# Bohr's contribution to the Copenhagen interpretation

mollwollfumble
TL;DR Summary
Bohr's papers on quantum mechanics
I wish to learn about Bohr's contribution to the Copenhagen interpretation by reading his original papers on QM, preferably in English.

Which papers are most important?

Gold Member
There's a set of collections of his essays called "The Philosophical Writings of Niels Bohr". Volume I and II have the most important essays I think.

They're not an easy read in my experience. He tended to have very precise meanings in mind that he never fully states.

dextercioby
I wish to learn about Bohr's contribution to the Copenhagen interpretation by reading his original papers
The papers by Bohr relevant for the Copenhagen interpretation are those from 1927 (but partly published 1928). Lots of details are given in my post here. See also here.

bhobba and pinball1970
Gold Member
I wish to learn about Bohr's contribution to the Copenhagen interpretation by reading his original papers on QM, preferably in English.

I recommend to start at first with Jan Faye's entry on the "Stanford Encyclopedia of Philosophy":
https://plato.stanford.edu/entries/qm-copenhagen/

bhobba and Demystifier
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2022 Award
Summary: Bohr's papers on quantum mechanics

I wish to learn about Bohr's contribution to the Copenhagen interpretation by reading his original papers on QM, preferably in English.

Which papers are most important?
Be warned. Bohr is utmost enigmatic, and I don't think it's because of language ;-)).

Nugatory and bhobba
Be warned. Bohr is utmost enigmatic, and I don't think it's because of language ;-)).
It is becaue he must be understood in his context, and not in terms of today's notions.

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Well, my main criticism is that Bohr's nearly esoteric ideas lead still today to some more esoterics, as can be seen in this subforum of PF. Instead of discussing how QT is really used in physics, we still discuss old pseudo-problems of interpretation. For me Copenhagen is just fine, as soon as you just strip off the nonsensical and unnecessary collapse idea, which finally leads to the minimal interpretation, which is without any open physical problems in the realm, where QT is formulated today. The real physics problem is that there is no comprehensive "theory of everything". We still only have an effective description of space-time in terms of GR (which is basically a classical relativistic field theory of the gravitational interaction) and all the (known) rest as relativistic local QFT (in terms of the Standard Model). That's the true problem, not some fictitious "measurement problem".

Another thing is "complementarity", which is Bohr's invention, and as far as I know from reading books on the history of physics that's the one idea Bohr was most proud of. What the heck did he want to really say? I think it's the simple fact that the result of a measurement depends on which observable I measure and how I prepare the object I'm measuring. The standard example is the double-slit experiment:

If I want to resolve through which slit each particle has gone, I cannot get an interference pattern for the particle distribution on the screen when using an ensemble. If I want this interference pattern I can't know through which slit each particle has gone. Complementarity seems to say that this is what has been called the "wave-particle duality" of "old QT".

I think, however, that this is misleading and "new QT" has resolved this riddle about "wave-particle duality" precisely because of Born's probabilistic interpretation.

For the double slit I consider this to be clearly seen as follows: First of all to make sense of saying that a particle has gone through the one or the other slit, it has to be prepared to be sufficiently well localized when arriving at the slits to begin with. This implies that the wave function is a wave packet with a width that it much smaller than the distance of the slits. If it then goes through one of the slits at all, it's possible to say through which slit it came, by putting the photoplate (CCD screen) for particle detection close enough to the double slits. Then the spread of the wave function is still small enough to be able to localize the particle sufficiently such that the position of the particle behind the slit reflects through which slit the particle came.

However, with the very same setup you can also get interference, if you only put the photoplate far enough from the slits, because then the partial waves for the particle running through the one or the other slit have become broad enough to overlap and to interfere. There's no wave-particle duality nor some strange "complementarity": It's simply the broadening of the free-particle wave function leading to interference of partial waves going through one or the other slit. Yet you cannot interpret the particles as "being the waves" since each individual particle leaves only one spot on the screen (no matter whether in the "near-slit" or the "far-slit setup"). This brought Born to his probability interpretation.

Particularly, this is all consistent because of the probability interpretation and the Heisenberg uncertainty relation following from this probability interpretation: The narrower the slits are, the faster the partial waves overlap and the closer you must put your photoplate to the detector to still resolve through which slit the particle has come, i.e., the better localized the particles, that came through one of the slits at all, the more uncertain the momentum becomes and the faster the partial waves thus overlap.

