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Published 27 April 2018

Yin and Yang, Bohr’s Complementarity and Inclusive “Knowledge”

Royal Society Te Apārangi Fellow, Emeritus Professor Robert Nola responds to the recent article “On science and stamp collecting” from Professor Richard Blaikie reflecting on the role of social science and the humanities in physical science.

Professor Richard Blaikie raises two important issues in his recent news item. The first is Rutherford’s view of science that it is either physics or stamp collecting. This is, of course, quite wrong. I will save my criticism of Rutherford for another time. The second is an attempt to try to bring together the humanities and the sciences by invoking Bohr’s complementary principle and his alleged acquaintance with eastern systems of belief; Professor Dame Anne Salmond also suggests something similar. This I will discuss here.

It is said that Bohr, when he was offered a Danish knighthood, constructed his own coat of arms and put in it the well-known Taoist Yin-Yang symbol (yin dark, yang light). He also added in Latin ‘opposites are complementary’. The symbol also appears underneath a bust of Bohr at the University of Copenhagen. There can be no problem with this. But it is a rather low-level unproblematic interaction between the sciences and the humanities, or more accurately a bit of ancient eastern religious speculation. (A biographer, Abraham Pais, tells us that Bohr was never interested in religion, or philosophy for that matter.) The Yin-Yang symbol is on the South Korean flag and one can even buy a Yin-Yang looking soup!

Coat of Arms of Niels Bohr.svg

Coat of Arms of Niels Bohr

Pais tells us more of Bohr’s coat of arms: "the belief that Bohr’s view on physics were influenced by oriental philosophy is unfounded. These speculations have an amusing origin"1. Then he goes on to tell us that Bohr could find no satisfactory coat of arms until the wife of a co-worker of his, Hanna Kobylinski who was a Chinese historian, suggested that he use the Yin and Yang symbol. So it is not as if Bohr was drawing on any deep acquaintance he had with eastern systems of belief!

Much more problematic is what inference can be drawn from this about the relationship between sciences and humanities. Does Bohr use some Taoist and/or Buddhist thought in his scientific publications? Well, it is said that he did mention these in his lectures. But did they appear in his papers? I have not read them all (too many!) but of the few I have read nothing of that sort appears in them. So I leave it to Bohr scholars to tell us whether or not Bohr puts into his scientific work the Yin-Yang doctrine. It is hard to get any evidence for this.

However there is a paper by Bohr ‘Discussion with Einstein on Epistemological Problems in Atomic Physics’ with a response by Einstein. If these two are going to have a “rap” about epistemology then surely this is the chance to cast their intellectual nets widely and introduce the Yin-Yang doctrine. There is no mention of it.

But there is a significant remark by Einstein who says of "Bohr’s principle of complementarity, the sharp formulation of which, moreover, I have been unable to achieve despite much effort which I have expended on it"2. This is a polite way of Einstein saying that he cannot make much sense of it and when he does, he does not accept it. And many other scientists did, and still do, hold this view. If Einstein cannot get a good formulation of Bohr’s principle of complementarity it is unlikely I will. But here goes.

The basic idea is of a “something” which has dual aspects which are normally exclusive of one another. In the 1920s Bohr began to think of the radiation emitted by atoms as having the dual features of a particle and of a wave. These cannot hold together at the same time, but Bohr claims that they can “complement” one another as one moves from the particle account to the wave account and back again. This is vague enough on what complementarity is, but it has to do.

In 1927 along came Heisenberg’s uncertainty principles which Bohr was glad to regard as a further instance of complementarity. An electron can have both momentum and position but any increase in accuracy of our knowledge of one of these has to be traded off with a decrease in our knowledge of the accuracy of the other. Again an exclusive duality. Though Heisenberg’s uncertainty principles are well founded in quantum mechanics, Bohr’s principle of complementarity is not; it is broader, vaguer and more “philosophical” (not necessarily a term of praise when used by scientists). This underlines the problem Einstein and many others had with formulating the principle of complementarity.

Bohr first aired his views on complementarity in his autumn 1927 Como lecture. There we get further examples of complementarity. First, in a way which is somewhat obscure, Bohr writes of ‘the complementary nature of the space-time description and the claims of causality’. Second, he finishes with the conjecture that complementarity is involved in the ‘general difficulty in the formation of human ideas inherent in the distinction between subject and object’. Whatever this is, it goes well beyond the issues in quantum mechanics which lead to complementarity in the first place.

