Constituents of Matter / 5 / 5.2 / 5.2.1

5.2.1 Bohr’s Complementarity Principle

Constituents of matter are objects of explanation. An object of explanation is not given to us to observe, in the way objects of experience are, but rather it is something we must give to ourselves in order to explain the behavior of objects of experience; should it fail to do so, it would be discarded. Since the Being of such an object thus resides in its power to explain a specific experiential situation involving specific objects of experience, the manner of being proper to it is always to refer to that specific experiential situation; objects of explanation are referential.

Now, let us consider a predicated measurement designed to observe such a constituent of matter. The object we must give ourselves in order to explain the outcome of such a measurement must, being itself an object of explanation, refer to the specific experimental situation involved. Two different observations performed on a constituent of matter which had been manufactured in the same way for both observations should therefore be explained by two different objects. For instance, if we manufactured an electron for the purpose of subsequently observing its momentum, the object of explanation which explained the outcome would be an electron with the measured momentum assigned to it. If, on the other hand, we manufactured the electron for the purposes of subsequently observing its position, the object of explanation which explained this outcome would be an electron with the measured position assigned to it. As we said above, these two objects need not be the same because they refer to different experimental apparatuses. But, if one insisted that the electron was instead an object of experience rather than an object of explanation, the two objects would have to be the same. We would then have to say that the electron was given to us with a specific value for its momentum and a specific value for its position and that the observations only served to ascertain those pre-existing values. Therefore, trying to capture the referentiality of an electron of explanation with the language proper for an electron of experience would require that the electron acquire a behavior deemed strange for an object of experience: it would have a momentum in the first observation but, because unobserved, it would have no position, and it would have a position in the second observation but, again because unobserved, it would have no momentum. Momentum and position would then be complementary, rather than simultaneous, physical properties of the electron.

For us therefore the Complementarity Principle results from the founding father’s attempt to describe the referentiality of an object of explanation in a language proper for an object of experience which lacks that feature. Bohr’s opaque assertion that different observations set up “different conditions for defining the possible types of predictions which regard the subsequent behavior of the first system” was nothing else but his awkward attempt to express a distinct sense that quantum observations were referential. For the founding fathers of quantum mechanics, on the other hand, the ultimate justification for the Complementarity Principle was that, as Feynman said, it managed to save the theory; there was no deeper reason for it than the fact that it worked. Understanding the ontological structure proper for constituents of matter provides a foundation for the strangeness of quantum mechanics which would otherwise appear to be merely contingent. It explains why the theory must include this principle at all.