The conclusion is that all pseudo problems vanish by simply accepting the probability interpretation. There's no need for "wave-particle dualism" or "complementarity" or any other esoteric philosophical ideas but only the profound conclusion from the study of quantum phenomena for the last 119 years that nature behaves, at least as far as we can observe phenomena, is intrinsically probabistic.

bhobba
Instead of discussing how QT is really used in physics, we still discuss old pseudo-problems of interpretation.
Those like me who discuss it seriously are convinced that there is something to be explained. For them it is a true problem.

Isn't it strange that you continue to contribute to these discussions - though you find its only a pseudo-problem not worth the time.

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I think, it's important in a forum like this, to emphasize that these are pseudo problems and that there is enough very interesting physics concerning QT out there, even related to these foundations (like Bell experiments). However, I've the impression that this field has rapidly developed even furthert, i.e., it's no longer about "foundational problems", but rather it starts to become even an issue of engineering! In other words the field is well-established enough and the tools and methods (particularly real-world experiments rather than philosophical speculations!) are so well developed now that it's ripe for harvesting the results of fundamental research for real-world applications like quantum cryptography or quantum computers (of which some amazing progress have been made in recent years; afaik a 50 qbit machine is available already now). This shows that there's not only no problem at all with these issues, i.e., quantum theory describes everything in accordance with application, but it can be even handled to a degree to provide new technology.

It's dangerous to leave the impression that this is all about esoterical philosophical problems while in reality it's a fascinating real science (and engineering!) subject. That's why I try to counterbalance the philosophical gibberish with some down-to-earth physics, but it's obviously a Quixotic endeaver :-(.

bhobba and DarMM
Gold Member
it's no longer about "foundational problems", but rather it starts to become even an issue of engineering
It's crazy that something that was a challenge to human conception in the 1960s like contexuality is now being put to practical use to speed up calculations!

vanhees71
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2022 Award
That's how fundamental research is working. Today's fundamental challenges are the basis for tomorrow's applications. If Maxwell had not wondered about the inconsistency between Ampere's Law and charge conservation for time-dependent charge densities, maybe we'd not have found electromagnetic waves at the time and we'd not have radio, TV, cell phones, and all that today!

Such developments have been found either by experiment or theory in a mutual fruitful collaboration and interaction, but never through philosophical speculations. Of course, sometimes pseudo problems, as raised in the EPR paper at the time (about which Einstein was quite unhappy), become fruitful, because it leads to further thinking also of scientists like, in this case, Bell, who translated the philsophy into a real physics problem, which then could be addressed by experimentalists like, in this case, Aspect. This was the beginning of a new branch of physics, which is now called "quantum information" (of course, with cross relations to the also quite newly developed information theory founded by Shannon in the late 1940ies) and is becoming one of the most promising technological developments in recent years. Last but not least it's now also funded heavily on both sides of the atlantic!

mollwollfumble
Thank you all, that's a much better collection of answers than I expected.

> I recommend to start at first with Jan Faye's entry on the "Stanford Encyclopedia of Philosophy":
Copenhagen Interpretation of Quantum Mechanics (Stanford Encyclopedia of Philosophy)

Done.

> The papers by Bohr relevant for the Copenhagen interpretation are those from 1927 (but partly published 1928). Lots of details are given in my post here. See also here.

> Be warned. Bohr is utmost enigmatic, and I don't think it's because of language ;-)).

I've read a few of his papers now, and I have agree. Much easier for me to read other's analyses of Bohr's contribution.

Follow on question. What is your opinion of http://www.phys.tue.nl/ktn/Wim/muynck.htm as a guide to interpretations of QM? It's not complete, obviously, but is it accurate?

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If you want to read something really good by the founding fathers, avoid Bohr and Heisenberg and turn to Born, Dirac, Pauli, and Schrödinger ;-)).

bhobba and jedishrfu
Mentor
I've read a few of his papers now, and I have agree. Much easier for me to read other's analyses of Bohr's contribution.

