Perhaps there isn't enough written by Niels Bohr to secure a faithful interpretation of his philosophy of quantum physics. That's despite the fact that Bohr did say a lot more than many other physicists on philosophical issues.
In more precise terms, Bohr never published a paper on the philosophy of quantum mechanics. (Though, clearly, he published much on physics itself - sometimes with philosophical interjections.)That's not a surprise: Bohr was a physicist, not a philosopher. So, again, because of that lack of technical philosophical detail, and philosophical argument, it's no wonder that philosophers argue about what exactly Bohr's philosophical position was. It's also worth noting that many of his philosophical remarks on quantum mechanics came after he'd done his important and relevant work in physics. (His published philosophical positions came largely after the 1920s - and sometimes a lot later than that.)
Thus it may also seem odd that there are books on “Bohr's philosophy”. And we also have books actually of (rather than about) Bohr's philosophy - such as The Philosophical Writings of Niels Bohr (in three volumes). Nonetheless, the problems I've just highlighted are summed up by one reviewer of this book. He writes:
“Niels Bohr's view of the world is always something to keep in mind. But this book's title is misleading: if you're trying to understand how the (back then) new discoveries and theories in physics may have affected and /or influenced Niels Bohr from a non-theoretical, philosophical point of view, you won't find it here.”
To sum up Bohr's philosophical position. I would say that “subjectivist” is a far better term than “idealist” to sum up Bohr's philosophy of quantum physics. And, in turn, “anti-realist” is far better than “subjectivist”. The term “idealism”, for one, comes with far too much philosophical and historical baggage to affix it to Bohr's positions. In addition, it's clear that Bohr fused subjectivism with intersubjectivism. (The latter position is something that the logical positivist Rudolf Carnap had to take on board in order to escape from the possible “solipsism” of his 1928 Aufbau position.) Indeed, as a physicist, Bohr could hardly not have fused subjectivism and intersubjectivism.
i) Niels Bohr's Ant-Realism?
Although I'm shoehorning terms from late-20th-century analytic philosophy into physics here, it can be said that Niels Bohr perfectly expressed an anti-realist position on physics (without ever actually using the term “anti-realism”) in the following:
“Physics is not about how the world is, it is about what we can say about the world.”
And in his The Unity of Human Knowledge (1958-1962), Bohr offers us a slightly more detailed anti-realist account of physics:
“Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods of ordering and surveying human experience.”
Of course Bohr wasn't alone. For example, the Swiss-American theoretical physicist, Wolfgang Pauli, went further when he rejected the opposition (i.e., between reality itself and what we can can know about reality) entirely when he stated the following (as quoted by N. David Mermin):
“One should no more rack one's brain about the problem of whether something one cannot know anything about exists all the same, than about the ancient question of how many angels are able to sit on the point of a needle.”
In other words, “how Nature is” amounts to no more than a metaphysician's dream. All we have is “what we can say about Nature”. And, at the quantum-mechanical level, what we can say is what we can say with mathematics. Consequently, just about everything else is analogical and/or imagistic. Indeed the analogical stuff can (or does) often mislead us. And perhaps that's also partly the source of quantum mechanic's "weirdness".
Despite all that, the quotes above don't in any way suggest that (as the philosopher Michael J. Loux puts it about idealism and anti-realism) physicists will “we make it all up”. (Or, as some say about postmodernism, that “anything goes”.) And neither does it commit physicists to idealism. What it does tell us is that we don't have something “a priori given”: what is given is mediated through “human experience”. How can it be otherwise? Nonetheless, not all of this need to be entirely “subjective” either - it is often intersubjective. As Bohr himself puts it:
“In this respect our task must be to account for such experience in a manner independent of individual subjective judgement and therefore objective in the sense that it can be unambiguously communicated in ordinary human language.”
This means that even if a multitude of different minds come to state various things about x, they're still (at least roughly) saying similar things about x. So, yes, minds matter. But that doesn't mean that anything goes or that physicists make it all up. After all, if a multitude of minds agree that the earth isn't flat, then surely that doesn't also mean that all those minds make that fact up.
It's true that Niels Bohr did seem to argue that our experimental results didn't reflect a reality which existed independently of our measurements. Yet surely if there were a different reality, then there would be different results. This isn't of course to say that there's a perfect mirroring of that reality with our results. Though there is a mind-independent reality which somehow determines or causes our experimental results. This, of course, isn't the claim that our theories or terms perfectly mirror (or mirror at all) that mind-independent reality. The claim is very simple and somewhat obvious: the mind-independent world can't be factored out.
So Bohr was wrong to say that nothing exists until it is measured. However, in a strong sense, it may as well not do. That is, we can say very little about that x as it is before it's measured or experimented upon. In that sense, it serves almost no purpose. Yet it still has causal effects on what we say. It's still an x which has specific results on our experiments and measurements. Thus if that x were substituted with y, then we'd have different experimental results and different measurements. In that sense, the noumenon (to use a Kantian term) that is x is obviously of vital importance.
In the same vain. Simply because our measurements and experiments produce changes in what's been measured, that doesn't also mean that what's been measured is somehow factored out. And neither does it mean that there's no relation between what's being measured and what we say about what's being measured. Again, there will be no perfect mirroring because such a notion hardly makes sense. However, there may be very strong relations between what we say and what we measure. Those relations may be symmetrical, isomorphic, etc. in nature. There may also be various correspondences (though not mirrorings) or modelings of various kinds.
Bohr himself went further and into detail on this.
