‘The Institute’ describes the work undertaken at Bohr’s headquarters in Copenhagen. It prospered under the ‘Copenhagen Spirit’, as Heisenberg called Bohr’s approach to quantum problems, and money from the Rockefeller, Carlsberg, and Rask-Ørsted Foundations and the Danish government. The Institute saw a decline in the early 1930s owing to Bohr’s doubts about certain productive ideas in nuclear physics that his philosophy seemed to rule out. Their productivity changed his mind and he raised money to build the large apparatus for experimental investigation of the nucleus. At the same time, he applied complementarity to biology and other domains outside of physics, to the dismay of Einstein, who continued to try to find ways to contravene what he called Bohr’s ‘tranquilizing philosophy’.
‘The Institute’ describes the work undertaken at Bohr’s headquarters in Copenhagen. It prospered under the ‘Copenhagen Spirit’, as Heisenberg called Bohr’s approach to quantum problems, and money from the Rockefeller, Carlsberg, and Rask-Ørsted Foundations and the Danish government. The Institute saw a decline in the early 1930s owing to Bohr’s doubts about certain productive ideas in nuclear physics that his philosophy seemed to rule out. Their productivity changed his mind and he raised money to build the large apparatus for experimental investigation of the nucleus. At the same time, he applied complementarity to biology and other domains outside of physics, to the dismay of Einstein, who continued to try to find ways to contravene what he called Bohr’s ‘tranquilizing philosophy’.
Meanwhile, on 18 September 1931, Heisenberg wrote to Bohr that ‘my work seems to be somewhat gray on gray’ (Pauli, 1985). By 1931 the bright future for quantum physics that seemed just over the horizon in the fall of 1927 paled with the formulation of relativistic quantum mechanics. Besides the as yet uninterpreted negative energy states, there was the electron's divergent self-energy and the continuous energy spectrum of β-particles in the supposedly two-body final state of nuclear β-decay, which implied that energy was not conserved in nuclear reactions (Bohr, 1932) and that perhaps quantum mechanics was not valid within the nucleus (to be discussed in Chapter 5). Measurement problems in a quantum theory of the electromagnetic field At the 20–25 October 1930 Solvay Conference, Bohr, Dirac, Heisenberg and Pauli concurred that fundamental difficulties in quantum electrodynamics might be clarified by investigating measurability of electromagnetic field quantities. Upon his return to Copenhagen from Brussels, Bohr continued discussing field measurements with Lev Landau, who happened to be visiting at Bohr's institute. In December 1930 Landau went on to Zürich, where he interested Pauli's assistant Rudolf Peierls in field measurements. Their deliberations led to a joint 1931 publication entitled ‘Extension of the uncertainty principle to relativistic quantum theory’ (Landau and Peierls, 1931). Measurement of an electric field can be accomplished through measuring the change in momentum of a charged test body placed in the field.
Wonderful news from Copenhagen Heisenberg and Schrödinger invented mathematical formalisms that provided correct answers to all the various problems of (non-relativistic) quantum theory. It was Bohr who uncovered the underlying significance of the theory; in particular he showed how the profound conceptual difficulties encountered as the theory developed – the so-called wave–particle duality, the totally unclassical nature of the uncertainty principle – could be neutralised by a revision, or more accurately a generalisation, of our use of physical concepts. Such was the generally accepted view of things from soon after 1927, when Bohr first put forward his ideas on complementarity at an international meeting of physicists at Como, through the 1930s, when it was felt that Bohr destroyed Einstein's contrary opinions, and certainly up to the time of Bohr's death in 1962. Bohr's analysis is usually spoken of as the Copenhagen interpretation of quantum theory, though it is instructive to realise that such a term was frowned upon by those closest to Bohr, since it appears to suggest that the interpretation is just one among (conceivably) many. Let us examine, for example, the words of Léon Rosenfeld, Bohr's disciple and long-term collaborator, as expounded at a conference in 1957 [52]. Rosenfeld maintained that any idea of ‘interpreting a formalism’ was a ‘false problem’, that in a good theory, the ‘ordinary language (spiced with technical jargon for the sake of conciseness)’ in which it is described, is ‘ inseparably united … with whatever mathematical apparatus is necessary’, that ‘we are here not faced with a matter of choice between two possible languages or two possible interpretations, but with a rational language intimately connected with the formalism and adapted to it, on the one hand, and with rather wild, metaphysical speculations … on the other’.
