”It might perhaps interest you that in 1927 when Niels Bohr tried to interpret quantum theory in philosophical terms, he thought over it for months and discussed with friends which words would be most appropriate to render it, and finally indicated it with the word ‘complementarity'..."
HEISENBERG PROVIDES HIS RECOLLECTIONS OF THE ORIGINS OF NIELS BOHR’S CONCEPT OF “COMPLEMENTARITY”.
A UNIQUE ITEM, OF UTMOST IMPORTANCE FOR THE LIGHT IT SHEDS ON THE PHILOSOPHICAL FOUNDATIONS OF QUANTUM MECHANICS AS SEEN BY THE PIONEERS IN THE FIELD.
It is by now a commonplace observation that until their properties are determined by a particular experiment or measurement, quantum systems can have a dual or multiple nature — light can be both a wave and a particle, an electron can follow two distinct and inconsistent paths, and so forth. Niels Bohr, in a famous 1928 paper, “The Quantum Postulate and the Recent Development of Atomic Theory,” described this aspect of quantum theory as “complementarity.” As explained by historian of science Max Jammer, in his magisterial book “The Conceptual Development of Quantum Mechanics”:
”… Bohr asked himself how it is possible that [the equations governing radiation] relate together characteristics of radiation which, strictly speaking, are contradictory to each other: particle attributes energy and momentum, necessary for the description of the interaction of radiation with matter … and frequency and wave number, necessary for the description of the propagation of light, as in the phenomena of interference and diffraction...
"It was this mutual exclusion and, at the same time, indispensability of fundamental notions and descriptions which led Bohr to the conclusion that the problem with which quantum physics found itself confronted could not be solved by merely modifying or reinterpreting traditional conceptions. What was needed, he concluded, was a new logical instrument. He called it “complementarity,” denoting thereby the logical relation between two descriptions or sets of concepts which, though mutually exclusive, are nevertheless both necessary for an exhaustive description of the situation.”
Complementarity is one of the fundamental pillars of the so-called, and much-debated, “Copenhagen Interpretation” of quantum theory that is associated with Bohr and his followers.
One of the profoundest philosophical questions posed by quantum theory is the nature of the underlying reality that is reflected in systems exhibiting complementarity. Does a quantum system have only one true underlying nature, governed by “hidden variables” that for some reason cannot be measured or determined? In other words, is complementarity merely an artifact of our incomplete knowledge of a system? Alternatively, do both (or all) of the complementary properties of a system coexist until the system is exposed to a particular measurement or experiment, which forces it to “make a choice”? Or do the relevant properties (e.g., wave or particle nature) simply not exist at all until an experiment is performed?
Closely linked to these questions of the fundamental nature of the underlying reality of a quantum system are important terminological issues. Both Einstein and Bohr recognized the importance of finding appropriate words and phrases to describe quantum phenomena, while avoiding meaningless terminology and misleading analogies to more conventional phenomena. (This is made quite clear in, for example, their contributions to Paul Schilpp’s Festschrift “Albert Einstein: Philosopher-Scientist”. In his own essay in that collection, “Discussion with Einstein on Epistemological Problems in Atomic Physics,” Bohr stated that “the difficulties have their root not seldom in the preference for a certain use of language ….”)
Hence, Heisenberg’s recollections in the letter offered here of the early discussions of the term “complementarity” are of the utmost importance. (Heisenberg was by no means merely an onlooker in those discussions. The historical and conceptual relationship between Heisenberg’s own principle of “uncertainty,” and Bohr’s principle of “complementarity,” is a much-discussed issue in the history of quantum theory. See Jammer.)
In this letter from 1973, Heisenberg responds to a letter that had been sent to him by Herr Pfarrer Dr. Dr. Otto-Hubert Kost, pastor (Pfarrer) of the Hannover Christuskirche. (Dr. Kost’s letter is also included here.) Dr. Kost noted that Heisenberg had used the term “dualism” ("Dualismus") in discussions of complementarity, and suggested that “duality” (“Dualität”) might be a more appropriate term. (The distinction between the two terms may be more meaningful in German than in English.) Heisenberg’s response quickly passed over the sterile distinction between “duality” and “dualism,” and went on to recount (as translated from the original German):
”It might perhaps interest you that in 1927 when Niels Bohr tried to interpret quantum theory in philosophical terms, he thought over it for months and discussed with friends which words would be most appropriate to render it, and finally indicated it with the word ‘complementarity.’ Many possible expressions/terms were discussed at that time, and I recall that the word ‘duality’ was suggested as a possibility. However, Bohr did not want to see ‘two’ expressed so directly in the word, and hence finally decided to choose the concept ‘complementarity.’ This is because this concept suggests on the one hand that it deals with many images that complement one another but that, on the other hand, the intention of a unity is also expressed with the images. It was naturally always our opinion that dualism is only apparent, and that this truth is well-represented to us in the word ‘complementarity.’
[signed] W. Heisenberg”
WITH: Beautiful classic large photogravure of Heisenberg signed in full [“Werner Heisenberg”] beneath the image in ink. Signed images of Heisenberg are rare.
Also included are the original mailing envelope for the letter and the original letter from Dr. Kost.
The Nobel Prize in Physics 1932 was awarded to Werner Karl Heisenberg "for the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen."
Munich: 4 December, 1973. One sheet (approx. 8.25x11.75 in) of Heisenberg’s Max Planck Institute letterhead. With original mailing envelope and original letter by Dr Kost (one sheet). WITH: Signed photogravure. One sheet approx. 8x11.5 in; image approx. 5.5x7.5 in). All housed in custom box. Very light usual folds to letters, in fine condition.
Price: $20,000 .