the recent book "Foundations of Quantum Mechanics" (2022), explained Copenhagen interpretation in a very lucid way. I particularly like the following part:

"Narratives, But No Serious Ones

When calculating predictions that can be compared to experimental data, adherents of CI often tell a story about the physical meaning of the mathematical elements of the calculation. This story may involve particles or waves, may be imprecise, may conflict with other stories told on another occasion, or may contain several parts conflicting with each other. But this story is not intended to describe what actually happens. On the contrary, CI insists that such narratives should not be taken seriously. They are just metaphor, or allegory, or analogy; they just serve as a mnemonic for the calculation, as a help for remembering the correct formulas or for setting up the corresponding formulas in similar calculations. In contrast, theories such as Bohmian mechanics or GRW or many-worlds are hypothesizing about what actually happens in nature and correspondingly aim at providing a single, coherent story that fits all experiments and situations."
 
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So when Bohr and other adherents of Copenhagen/standard/orthodox/minimal type of interpretations talk about interpretations, one should not take them very seriously. One should take seriously their calculations, and their measurable predictions, but not their narratives in which they try to explain what that means. After all, they don't take very seriously their own narratives themselves.
 
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  • #4
The collapse contradicts the formalism of relativistic microcausal QFT. So naive collapse interpretations are self-contradictory. I always thought the still big "fan club" of Bohr always claim Bohr never had introduced a collapse. Einstein's advice about theorists is wise: "Don't listen to their words, look at their deeds."
 
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  • #5
Well in his essays Bohr never mentions collapse. The closest he gets is discussing filtering, that's about it.

This is no different from the history of physics in general though. Even in Classical Mechanics the "folk history" is completely different from the actual history of what Langrange, D'Alembert, Hamilton, etc did. In my general experience you have to read the original sources and proper academic histories, which are surprisingly uncommon in physics, instead of the storybook version most texts will recount.
 
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  • #6
vanhees71 said:
I always thought the still big "fan club" of Bohr always claim Bohr never had introduced a collapse. Einstein's advice about theorists is wise: "Don't listen to their words, look at their deeds."
Could you expand on what you mean here?
 
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I think Einstein meant, you should look at what the theorists really have achieved than listening to their "philosophy", and for me it's completely ununderstandable, why Bohr was such a celebrity in connection with "modern quantum mechanics", where he wrote a lot of philosophical rather than solid physics papers with vague gibberish like "complementarity" and a "divide in a classical and a quantum dynamics" ("Heisenberg cut"). His influence was rather destructive to people who dared to question this "Copenhagen doctrine". This was broken only in the early 80ies, when Bell's work of the 60ies slowly became to be realized in the lab. Bell himself warned young colleagues to touch the issue, because he thought it could be destroying their careers, and he emphasized that he could dare to go into the foundational-QM issues only because he had a solid tenured job.
 
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  • #8
vanhees71 said:
for me it's completely ununderstandable, why Bohr was such a celebrity in connection with "modern quantum mechanics", where he wrote a lot of philosophical rather than solid physics papers
It's also interesting how there are many people who were a major influence on the emerging theory, but don't get included in the folk story. For example Jordan, Majorana, Kramers, etc.
I think it's a common tendency to concentrate history into a few "mythic" figures.
 
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Jordan is the most underestimated, but it's due to his idiosyncratic political behavior during (and even before!) 1933-1945:

https://physicstoday.scitation.org/doi/10.1063/PT.3.5158
https://physicstoday.scitation.org/do/10.1063/PT.6.4.20230104a/full/

What were Majorana contributions? I must admit, I don't know. Of course I'm aware of his work in connection with relativistic theory ("Majorana fermions" and all that).

Kramers role was to have been involved with Bohr in a famous failure shortly before modern quantum theory was discovered by Born, Jordan, based on Heisenberg's Helgoland idea. In this paper the authors gave up the conservation laws, particularly energy conservation. This was famously refuted shortly thereafter by Bothe's coincidence method applied to Compton scattering.
 