Part I. Niels Bohr. Communications delivered to the Royal Danish Academy of sciences and letters, October 3, 1985 (edited by E. Dal). Niels Bohr and The Royal Danish Academy of Sciences and Letters (B. Strmgren). Epistemological considerations (H.B.G. Casimir). Part II. The Lesson of Quantum Theory. Proceedings of the Niels Bohr centenary symposium, October 4-7, 1985 (edited by J. de Boer and O. Ulfbeck). The Lesson of Quantum Theory. Measurement in quantum theory and the problem of complex systems (P.W. Anderson). Quantum mechanics at the macroscopic level (A.J. Leggett). Strange quantum numbers in condensed matter and field theory (J.R. Schrieffer). The study of the nucleus as a theme in contemporary physics (B.R. Mottelson). ``Complementarity'' between energy and temperature (J. Lindhard). Radiation by uniformly moving sources (V.L. Ginzburg). Does elementary particle physics have a future? (S.L. Glashow). Observability of quarks (J.D. Bojrken). The transformation of elementary particle physics into many-body physics (L. Van Hove). Physics in terms of difference equations (T.D. Lee). Some lessons of renormalization theory (A.S. Wightman). Particles, fields and now strings (S. Weinberg). Collective and planetary motion in atoms (R.S. Berry). Unity of Knowledge. The origin and evolution of large-scale structure in the universe (W.L.W. Sargent). Molecular replication and the origins of life (L.E. Orgel). Physics, biological computation and complementarity (J.J. Hopfield). Complementarity in linguistic observation, description and explanation (W.U. Dressler). Niels Bohr. Great connections come alive: Bohr, Ehrenfest and Einstein (M.J. Klein). Niels Bohr and nuclear weapons (M. Gowing). Niels Bohr: The man and his legacy (J.A. Wheeler). Mellem Spog og Alvor: Personal recollections about Niels Bohr (H.B.G. Casimir). Part III. Jest and Earnest. AAddresses presented at the evening gathering in Ny Carlsberg Glyptoteket, October 7, 1985. (edited by J. de Boer and O. Ulfbeck). Parentesen (A. Aaboe). Truth and clarity (R. Peierls). Niels Bohr, the quantum and the world (V.F. Weisskopf).
“Bohr was primarily a philosopher, not a physicist, but he understood that natural philosophy ... carries weight only if its every detail can be subjected to the ... test of experiment” (Heisenberg in [59], p. 95). As a result his approach differed from that of theschool-philosopherswhom he regarded with a somewhat “sceptical attitude, to say the least” ([59], p. 129) and whose lack of interest in “the important viewpoint which had emerged during the development of atomic physics” he noticed with regret ([59], p. 183).But it also differed, and to a considerable degree, from the spirit of what Professor T. S. Kuhn has called a “normal science.” Looking at Bohr's method of research we see that technical problems, however remote, are always related to a philosophical point of view; they are never treated as “tiny puzzles” whose solution is valuable in itself, even if one has not the faintest idea what it means, and where it leads: “For me” Bohr writes to Sommerfeld in 1922 ([59], p. 71) “[the quantum theory] is not a matter for petty didactic details, but a serious attempt to reach ... an inner coherence.” Emphasis is put on matters of principle ([59], p. 36) and minor discrepancies, or “puzzles” in the sense of Kuhn, instead of being deemphasized, and assimilated to the older paradigm,are turned into fundamental difficultiesby looking at them from a new direction, and by testing their background “in its furthest consequences by exaggeration” ([59], p. 329). A noteworthy example of Bohr's “non-normal” and rather metaphysical approach to physics is his scepticism in the face of the success of his own atomic model.
The theoretical and experimental context within which the “Copenhagen Interpretation” of quantum theory was generated is underemphasized by recent critics of the Bohr-Heisenberg philosophy. When an interpretation of a theory has been as successful as this one has been, there is little practical warrant for the “alternative interpretations” which have, since Bohm, been receiving prominence. Indeed, these are not even genuine alternatives; although rich in provocative prose, they provide not a scrap of algebra with which to organize the practical physicist's thinking. Several objections to the Bohr interpretation are critically examined, as is also a particular use of the correspondence principle which has seemed to cast doubt on the Copenhagen ideas.
The Copenhagen Interpretation of quantum mechanics has become the standard way of explaining the physical philosophical meaning of the mathematics of microscopic behavior. It developed from the thought of people who attended the Institute for Theoretical Physics of the University of Copenhagen during the 40 years that Niels Bohr directed it, from 1920 to 1962. Bohr took a large part in developing the Copenhagen Interpretation, he didn't escape all controversy over it (although the quote above is a bit of an exaggeration). Bohr did die before the experimental theoretical developments that have transposed the debate over the appropriateness adequacy of the Copenhagen Interpretation into a new key, he may well have been lucky to have escaped when he did. There is no authoritative codification of CI [the Copenhagen Interpretation], write T Gornitz C. E von Weizsacker of the Max Planck Institute in Starnberg, West Germany in' their formal presentation to the recent Loyola Conference on Mathematical Interpretational Problems in Relativistic Quantum Theory, and under the conditions of its origin there probably could not be one.... All prominent authors of the time who wrote on these questions, like Bohr, [Werner] Heisenberg, [Wolfgang] Pauli, J[ohn] v[on] Neumann on one side, [Albert] Einstein [Erwin] Schrodinger on the other, fell, so we feel, into some stammering when they tried to express their own positions. Nevertheless there is what might be called a Copenhagen attitude. It concentrates on observable phenomena, makes statistics probability central to its understanding, considers quantum mechanics primarily a theory of knowledge or information becomes a kind of physica negativa (analogous to a theologia negativa) in trying not to say too much about things that can't be seen or felt. For example, it doesn't much care whether the orbits of electrons in atoms are real or not and, in fact, tends to regard them as unreal. It appears in most textbooks as the standard way of looking at things.