What was Niels Bohr’s interpretation of quantum mechanics?

In asking this seemingly innocent question, we have sinned twice. First, we have neglected that Bohr’s thought was a moving target: Finely sifting his words, one can make a good case that his thinking changed, not so much in the big ideas he advocated but in how he advocated for them. Second, more fundamentally, the idea of an “interpretation of quantum mechanics” is a modern one. It presumes that the mathematics of quantum theory is established, agreed upon and empirically validated beyond all reasonable doubt, and that what we lack is only a narrative about how these equations tie back to nature. But Bohr was a pioneer, and his heyday was the age of ferment, when those equations and their interconnections were being hashed out, co-evolving with the individually unstable and mutually contradictory worldviews of all the pioneers. To ask for Bohr’s “interpretation” is to demand an anachronism.

So, then, let us contemplate Bohrian thinking at its most refined and battle-tested. If we focus upon Bohr’s later writings, after the hurly-burly of the Einstein—Podolsky—Rosen affair, we can articulate a mature Bohr-ism that provides the most fruitful position for analysis. Bohr has a reputation for opacity, typically blamed on his writing style but also due on a more subtle level to the fact that he is often concerned with issues other than what a modern reader expects to find in a “quantum foundations” essay. If one jumps in and opens one’s eyes beneath the surface, there will be more to see than the rumors foretold. His 1938 Warsaw lecture is a good place to start.
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