It seems today a lot of people are happy to think about quantum mechanics in terms of collapse theories, without thinking about where in the process the collapse occurs or what triggers it etc. This is (according to my understanding) because decoherence itself (which is part of quantum theory not of the interpretation) would guarentee one would not be able to carry out experiments that would reveal where in the von Neumann chain the collapse occurs. See also the answers I got to my question here Position of collapse in von-Neumann chain where decoherence was stated as the reason why the point of collapse could not be determined.

But why was the Copenhagen interpretation widely accepted before decoherence was discovered? Did the proponents have the same intuitive idea of decoherence?

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    $\begingroup$ The interpretation of the interpretations I use is: decoherence is a facet of quantum mechanics which is sufficient to explain all observed measurements if everything is governed by QM. The interpretations, such as Copenhagen, instead start from the assumption that classical observers indeed exist, and explore the question of what these classical observers might indeed measure. It's not an answer to your question, but I think it does point in the general direction. $\endgroup$
    – Cort Ammon
    Jul 6, 2017 at 18:05
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    $\begingroup$ "It seems today a lot of people are happy to think about quantum mechanics in terms of collapse theories, without thinking about where in the process the collapse occurs or what triggers it etc." That seems, to me, not to be the case. The nature of collapse is not only a point of frequent discussion, but is actually the subject of active research (1 2). I can give more links if you're interested. $\endgroup$
    – DanielSank
    Jul 6, 2017 at 18:22
  • $\begingroup$ In light of our policy on history questions and the nature of the question and answer here, I'm declining a request to migrate this question to History of Science and Mathematics and removing a comment discussion about its on-topic-ness. If you have strong feelings about this decision, let's talk in Physics Chat. $\endgroup$
    – rob
    Jul 9, 2017 at 22:30

1 Answer 1


The issue with the Copenhagen interpretation is that when people claim that they support it, it doesn't actually mean that they support the stuff Bohr and Heisenberg were saying, but rather something they think is a "standard way". Their "standard way" can differ A LOT because a lot of physicists actually don't think about this stuff enough. And it's a good thing. Because many of them don't know enough about this topic (quite often it's perfectly ok for a working physicist to know QM on the level of particle in one-dimensional well, which is not enough for "interpretational" talks) Worse than that is constant misrepresentation of the Copenhagen interpretation by "interpreters" and philosophers.

If you want to understand what Copenhagen interpretation actually was the shortest way is to read Chapter 3 of the book by Werner Heisenberg "Physics and Philosophy" which is entitled... "The Copenhagen interpretation of quantum theory".

I would like to present the following excerpts, make from them what you want.

When we now come to the next observation. the result of which should be predicted from the theory, it is very important to realize that our object has to be in contact with the other part of the world, namely, the experimental arrangement, the measuring rod, etc., before or at least at the moment of observation. This means that the equation of motion for the probability function does now contain the influence of the interaction with the measuring device.

This influence introduces a new element of uncertainty, since the measuring device is necessarily described in the terms of classical physics; such a description contains all the uncertainties concerning the microscopic structure of the device which we know from thermodynamics, and since the device is connected with the rest of the world, it contains in fact the uncertainties of the microscopic structure of the whole world. These uncertainties may be called objective in so far as they are simply a consequence of the description in the terms of classical physics and do not depend on any observer. They may be called subjective in so far as they refer to our incomplete knowledge of the world.


We have to add some comments on the actual procedure in the quantum-theoretical interpretation of atomic events. It has been said that we always start with a division of the world into an object, which we are going to study, and the rest of the world, and that this division is to some extent arbitrary.

It should indeed not make any difference in the final result if we, e.g., add some part of the measuring device or the whole device to the object and apply the laws of quantum theory to this more complicated object. It can be shown that such an alteration of the theoretical treatment would not alter the predictions concerning a given experiment. This follows mathematically from the fact that the laws of quantum theory are for the phenomena in which Planck's constant can be considered as a very small quantity, approximately identical with the classical laws. But it would be a mistake to believe that this application of the quantum-theoretical laws to the measuring device could help to avoid the fundamental paradox of quantum theory.

The measuring device deserves this name only if it is in close contact with the rest of the world, if there is an interaction between the device and the observer. Therefore, the uncertainty with respect to the microscopic behaviour of the world will enter into the quantum-theoretical system here just as well as in the first interpretation. If the measuring device would be isolated from the rest of the world, it would be neither a measuring device nor could it be described in the terms of classical physics at all.

  • $\begingroup$ I really like your presentation here, but one quick comment: what kind of working physicist would only know quantum mechanics to the extent of a particle in a one dimensional well? I'm pretty sure every undergraduate physics degree requires much deeper understanding than that (even if they don't know enough for "interpretational" talks, as this typically is a different area of study completely). $\endgroup$ Jul 7, 2017 at 5:26
  • $\begingroup$ @BBobKnighton Well,probably somewhat deeper than that. But many are familiar with closed quantum systems whereas open quantum systems rapidly vanish from their mind. I met a lot of people for whom density matrix is only some obscure object they had exam about many years ago. Even met once such a guy who work on some open quantum systems)))) $\endgroup$
    – OON
    Jul 7, 2017 at 5:46

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