The physics vs. philosophy slow-motion blogfight continues, the latest major contribution being Sean Carroll’s “Physicists Should Stop Saying silly Things About Philosophy. I’ve been mostly trying to stay out of this, but when I read through the comments at Sean’s post to see if anybody offered any specific examples of problems that could’ve been avoided by talking to philosophers, I was kind of surprised to find a lot of people talking up Niels Bohr. (Likewise Ashutosh Jogalekar’s Philosophy Begins Where Physics Ends….) If you’re trying to talk up the virtues of philosophy over “pure” physics, I’m not sure Bohr’s your guy. Not because Bohr wasn’t philosophically inclined– Werner Heisenberg famously described Bohr as “primarily a philosopher, not a physicist”– but because I’m not convinced he was all that good at it.
If you asked me to draw up a list of the primary virtue of philosophy, I’d end up pointing toward a kind of intellectual precision. The thing that the best philosophers seem to do that is worth emulating is to think extremely carefully about what things mean, and work through the implications of those meanings. (This is also the thing that makes philosophy so tedious to read, because there’s a hundred pages of term-defining before you get to the interesting new stuff, but that’s a separate issue…) Philosophy at its best goes back to first principles, and verifies that those are, in fact, the first principles, before embarking on a clear and logical progression from those principles through their application to interesting problems.
And, you know, clarity of thought and expression just wasn’t Bohr’s thing. He was famous for his dithering equivocation (Graham Farmelo’s Dirac biography tells a story about the one time Dirac was asked to work with Bohr on a paper. After several false starts and much waffling, Dirac offered “In school, I was taught that one should not begin a sentence until one knows where it will end.”) This shows up most spectacularly in Bohr’s attempted response to the Einstein, Podolsky, and Rosen paper of 1935. Admittedly, this is a tough problem to answer, and he was a bit rushed in producing it, but Bohr’s paper is a hopeless muddle, and his responses didn’t get much better when he had more time to think about them.
His primary philosophical contributions are the Copenhagen Interpretation and the principle of “complementarity,” the idea of incompatible pairs of observables like the particle and wave natures of a quantum object. Nobody other than Bohr seems to have done much with complementarity other than attribute it to Bohr, because it doesn’t seem to have a very precise definition. It’s tempting to cynically suggest that complementarity meant basically whatever Bohr needed it to mean at the time he invoked it. (Later in life, he apparently tried to extend his philosophy of complementarity outside the physical sciences, which almost nobody talks about because it’s kind of embarrassing)
As for the Copenhagen Interpretation, that’s also kind of a mess, particularly during Bohr’s day, a shambling creature stitched together from ad hoc rules. It demands an absolute separation between microscopic systems obeying quantum rules and macroscopic ones that behave classically, and a quasi-magical collapse of the wavefunction that was never adequately defined.
Bohr was also somewhat prone to weird and poorly motivated enthusiasms. For a while during the “old quantum theory” period, he was adamantly opposed to the quantization of light, for reasons that seem to have been mostly aesthetic (this proved untenable, but not until after some fairly bad work). Having succeeded with one revolutionary overthrow of classical physics on grand philosophical terms, he kept holding out hope for another. He wanted to discard the idea of energy conservation as anything other than a statistical principle in order to deal with the problem of beta decay (Pauli solved this by introducing the neutrino). And when the problems of QED became clear, Bohr kept pushing for a grand and revolutionary solution, when the actual answer proved to be surprisingly conservative– the basic idea of renormalization was kicked around in the early 1930’s, but demanded a more systematic and mathematically sophisticated approach that wasn’t widely appreciated until the late 1940’s.
Bohr’s philosophical pursuits are important mostly in an inspirational sort of sense. That is, they became useful once other people got hold of them and sharpened Bohr’s hopelessly vague notions into something, well, philosophical. The best example is obviously Bell, who worked through the implications of the Local Hidden Variable models preferred by people like Einstein, contrasting them to the more orthodox quantum theory, and ending up with something precise, mathematically sound, and most importantly testable. Bell was inspired in large part by the work of David Bohm on an alternative interpretation of quantum mechanics where particles have well-defined trajectories shaped by a non-local guiding potential. And the Copenhagen Interpretation remained kind of muddled until things like Everett’s Many-Worlds Interpretation (which Sean evangelizes for here) and the general program of research into decoherence (which I tend to associate with Zurek, but he’s one of many) forced it to take on a more definite shape.
So, while Bohr was unquestionably trying to do philosophical sorts of things, I’m not sure I’d claim that any of them were all that successful. Now, I suppose you can claim this as support for the pro-philosophy camp in the sense that the things Neil deGrasse Tyson and others have called out as airy piffle aren’t any more airily piffilicious than Bohr’s work, and that ended up all right. Which, you know, I guess you can do, but it doesn’t mean that Bohr’s stuff wasn’t airy piffle. If you want to valorize people for thinking philosophically about physics, you’d be better off going with folks like Bell and Bohm, who brought some clarity and precision to the subject, like philosophers are supposed to do.