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  • #10
bhobba said:
I was reading an article 'What Einstein Really Thought about Quantum Mechanics' in Scientific American. There they mentioned something I didn't know - Bohr did not believe in Wave Function Collapse as an issue (I don't either - but that's just my opinion, so means nothing). I found this surprising because just about everyone cites it as an issue eg Sabine Hossenfelder.
In order to avoid any 'nonsense' that might have been circulated about Bohr's thinking, I recommend to read “Niels Bohr and the Philosophy of Physics: Twenty-First-Century Perspectives” (edited by Jan Faye and Henry J. Folse, published 2017).
 
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vanhees71 said:
What were Majorana contributions? I must admit, I don't know. Of course I'm aware of his work in connection with relativistic theory ("Majorana fermions" and all that).
Well first of all just that. Everybody knows the name, but himself as an actual person is rarely discussed in the history of physics, which is odd because I think it's one of the more interesting stories. Outside of that he was the first to make major progress in the treatment of the spectroscopy of heavier elements and the first to correctly identify and treat the neutron.

vanhees71 said:
Kramers role was to have been involved with Bohr in a famous failure shortly before modern quantum theory was discovered by Born, Jordan, based on Heisenberg's Helgoland idea. In this paper the authors gave up the conservation laws, particularly energy conservation. This was famously refuted shortly thereafter by Bothe's coincidence method applied to Compton scattering.
Well for example Kramer's 1924 dispersion theory paper and his January 1925 paper with Heisenberg was the biggest influence on Heisenberg's 1925 Helgoland paper.
 
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LittleSchwinger said:
Well first of all just that. Everybody knows the name, but himself as an actual person is rarely discussed in the history of physics, which is odd because I think it's one of the more interesting stories. Outside of that he was the first to make major progress in the treatment of the spectroscopy of heavier elements and the first to correctly identify and treat the neutron.
He also considered field quantization already in the "Dreimännerarbeit". It had to be rediscovered by Dirac in 1927/28, because it was ignored at the time.
LittleSchwinger said:
Well for example Kramer's 1924 dispersion theory paper and his January 1925 paper with Heisenberg was the biggest influence on Heisenberg's 1925 Helgoland paper.
That's true. Of course Kramers contributed a lot more later to "new QM" like the WKB approximation etc.
 
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  • #13
vanhees71 said:
He also considered field quantization already in the "Dreimännerarbeit". It had to be rediscovered by Dirac in 1927/28, because it was ignored at the time.
Sorry, in case I wasn't clear, in the above I referred to Majorana not Jordan.
 
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I see. I must say, I don't know much about Majorana's contributions to early QT.
 
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vanhees71 said:
I see. I must say, I don't know much about Majorana's contributions to early QT.
Yeah, unfortunately he was very private and quiet, mostly known to only people in Fermi's group and died young under mysterious circumstances.
 
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Indeed, most writings on him are speculations about his mysterious disappearance rather than about his science :-(.
 
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LittleSchwinger said:
It's also interesting how there are many people who were a major influence on the emerging theory, but don't get included in the folk story. For example Jordan, Majorana, Kramers, etc.
I think it's a common tendency to concentrate history into a few "mythic" figures.
You could argue both cases. E.g. if Bohr had never lived, then QM would have progressed along parallel lines in a similar time frame. Or, it could have held up progress for a decade or more.

The case is less clear, IMO, if Einstein had never emerged from the patent office. Somebody would have produced SR, surely? But, how long after 1905?
 
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  • #18
I think, Bohr had in fact pretty little influence on the development of modern quantum mechanics, except that he provided opportunities for the scientists to work in his famous institute. I think at least Schrödinger's wave mechanics would have been also developed without the presence of Bohr, since it was triggered by de Broglie's work with considerable support by Einstein. One can even speculate, whether we'd have been spared all these philosophical debates. On the other hand, it triggered also the scientific case in terms of Bell and the development of all the scientifically fruitful developments celebrated by the recent Nobel Prize in physics and the development of new technologies like quantum cryptography, quantum informatics, etc.
 