Indeed. I often think only his good friend Einstein really understood him. Heisenberg for example didn't completely - Bohr had to correct him on his understanding of the uncertainty principle. He was the opposite of Dirac because Dirac couldn't really care - he was more a mathematician, Bohr a philosopher:
http://repository.ias.ac.in/98484/1/bohr.pdf
From the modern vantage the issues that concerned Bohr and Einstein were on the wrong track:
https://physicstoday.scitation.org/doi/10.1063/1.2155755
The real issue that has not been 100% solved is exactly how does a classical world emerge from the quantum. Great progress has been made but some issues still remain (and may never be resolved - we really do not know):
Follow on question. What is your opinion of http://www.phys.tue.nl/ktn/Wim/muynck.htm as a guide to interpretations of QM? It's not complete, obviously, but is it accurate?

The interpret ion favored by me, Vanhees and some others on this forum is the Ensemble interpretation. The above articles take on it is:
'Nevertheless, probably due to the Copenhagen adoption of completeness in the wider sense (as a consequence of not sufficiently distinguishing it from completeness in the restricted sense), generic ensemble interpretations' were rejected by the physics community and the individual-particle interpretation' adopted. It is highly questionable whether, in view of present-day experimental evidence, the same choice is still possible.'

True - Einstein in response to Bohr favored the Ensemble interpretation. But in 1970 it was revived by Ballentine:
http://www.informationphilosopher.com/solutions/scientists/ballentine/PR70.pdf
The paper has some issues I will let the reader figure out, but has been greatly expanded in his wonderful textbook - Quantum Mechanics - A Modern Approach. That book has some, IMHO, minor issues that I will again let others spot, It must be mentioned that at least one science adviser here thinks they are major. Obviously I do not but will not argue about it - make up your own mind.

Thanks
Bill

vanhees71
The above articles take on it is:
'Nevertheless, probably due to the Copenhagen adoption of completeness in the wider sense (as a consequence of not sufficiently distinguishing it from completeness in the restricted sense), generic ensemble interpretations' were rejected by the physics community and the individual-particle interpretation' adopted. It is highly questionable whether, in view of present-day experimental evidence, the same choice is still possible.'
In direct opposition to this, I find it highly questionable that only an 'ensemble interpretations' is adopted by some, when the 'individual-particle interpretation' is the only natural one to explain continuous measurement experiments done on single quantum systems, such as the one discussed here.

dextercioby
Mentor
If you want to read something really good by the founding fathers, avoid Bohr and Heisenberg and turn to Born, Dirac, Pauli, and Schrödinger ;-)).

Especially Dirac. His mathematical approach is clear. I sometimes think of the original founders he was the one that got it right. He also had an interesting view on science itself detailed in discussions with Heisenberg:
http://philsci-archive.pitt.edu/1614/1/Open_or_Closed-preprint.pdf
I personally side with Dirac - Heisenberg for me is too much like Kuhn who I do not have a lot of time for personally - I tend to side with Weinberg:
https://www.physics.utah.edu/~detar/phys4910/readings/fundamentals/weinberg.html'All this is wormwood to scientists like myself, who think the task of science is to bring us closer and closer to objective truth. But Kuhn's conclusions are delicious to those who take a more skeptical view of the pretensions of science. If scientific theories can only be judged within the context of a particular paradigm, then in this respect the scientific theories of anyone paradigm are not privileged over other ways of looking at the world, such as shamanism or astrology or creationism. If the transition from one paradigm to another cannot be judged by any external standard, then perhaps it is culture rather than nature that dictates the content of scientific theories. '

The issue for me that Dirac has science as about truth - not sociological paradigm shifts. But that is philosophy which we really do not, by forum rules, deal with here. We did at one time but currently we do not have any experts in it as mentors - the one we had left unfortunately.

If that sort of thing really interests you then the following program with dual honors in physics and philosophy may appeal:
Thanks
Bill

vanhees71
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2022 Award
In direct opposition to this, I find it highly questionable that only an 'ensemble interpretations' is adopted by some, when the 'individual-particle interpretation' is the only natural one to explain continuous measurement experiments done on single quantum systems, such as the one discussed here.
As you bring this "quantum jumping" up again, just have a look at this (all standard QT, no discontinous jumps) (including the theory part which is in the supplements):

https://www.nature.com/articles/s41586-019-1287-z