The Copenhagen Interpretation of Quantum Mechanics Is Not Idealist in Nature
The Copenhagen interpretation of quantum mechanics — and even quantum mechanics generally — has been both loosely and strongly associated with the philosophical position of idealism (see here).
Of course, there are almost as many varieties of idealism (from subjective idealism to absolute idealism to pluralist idealism) as there are idealists. So this isn’t the place to run through all of them. In any case, the kind of idealism I have in mind will become clear in this essay itself.
So take as an example the following passage (written in 2005) from Harry Nielsen:
“ Almost unbelievably perhaps, [the academics who developed quantum mechanics] chose to interpret the strangeness of quantum behaviour by denying the existence of physical reality. And as a standard textbook interpretation of quantum mechanics, physicists have been taught for the last 80 years that physical reality therefore only exists as a result of the act of observation. This is the ‘Copenhagen interpretation’ of quantum mechanics, developed in the late 1920’s by Niels Bohr and Werner Heisenberg. To quote Heisenberg: ‘I believe that the existence of the classical ‘path’ can be pregnantly formulated as follows: The ‘path’ comes into existence only when we observe it’.”
The passage above doesn’t actually mention “idealism” — yet the general gist seems to be that the Copenhagen interpretation is idealist (or at least subjectivist). And it isn’t at all odd that this writer thought this way about this interpretation of quantum mechanics.
So it’s important to stress that the Copenhagen interpretation of quantum mechanics is not idealist. Or at least the Copenhagen interpretation itself can be interpreted as not being idealist. That said, it is difficult to explain why that’s so. Indeed not many physicists — or even philosophers — have fully clarified why that is so.
Finally, there are stronger arguments which assert that quantum mechanics — if not the Copenhagen interpretation itself — is entirely subjectivist in nature. Yet there are clear differences between subjectivism and idealism. Indeed, Harry Nielsen’s words above can be interpreted as only claiming that the Copenhagen interpretation is subjectivist, not idealist.
Various philosophers (such as Karl Popper) have also argued against those who claim that quantum mechanics is subjectivist , let alone idealist (see Popper here).
Arthur Mach’s Position
It is therefore convenient that there’s a short and simple passage (as quoted in Karl Popper’s The Logic of Scientific Discovery) from the Austrian physicist Arthur March (1891–1957) which perfectly clarifies this issue.
Firstly, Arthur March makes an explicit anti-idealist statement. (It’s not clear if that was his main intention — indeed, it probably wasn’t.) He wrote:
“Naturally nobody is so mad as to assert that a body ceases to exist the moment we turn our backs to it [].”
[March was referring to quantum bodies such as particles; not cups, trees or human beings.]
An idealist, on the other hand, would argue that
a body ceases to exist the moment we turn our backs to it.
And that’s because, to an idealist, “a body” — and everything else — only exists in the individual mind or in some kind of collective mind. Thus, it does indeed cease to exist when “we turn our backs to it”. (Unless, that is, it still exists in the mind of God — as with Bishop Berkeley immaterialism, in some other being’s mind or in a collective mind.)
Arthur March, on the other had, stated that it is “mad [] to assert” that idealist position. Yet March did preface that phrase with the following words:
“One may say perhaps without fear of being misunderstood [] that for the physicist a body has reality only in the instant in which he observes it.”
It’s worth noting the phrase “for the physicist”.
This isn’t an expression of a philosophical view or even of a layperson’s position. As a physicist, March was speaking entirely as a physicist.
So even though it may be problematic to stress observation alone, this is still the stress of a physicist qua physicist. And, almost by definition, a physicist is only concerned with the observations of — and experiments on — a (in March’s word) body.
Thus March, qua physicist, stated that the
“[body] does cease, in the moment, to be an object of inquiry for the physicist”.
Why is that?
It’s
“because there exists no possibility of saying anything about it which is based on experiment”.
March’s statement above is true by definition. That is, what comes before the experiment can’t actually be part of the experiment. Or if someone does “say[] anything about it”, then what he says can’t be based on the experiment, and is therefore not itself physics. Indeed, even the mathematical formalism and/or the wave function required for the experiment has nothing to “say” about any bodies before the experiment.
The wave function has just been mentioned.
Thus, the standard interpretation of the wave function is that — in itself — it has no physical interpretation. It is purely a mathematical function. A tool. Something physicists use to do things with. Of course, it can then be given a physical interpretation.
Take the Schrödinger equation.
This doesn’t tell us what the wave function is — other than in purely mathematical terms. The physical meaning of the equation — i.e., what entities, etc. are posited — is a product of the interpretation of quantum mechanics the physicist adopts.
And even when physicists do “guess” at the (as it were) before-observation, that too is only a use of the wave function; which, again, is a large abstract mathematical object made of smaller mathematical objects.
So, basically, Arthur March, qua physicist, wasn’t interested in what might have been the case before the experiment. And, presumably, he wasn’t that interested in what was the case — i.e., in the experimental area — after he took off his white coat and packed up his bags to go home. Alternatively, if he was interested in both the before and after of the experiment, then it was as a philosopher or a layperson, not as a physicist.
Conclusion
As hinted at in the introduction, it’s at least partly understandable that some people see the Copenhagen interpretation as being philosophically idealist in nature. In the passage in the introduction, for example, Harry Nielsen quoted Werner Heisenberg.
So I’ll help Nielson’s cause along by also quoting Heisenberg stating the following:
“[A]n ‘objective’ physics in this sense, i.e. a sharp division of the world into object and subject has indeed ceased to be possible.”
Such passages from Heisenberg and others have provided grist to the mill for various idealists, mystics, New Agers and whatnot. Yet even here (on my reading of idealism at least) an idealist simply can’t make any distinction at all between “object and subject”. (Of course, it’s possible that the word “object” can be defined in such as way that it squares with idealism.) That’s because (to an idealist) literally everything is in the individual’s (or experimenter’s) mind. (Either that, or everything is in a collective mind or in the mind of God.) In other words, there is simply no object in idealism — we only have a subject or subjects.