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vanhees71 said:
........ why Bohr was such a celebrity in connection with "modern quantum mechanics", where he wrote a lot of philosophical rather than solid physics papers with vague gibberish like "complementarity" and a "divide in a classical and a quantum dynamics" ("Heisenberg cut"). His influence was rather destructive to people who dared to question this "Copenhagen doctrine".
With all due respect, are you really serious about your comment?

In “BOOJUMS ALL THE WAY THROUGH: Communicating Science in a Prosaic Age” (first published 1990), N. David Mermin remarks in chapter 14 “The philosophical writings of Niels Bohr”:

But against the sometimes maddening frustration brought about by a study of these ponderous essays is the indisputable fact that nobody has succeeded in saying anything manifestly better in the sixty years since Bohr started talking about complementarity. How he could have known that they would fail, right from the start, is yet another puzzle. As a philosopher Bohr was either one of the great visionary figures of all time, or merely the only person courageous enough to confront head on, whether or not successfully, the most imponderable mystery we have yet unearthed.
 
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  • #20
Without Bohr the field of discussing Bohr would have been held back decades :smile:
 
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  • #21
PeroK said:
You could argue both cases. E.g. if Bohr had never lived, then QM would have progressed along parallel lines in a similar time frame. Or, it could have held up progress for a decade or more.

The case is less clear, IMO, if Einstein had never emerged from the patent office. Somebody would have produced SR, surely? But, how long after 1905?
This is ridiculous. Both made discoveries that were not foreshadowed in any way. Why are you so sure that they would have been discovered without them? But we should also be clear that both delivered their groundbreaking work in young years, and especially not past 1926. Bohr had the wisdom to support young researchers, and this is a second important part of his lasting contributions. Einstein occasionally also managed to support other researchers, like Bose or de Broglie. Don't underestimate that part.
 
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SR was more or less already there, particularly by Poincare. I think it wouldn't have taken long without Einstein to realize that there's no need for an aether at all.

Bohr's great achievement was the old quantum theory of atoms, paving the way to the modern development, when the failure of this concept was obvious pretty soon in the vigorous research on spectroscopy. The idea of the new quantum theory came from Heisenberg in 1925 and, independently, from Schrödinger in 1926, not from Bohr.

I agree with the importance of Bohr's establishment of his famous institute and the mentoring of many young scientists.
 
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vanhees71 said:
Indeed, most writings on him are speculations about his mysterious disappearance rather than about his science :-(.

That is true. I know him from a television special about strange disappearances.

Thanks
Bill
 
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vanhees71 said:
SR was more or less already there, particularly by Poincare. I think it wouldn't have taken long without Einstein to realize that there's no need for an aether at all.

In another forum where the question of if anyone nearly had SR before Einstein was asked, I mentioned Poincare. But he was 'stuck' in old thinking and even asked Einstein what the dynamical principle behind SR was. Of course, Poincare had the mathematical ability to invent even the modern version, but the issue was his worldview. Einstein, of course, was the bridge between the old and modern worldviews. I laughed when someone responded that Minkowski came up with the geometrical interpretation - not Einstein. That was the point - Poincare has the mathematical ability to do that, and I am a bit surprised Minkowski did it before him purely on mathematical grounds. But perhaps his preconceptions may have held him back. Even there, Poincare came tantalisingly close:
https://shs.hal.science/halshs-01234449/document

Thanks
Bill
 
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  • #26
As far as I have concluded from recall reading somewhere sometimes there was a tension between Bohr and Heisenbergs view, where Heisenberg emphasised the cut between observer and system, where the "collapse may have been more emhpasises", while Bohr was more thinking of the "classical vs quantum" cut, but IMO the unification of these views is that, as long as all the "Heisenberg observers" share the same common classical reality, they can communicate and make a joint agreement about things. In this sense the set of all possible Heisenberg observers, seems to be the same as the classical observer of Bohr. This is how at least I always thought of it, and they are thus not really in contradiction. Both views make sense and complement each other. So together this nifcely defines the Copenhagen interpretation.

