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In many books one finds different explanations. Specifically popular seems to be that he "argued against the Copenhagen interpretation". But what did he really intend to communicate?

I for myself have tried to come up with an answer to this question. Please feel free to present the correct answer and / or critique my version:

In my point of view, Schroedinger tried to discuss the current situation in quantum physics (at that time) which seems to be somewhat equivalent to what we now call the Copenhagen interpretation (?).

He discussed how one could understand the "Verwaschenheit" or "Unschärfe" (the fuzziness/blurriness) of quantum physics, which today might be called the probabilistic character of quantum mechanics (?).

He although seemed not to be as worried about the probabilistic features itself, but more how one could use a realistic interpretation on this (since he and Einstein seemed to have a realistic point of view when it comes to the interpretation). In his famous paper from 1935 (where he introduced the cat Gedankenexperiment) he claimed that it was impossible to interpret this blurriness as just "having not enough information" (e.g. a Gibbs Ensemble) and also argued that viewing this blurriness as a objective property of quantum systems is inadequate with a realistic point of view. This (latter specific point) was made clear by the cat Gedankenexperiment as the blurriness as an objective property would lead to something that doesn't match out intuition in the macroscopic world.

So in the end, he tried to make clear, that a realistic interpretation didn't work for any of the 2 possible ways (Gibbs Ensemble vs. objective property) the current situation of quantum mechanics (in other words the Copenhagen interpretation) was able to provide. The thought experiment was used to "cross one of the 2 possible ways out of the equation".

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    $\begingroup$ I’m voting to close this question because this question belongs to physics SE. $\endgroup$ Feb 20 at 22:09
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    $\begingroup$ This question has already been moved from physics SE because of members there saying it belongs here. $\endgroup$
    – manuel459
    Feb 21 at 10:27
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    $\begingroup$ In Einstein's Unfinished Revolution Lee Smolin discusses some of the history adjacent to this question. He takes a framing of realists like Einstein and Schrodinger vs antirealists like Bohr and Heisenberg. $\endgroup$
    – Galen
    Feb 22 at 2:32

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One can simply read what Schroedinger said, English translation of his paper The Present Situation in Quantum Mechanics (1935) by Trimmer is available on Jstor. The cat paradox is presented as part of the argument that granting reality to the wave function, "blurring" the real, as Schroedinger puts it, is absurd when applied to macroscopic objects, a rhetorical reductio of that idea. Since the Copenhagen interpretation did not treat the wave function this way (it was rather the most complete description an observer can have) this particular argument was not directed against it:

"The other alternative consisted of granting reality only to the momentarily sharp determining parts - or in more general terms to each variable a sort of realization just corresponding to the quantum mechanical statistics of this variable at the relevant moment. That it is in fact not impossible to express the degree and kind of blurring of all variables in one perfectly clear concept follows at once from the fact that Q.M. as a matter of fact has and uses such an instrument, the so-called wave function or $\psi$-function, also called system vector. Much more is to be said about it further on. That it is an abstract, unintuitive mathematical construct is a scruple that almost always surfaces against new aids to thought and that carries no great message.

[...] But serious misgivings arise if one notices that the uncertainty affects macroscopically tangible and visible things, for which the term "blurring" seems simply wrong... One can even set up quite ridiculous cases. A cat is penned up in a steel chamber... [description of the paradox follows]. It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a "blurred model" for representing reality".

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  • $\begingroup$ Let me please ask two questions to get this right: 1) When Schroedinger says "which can then be resolved by direct observation" - is he driving at the collapse of the wavefunction by "observation as a measurement" or simply the contradiction to observations of daily life? 2) Many say Schroedinger wants to express that quantum mechanics was incomplete. How does this associate with him trying to deny granting a reality to the wave function? $\endgroup$
    – manuel459
    Feb 23 at 11:52
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    $\begingroup$ @manuel459 1) He refers to absurdity of observing superpositions of dead and alive cats and the like before collapse. 2) That is the part where he, like Einstein, opposes Copenhagen. However, his target with the cat is not Copenhagen itself, but rather a combination of completeness claim with a naive realist view of the wave function (which Copenhagen says is a complete description). It is similar to Einstein's point in EPR that Copenhagen's view is incompatible with local realism that he advocated. $\endgroup$
    – Conifold
    Feb 23 at 13:16
  • $\begingroup$ Thank you. So by arriving at a contradiction from the assumption, that the wave function is a complete description leads Schroedinger to the conclusion, that quantum mechanics is incomplete. Correct? Just to be sure. $\endgroup$
    – manuel459
    Feb 23 at 15:27
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    $\begingroup$ @manuel459 From two assumptions, completeness and realism in a classical observer-removed sense that Copenhagen gave up. Einstein and Shroedinger thought that parting with it was too high a price, hence the hope for "completion". $\endgroup$
    – Conifold
    Feb 23 at 18:50
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    $\begingroup$ @manuel459 It is only alluded to. In section 9 he states that "rejection of realism also imposes obligations" and proceeds to flesh out their unpleasantness. In section 15 he points out incompatibility of "sharp time" with relativity, mentions that Dirac's relativistic theory of electron "very strongly transcends the conceptual plan of Q.M. (that which I have attempted to picture here)", and suggests that the "plan" is "after all only a convenient calculational trick, but one that today, as we have seen, has attained influence of unprecedented scope over our basic attitude toward nature". $\endgroup$
    – Conifold
    Feb 24 at 11:30

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