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Causality and Complementarity

1948 to 1950
University of Edinburgh

Noted atomic physicist Niels Bohr gave the 1949 Gifford Lectures at the University of Edinburgh under the title “Causality and Complementarity.”  The lectures remain unpublished, but the audio recordings of 9 of the 10 lectures [lecture 2 is unfortunately missing] are maintained at the Niels Bohr Archive ( A brief summary of Bohr’s Gifford Lectures is published in Complementarity beyond Physics (1928–1962); ed. David Favrholdt; The Complete Works of Niels Bohr, vol. 10 (Amsterdam: Elsevier, 1999): 174–181. A series of supplementary papers has been published under the same title [Niels, Bohr, Causality and Complementarity, The Philosophical Writings of Niels Bohr, Volume IV, eds. Jan Faye and Henry J. Folse (Woodbridge, CT: Ox Bow Press, 1998)], but these do not include the text of Bohr’s Gifford Lectures.

Bohr’s initial lectures examine the history and progression of natural philosophy and atomic theory. Bohr recounts the predominant scientific models extending from ancient Greece through to the modern development of quantum theory based on the progressive examination and description of natural phenomena, particularly motion and movement. He describes how the Aristotelian explanation of physics—which focused on the search for causality that tied together the Aristotelian scientific enterprise with a broader search for the purpose of life—dominated for almost 2,000 years. Galileo’s and Newton’s observations of motion required a new explanatory frame that could not be contained by the previous categories of Aristotelian natural philosophy. Newton’s explanations remained the predominant scientific paradigm until modern atomic observations once again required the development of a new frame of scientific analysis and description. 

The middle lectures, then, provide a description of these new atomic discoveries associated with Bohr’s quantum theory. Bohr engages in a detailed discussion of the various phenomena that led to the formation of the new quantum theory of mechanics, particularly focusing on the observations of subatomic particles and photons. The complex movement of subatomic particles requires a new statistical analysis (or analysis of probability) rather than a causal explanation of movement. Bohr further describes how the method of observation affects the phenomena observed and how these different observations may even seem mutually exclusive. However, the various observations should be understood as complementarity properties of the object. None of the observations are able to account for the full scope of phenomena of an object, but when understood as complementary, they can lead to a more accurate description of the observed phenomenon.

Bohr’s final lectures suggest that quantum mechanics require the development of a new language of description—one that implements new words to communicate the richness of physical experiences while also remaining cognizant of the fact that they do not contain the whole description of the event. Such descriptive language is to be comparative rather than deterministic.  Bohr ultimately concludes that this has epistemological significance beyond the field of physics.  In addition to its implications for understanding the origins of organic life, Bohr suggests that such scientific observations require the appropriate balance of differences across the spectrum of human experience. To illustrate this, Bohr uses the example of seriousness and humor, which, though seemingly mutually exclusive, are complementary descriptions of human attitude and which require a fine balance in the daily life of individuals. Ultimately, Bohr’s theory of complementarity is a theory of reconciliation, suggesting that it is possible to include different descriptions of human life and ideology that, even though they may seem mutually exclusive, are complementary descriptions of the nature of reality from different observational standpoints. 


Causality and Complementarity

  • Eric Covington, University of St Andrews