This is the article by A. Einstein, B. Podolsky, and N. Rosen to which E. Schrodinger was responding when he posited the thought experiment made famous under his name.
The Einstein, Podolsky, Rosen article is available here. The question of the completeness of a model is a very interesting question, and has importance in philosophy, politics, and art.
Schroedinger’s response to the paper above is THE PRESENT SITUATION IN QUANTUM MECHANICS:
A TRANSLATION OF SCHRÖDINGER’S “CAT PARADOX PAPER“.
Don’t be afraid of not understanding the terminology. There are important issues here about how we understand reality.
Schroedinger raises a point:
But it is quite probable that the adaptation of thought to experience is an infinite process and that “complete model” is a contradiction in terms, somewhat like “largest integer.
I believe that this is correct; that the idea of creating a model that is “perfect” or that mathematically defines from a given moment the complete state of the universe in the next moment is a misunderstanding of the existence of the universe and philosophically abhorrent in terms of conscious action. Further, it appears to me to be another form of chasing after “God” to chase after the “perfect” model.
Schroedinger is involved in a direct attack on idealist rather than materialist concepts of knowledge.
In critically looking at the current established views in physics, Schroedinger sees:
The rejection of realism has logical consequences. In general, a variable has no definite value before I measure it; then measuring it does not mean ascertaining the value that it has. But then what does it mean? There must still be some criterion as to whether a measurement is true or false, a method is good or bad, accurate, or inaccurate – whether it deserves the name of measurement process at all. Any old playing around with an indicating instrument in the vicinity of another body, whereby at any old time one then takes a reading, can hardly be called a measurement on this body. Now it is fairly clear; if reality does not determine the measured value, then at least the measured value must determine reality – it must actually be present after the measurement in that sense which alone will be recognised again. That is, the desired criterion can be merely this: repetition of the measurement must give the same result. By many repetitions I can prove the accuracy of the procedure and show that I am not just playing. It is agreeable that this program matches exactly the method of the experimenter, to whom likewise the “true value” is not known beforehand. We formulate the essential point as follows:
The systematically arranged interaction of two systems (measured object and measuring instrument) is called a measurement on the first system, if a directly-sensible variable feature of the second (pointer position) is always reproduced within certain error limits when the process is immediately repeated (on the same object, which in the meantime must not be exposed to any additional influences).
This is the point. Whenever one has a complete expectation-catalog – a maximum total knowledge – a psi-function – for two completely separated bodies, or, in better terms, for each of them singly, then one obviously has it also for the two bodies together, i.e., if one imagines that neither of them singly but rather the two of them together make up the object of interest, of our questions about the future.
But the converse is not true. Maximal knowledge of a total system does not necessarily include total knowledge of all its parts, not even when these are fully separated from each other and at the moment are not influencing each other at all. Thus it may be that some part of what one knows may pertain to relations or stipulations between the two subsystems (we shall limit ourselves to two), as follows: if a particular measurement on the first system yields this result, then for a particular measurement on the second the valid expectation statistics are such and such; but if the measurement in question on the first system should have that result, then some other expectation holds for that on the second; should a third result occur for the first, then still another expectation applies to the second; and so on, in the manner of a complete disjunction of all possible measurement results which the one specifically contemplated measurement on the first system can yield. In this way, any measurement process at all or, what amounts to the same, any variable at all of the second system can be tied to the not-yet-known value of any variable of the first, and of course vice versa also. If that is the case, if such conditional statements occur in the combined catalog, then it can not possibly be maximal in regard to the individual systems. For the content of two maximal individual catalogs would by itself suffice for a maximal combined catalog; the conditional statements could not be added on.
The limitations on knowledge, and the approaches that he is challenging, are areas that I want to return to in more depth. I don’t pretend to understand the underlying physics of the articles, but I get the gist of the argument.
The simple procedure provided for this by the non-relativistic theory is perhaps 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.
Schroedinger is arguing for the existence of reality, and reminding us that the model is not reality itself, but a convenient tool for exploring reality. This echoes in my own philosophical foundations with Marx:
The question whether objective truth can be attributed to human thinking is not a question of theory but is a practical question. Man must prove the truth — i.e. the reality and power, the this-sidedness of his thinking in practice. The dispute over the reality or non-reality of thinking that is isolated from practice is a purely scholastic question.
The materialist doctrine concerning the changing of circumstances and upbringing forgets that circumstances are changed by men and that it is essential to educate the educator himself. This doctrine must, therefore, divide society into two parts, one of which is superior to society.
The coincidence of the changing of circumstances and of human activity or self-changing can be conceived and rationally understood only as revolutionary practice.
There is a lot more to look at here.