The only problem with this, is when you put in classical objects as "quantum systems", but then - it is not the puropose of QM, the purpose of QM is a theory of the subatomic world we only indirectly can access.

/Fredrik
 
  • #27
But QT describes everything, including macroscopic systems. That's what zillions of condensed-matter physicists work about with great success! There is also no hint yet that there's an upper limit in size, where all of a sudden the fundamental laws of QT get invalid and classical physics takes over. As I repeatedly quoted, even ~10 kg objects like the LIGO mirrors show quantum behavior. It's only a matter of the technical ability to prepare macroscopic systems such that quantum phenomena become observable.
 
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gentzen said:
Einstein occasionally also managed to support other researchers, like Bose or de Broglie. Don't underestimate that part.
Einstein awarded his Einstein Prize to Schwinger and Feynman.
 
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vanhees71 said:
But QT describes everything, including macroscopic systems. That's what zillions of condensed-matter physicists work about with great success! There is also no hint yet that there's an upper limit in size, where all of a sudden the fundamental laws of QT get invalid and classical physics takes over. As I repeatedly quoted, even ~10 kg objects like the LIGO mirrors show quantum behavior. It's only a matter of the technical ability to prepare macroscopic systems such that quantum phenomena become observable.
Zurek has some fun papers modelling Hyperion, a moon of Saturn, with QM:
  • Zurek, W.H.: Pointer basis of quantum apparatus: Into what mixture does the wave packet collapse? Phys. Rev. D 24, 1516–1525 (1981)
  • Zurek, W.H.: Environment-induced superselection rules. Phys. Rev. D 26, 1862–1880 (1982)
  • Zurek, W.H.: Preferred states, predictabilty, classicality, and the environment-induced decoherence. Prog. Theor. Phys. 89, 281–312 (1993)
  • Zurek, W.H.: Decoherence, chaos, quantum–classical correspondence, and the algorithmic arrow of time. Phys. Scr. T 76, 186–198 (1998)
 
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  • #30
vanhees71 said:
But QT describes everything, including macroscopic systems.
Except gravity, which you leave for last (as you think it has nothing todo with this), but this is where we disagree. I also think there are other indirect implicitations; we still have no GUT. I also think this is relevant to this. But, yes it's just my hunch, we you can label philosophy if you wish.

vanhees71 said:
It's only a matter of the technical ability to prepare macroscopic systems such that quantum phenomena become observable.
Yes! This is also my point, believe or not, but while you seem to think it's this is jus a practical limit or the scientific technolgoy at hand, I take it more seriously and suggest this: It is a matter of ability of the OBSERVER/AGENT to represent/control/process the part of it's environment in a way that makes it possible to implement the inference and measurements, produce statistics etc. The only reason why i insist that QM works best for SMALL systems, is that this is where we can keep this assymmetry.

For more general cases, I think QM/QFT needs to be relaxed and improved.

I kind of like your sticking to experimentally accessible things, but my "philosophy" is that this is more than just a practical matter of human technolgoy, I take this seriously which has led me to my own stance with interatting agent initerpretations.

From the practical perspective, I can't disagree with you.

/Fredrik
 
  • #31
Fra said:
Except gravity, which you leave for last (as you think it has nothing todo with this), but this is where we disagree. I also think there are other indirect implicitations; we still have no GUT. I also think this is relevant to this. But, yes it's just my hunch, we you can label philosophy if you wish.
Unless it is clear what I meant, the problem of hte GUT is to unify interactions at different scales, it's hard to "connect them" from the perspective of inference.... so even if gravit is at the extremal scale, we have "scale issues" already in the quest for GUT. This is in short why I see a common issue here. It's early signs, that simply get unsurmountable if you include gravity. For the GUT, we may overcome it, but perhasp there is an easier way, that will also reduce the number of free parameters.