In later lectures Bohr got carried away by the wide application of complementarity which he found nearly everywhere. For example, in a theory of word meaning: ‘the use of any word must stand in a complementary relationship to an analysis of its meaning’. Or in psychology where we are told: ‘the use which we make of words like "thought" and "feeling," or "instinct" and "reason" to describe psychic experiences of different types, shows the existence of characteristic relationships of complementarity conditioned by the peculiarity of introspection’. Hmm!

Are there no limits to the application of complementarity? The Danish historian of quantum mechanics, Helge Kragh3, tells us of an associate of Bohr’s, called Jordan, who extended complementarity to areas such as psychology, philosophy and biology, in particular vitalism in biology. The last of these Bohr strongly resisted. So there are limits to its application but they are not easy to discern.

What has this to do with Yin-Yang and its symbolism? This can also be understood to express a dualism of “opposites” which are also complementary. So it would appear that Yin-Yang is related to Bohr’s principle of complementarity, either as an instance of it or as an alternative expression of it.

Now did Bohr have prior knowledge of Yin-Yang and then apply it to his quantum mechanics? Or did his thinking about quantum mechanics, especially the problem of wave-particle duality, lead to the principle of complementarity of which Yin-Yang is a further example? One would have to leave this to Bohr scholars to try to determine what was going on in Bohr’s mind in the 1920s. But Kragh does say (p. 209): ‘There is little doubt that that the formulation of the principle was indebted to Heisenberg’s work with quantum uncertainties but the idea of complementarity was not merely a philosophical generalisation of Heisenberg’s principle. It grew out of reflections about quantum theory that Bohr has entertained before Heisenberg started his work’.

Bohr’s principle does have some correct instances, such as Heisenberg’s uncertainty results (and these are an established part of quantum mechanics). But it goes well beyond them into areas which have nothing to do with quantum mechanics and which might offer little in the way of confirmatory support, or be disconfirmatory.

Does anyone accept Bohr’s principle these days? One of the most popular interpretations of quantum mechanics is the “Copenhagen Interpretation”, a loose set of ideas which did include Bohr’s complementary principle (which was understood in different ways). But now there are over a dozen competing interpretations of quantum mechanics with their adherents. These can be in conflict with one another but without necessarily exhibiting complementarity.

With such a proliferation of theories, support for the Copenhagen interpretation and its complementarity principle has declined. If the Copenhagen interpretation is to be dropped, then the principle of complementarity goes as well. And if that goes, the Yin-Yang story goes as well. Other interpretations of QM need neither doctrine. In fact the principle of complementarity, far from explaining things, is a view which itself needs explaining as one digs further into quantum mechanics. It is not to be taken as a fundamental stopping point, or as an unexplained explainer, as Bohr seems to assume. In religious systems of belief the Yin-Yang of complementarity might be treated as an untestable dogma; in science it ought not be.

Perhaps one should not be surprised at the range of popular, bizarre accounts of quantum mechanics that have emerged, and to which Bohr’s principle might have contributed. The high-water mark of this might well be the 1975 book by Fritjof Capra, Tao of Physics.

Bohr’s principle has been invoked as a model for the relationship between the sciences and the humanities in which inclusiveness is meant to be paramount rather than division. But if one is going to advocate a kind of inclusiveness between the various bodies of human belief (not knowledge!) then it might be better to provide an independent justification of that without appeal to Bohr’s principle which does not have general acceptance in physics. But is inclusiveness always to be desired?

Though I am not a great fan of the Yin-Yang doctrine, I could take a cue from it and play up the conflict it supposes between incompatible dualisms; and then extend this, perhaps illegitimately, to our various systems of belief which in turn inform our actions. Sciences and humanities complement one another in that each offers a critical stance from which to assess the other. And this keeps both healthy. The humanities need criticism from the sciences given the way in which they have been devastated by postmodernist and other tendencies. And the sciences need criticism from the humanities given the way in which the world is being devastated by what we are doing to it through some of our science-based technologies. As Einstein said in a 1917 letter: ‘All of our exalted technological progress, civilization for that matter, is comparable to an axe in the hand of a pathological criminal’. What relief can the humanities give concerning Einstein’s gloomy picture?

robert nola

Emeritus Professor Robert Nola


Professor Richard Blaikie's article 'On science and stamp collecting'

Dame Ann Salmond's article 'Science of nature without culture' 


1. Abraham Pais (1991) Niels Bohr’s Times in Physics, Philosophy and Polity, Clarendon Press, p. 24.
2. P. A. Schilpp (ed.) (1959) Albert Einstein: Philosopher-Scientist, Harper Torchbook, Volume II, p. 674.
3. Helge Kragh (2002) Quantum Generations, Princeton University Press, p. 210