For example, he claimed that the spin of an electron or the momentum of an atom aren't things which reflect what is the case mind-independently. It's true that there's (obviously) some level of contingency in our descriptions and experimental results. However, the fact that physicists have stuck with words like “spin” and “momentum” (as well as the precise measurements of spin and momentum) must mean that they're getting at least something right.
Bohr also gave an explicitly anti-essentialist position when he said that
“our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, as far as possible, relations between the manifold aspects of our experience”.
Nonetheless, Bohr does (again) seem to needlessly play down the world and, correspondingly, play up mind or “our experience”.
For one, we can deny “real essence” yet also play down the idealist, subjectivist or phenomenalist implications of Bohr's words. After all, when it comes to elementary particles, the distinction between essential and contingent properties is hard to maintain in the sense that besides spatial, temporal and relational properties, properties such as spin, mass and energy are the only properties particles have. So, for example, when you give the spin or mass of an electron, you're not factoring out a whole host of contingent properties of that electron in one's descriptions.
The subjectivist import of Bohr's words is also apparent when he said that we
“only to track down, as far as possible, relations between the manifold aspects of our experience".
Why not say, instead, that we should track down the relations between the “manifold aspects of experience” and what (causally) gives rise to that experience? It's true that this is a difficult ontological and epistemological nut to crack. However, it's a nut which exists. Thus if it's literally all about the manifold aspects of our experiences, then why talk about “experiments”, “observations” or “electrons” at all? Why not give a purely phenomenological account of the specific experiences of physicists at particular times? Indeed why single out what physicists say in the first place – why not ask sociologists or fishermen?
Subjectivism vs. Intersubjectivism
Despite all the above, Bohr himself notes the problem with over-stressing what he calls the “subjective element”. Or at least he does so when it came to Albert Einstein's relativity theory. Here is an example:
“Today we know that 'simultaneity' contains a subjective element, inasmuch as two events that appear simultaneous to an observer at rest are not necessarily simultaneous to an observer in motion.”
The above is the “subjective element”. What follows is what we can call the intersubjective element of exactly the same situation:
“However, the relativistic description is also objective inasmuch as every observer can deduce by calculation what the other observer will perceive or has perceived.”
Still, Bohr ends on a non-classical note when he says that
“[f]or all that, we have come a long way from the classical ideal of objective descriptions”.
And Bohr makes roughly the same point elsewhere when he says that
“every physical process may be said to have objective and subjective features”.
“Admittedly, even in our future encounters with reality we shall have to distinguish between the objective and the subjective side, to make a division between the two. But the location of the separation may depend on the way things are looked at; to a certain extent it can be chosen at will.”
To put all that in more basic terms: it's clear that Bohr's position (or positions) could never be deemed to be entirely subjectivist. Indeed isn't that obvious? Though, as we can see in the quotes above, there is indeed an element of subjectivism (or experientialism) in Bohr's statements.
ii) The Case of Waves & Particles
As Bohr put it, the words “wave” and “particle” are “classical terms”:
“However far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms.”
Bohr extends what he says about classical terms and refers to “what we have done” in our “experiments”. He wrote:
“The argument is that simply by the word 'experiment' we refer to a situation where we can tell others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics.”
In a certain sense, we have nothing more than classical terms because even when we attempt to describe the “strange” goings-on of the quantum world, we still use classical terms. Why? Because there's nothing else we can use other than the mathematics.
Mathematics has just been mentioned. Bohr makes it clear that only mathematics gives us a true of picture of the quantum realm. He wrote:
"We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.”
This, again, is a roundabout of saying that only the mathematics is adequate when it comes to describing sub-atomic phenomena. Thus, by this definition, the words “particle” and “wave” simply can't do the full job. They can do part of the job; though not the full job. Indeed it's not just the word “particle” that's problematic, thinking of particles as things is too. Bohr wote:
"Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems."
In other words, it can't possibly be about the world “as it is in itself”. Every statement we make about the world comes with a lot of contingent baggage. There is of course a causal set of relations to any x – though those very same relations can be described in an indefinite number of ways.
So when Louis de Broglie argued (in 1924) that that every moving particle (yes, particle) can be equally described as either a wave or a particle, that may well be because this x is neither a wave or a particle. Nonetheless, describing x as a wave or a particle still helps both physicists and ourselves.
It's also the case that the words “particle” and “wave” carry far too much baggage. After all, the original wave-particle experiments had water waves in mind. Is that a good thing when it comes to talk about things happening at the subatomic level? Yes, it is if it helps us get of grip of things. The same goes for the word “particle”. Thus there are a host of good reasons as to why x should be seen as a particle. Yet there are also a host of equally good reasons as to why we shouldn't see it as a particle.
For instance, is a photon a particle? Well, they don't have mass for a start. And that's partly why the physicist Willis Lamb said:
“At the first of the 1960's Rochester Coherence Conferences, I suggested that a license be required for use of the word photon, and offered to give such license to properly qualified people.”
Lamb then went on to say that “[t]here are very good substitute words for 'photon' (e.g., 'radiation' or 'light')”.
Bohr himself argued that the reality behind our measurements and experiments is that there is neither a particle nor a wave. Indeed linguistic practicalities led me to want to say: The reality behind the particle/wave measurements and experiments is that there is neither a particle nor a wave. But we must still talk about something. However, that something isn't necessarily a thing as such (this is grammar speaking here). And we give that x (which doesn't need to be a thing, only a something) the name “wave” or “particle”.