/Fredrik
 
  • #32
Fra said:
Except gravity, which you leave for last (as you think it has nothing todo with this), but this is where we disagree. I also think there are other indirect implicitations; we still have no GUT. I also think this is relevant to this. But, yes it's just my hunch, we you can label philosophy if you wish.
Gravity is completely negligible for the description of macroscopic bodies. It's treated semiclassically whenever one investigates the quantum behavior in the gravitational field of the Earth (e.g., the famous experiment with cold neutrons, which is well explained by treating the gravitational field as ##V(\hat{\vec{x}})=-m \vec{g} \cdot \hat{\vec{x}}## in the Hamiltonian).
Fra said:
Yes! This is also my point, believe or not, but while you seem to think it's this is jus a practical limit or the scientific technolgoy at hand, I take it more seriously and suggest this: It is a matter of ability of the OBSERVER/AGENT to represent/control/process the part of it's environment in a way that makes it possible to implement the inference and measurements, produce statistics etc. The only reason why i insist that QM works best for SMALL systems, is that this is where we can keep this assymmetry.
It's simply technologically simpler to handle small systems such that quantum behavior becomes observable. There's not a single hint that QT becomes invalid for macroscopic systems.
Fra said:
For more general cases, I think QM/QFT needs to be relaxed and improved.
There's no empirical evidence for this claim.
Fra said:
I kind of like your sticking to experimentally accessible things, but my "philosophy" is that this is more than just a practical matter of human technolgoy, I take this seriously which has led me to my own stance with interatting agent initerpretations.
The problem is that there is not the slightest hint for the necessity to claim that sufficiently large bodies cannot be described by QT. It's just a philosophical feeling that there must something be wrong with QT, for whatever reason you have this feeling.
 
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  • #33
vanhees71 said:
There's not a single hint that QT becomes invalid for macroscopic systems.
Yes, there is: macroscopic systems do not show quantum interference effects. That was Schrodinger's original point in his cat thought experiment.

The usual QT response is that if we are someday able to do, say, double slit experiments with macroscopic objects like rocks, we will find that they do show quantum interference effects. But that's not a claim based on actual evidence: it's a claim about what kind of future evidence we will find. There is no actual evidence that macroscopic objects show quantum interference effects, and mountains of evidence that they do not. That is a hint that QT might not be valid for macroscopic systems, or at the very least that there might be corrections at the macroscopic level to the QT that works well for microscopic systems.
 
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  • #34
vanhees71 said:
But QT describes everything, including macroscopic systems.

Exactly. And that is why I am dubious of the existence of wave-function collapse as anything but something that occurs in a theorist's calculations. This is bolstered by Gleason's Theorem, where the state is seen as merely a mathematical convenience to calculate probabilities (of course, an important one). My question is, if you think states collapse - do you think probabilities do as well? Note - you can take that position if you like, but those into probability usually don't.

Exactly how our classical world arises from such a view is an interesting question, still under investigation. The latest I have heard about it is the following I have posted before:

https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

And, of course, Peter is correct. Many, including me, think everything is quantum, but experimental justification is lacking.

Thanks
Bill
 
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  • #35
PeterDonis said:
Yes, there is: macroscopic systems do not show quantum interference effects. That was Schrodinger's original point in his cat thought experiment.
But don't coarse graining and decoherence predict the absence of interference effects for macroscopic collective coordinates? In addition quantum condensed matter correctly derives the macroscopic physical properties (e.g. friction, dielectric and many other coefficients).

So the predictions of QM for macroscopic bodies is that they should possess various effective constants that they have been shown to possess and also that interference should be absent experimentally. In fact the absence of interference is often directly related to the value of their frictional coefficients.

It would be one thing if quantum theory predicted the presence of interference effects and wrong values for the effective constants of materials, but as far as I can see it everything it is know to predict is observed in macroscopic bodies.

I mean even at a simple level quantum statistical mechanics correctly predicts how rapidly a macroscopic body heats up. Similarly simple you need QM to explain macroscopic stability, i.e. that macroscopic bodies don't energetically prefer dividing in smaller macroscopic bodies:

E. H. Lieb and R. Seiringer, The stability of matter in quantum mechanics, 1st ed. (Cambridge Univ. Press, Cambridge, UK, 2010).
 
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