Among philosophers in particular the principle of complementarity has been considered an unreasonable limitation of natural science, and Niels Bohr’s exposition of the inevitability of a statistical description when accounting for atomic phenomena has been regarded as a renouncement directly opposed to the very spirit of scientific investigation. Indeed, fundamental limitations in the range of applicability of the causal spacetime description of physical phenomena do exist, but quantum physics is precisely the generalization of the conceptual frame of classical physics which has converted these limitations into a springboard for investigations which are among the most fruitful in the history of science. Far from raising a barrier for scientific investigation, quantum physics has extended the scope of natural science to a degree that was unimaginable before its advent. Natural science now encompasses such different objects as the constituents of atoms, processes in remote galaxies, and psycho-chemical phenomena in man. Biochemistry and psychopharmacology, for instance, both rely on one of the most important results of quantum physics: the periodical system of the elements, which is fundamental to all modern chemistry. Furthermore atomic physics and chemistry are basic sciences for technology and thus contribute to the continuous development of instruments of observation for all branches of natural science.
Among philosophers in particular the principle of complementarity has been considered an unreasonable limitation of natural science, and Niels Bohr’s exposition of the inevitability of a statistical description when accounting for atomic phenomena has been regarded as a renouncement directly opposed to the very spirit of scientific investigation. Indeed, fundamental limitations in the range of applicability of the causal spacetime description of physical phenomena do exist, but quantum physics is precisely the generalization of the conceptual frame of classical physics which has converted these limitations into a springboard for investigations which are among the most fruitful in the history of science. Far from raising a barrier for scientific investigation, quantum physics has extended the scope of natural science to a degree that was unimaginable before its advent. Natural science now encompasses such different objects as the constituents of atoms, processes in remote galaxies, and psycho-chemical phenomena in man. Biochemistry and psychopharmacology, for instance, both rely on one of the most important results of quantum physics: the periodical system of the elements, which is fundamental to all modern chemistry. Furthermore atomic physics and chemistry are basic sciences for technology and thus contribute to the continuous development of instruments of observation for all branches of natural science.
From the preceding exposition it follows that the quantum statistical representation of an individual atomic system does not imply any renunciation regarding further knowledge about the system. On the contrary the statistical character of quantum physics directly reflects the fundamental non-causal appearance of quantal recordings, and through the discovery of the complementarity of quantal phenomena a complete but necessarily statistical description of the atom has been accomplished. Furthermore quantum physics has afforded a statistical description of macroscopic bodies through their representation as assemblies of huge numbers of identical atomic systems, and, apart from accounting in a more fundamental way for the properties already described in classical physics(the correspondence principle, p. 36), quantum statistics explains several macroscopic phenomena which cannot be understood within the conceptual frame of classical physics only. It must be maintained, however, that when observing the properties of individual atomic objects the macroscopic observational apparatus cannot be represented in terms of atomic constituents because this would destroy the unambiguity of the description. The observational apparatus fulfils its basic function of being the spacetime frame of reference for the quantal recordings precisely through its description in classical physics alone. However, when refraining from the analysis of individual quantal processes it is possible to apply quantum statistics to macroscopic phenomena, that is, the causality of classical physical phenomena can be interpreted in the quantum formalism as the statistical result of the interplay of enormous numbers of atomic processes. It is possible, then, to talk of statistical causality as a fundamental aspect of physical reality.
From the preceding exposition it follows that not only physics but natural science as a whole is basically dependent on ordinary language. The complementarity of quantum physics displays the fundamental interdependence of observation and description of natural phenomena, and the very objectivity of natural science relies directly on the differentiation of experiences into external and private phenomena afforded by ordinary language. Because of the extreme smallness of the quantum of action the complementarity of physical phenomena is not experienced either in daily life or in classical physics. But the interdependence of observation and description of phenomena prevails as much in classical science as in quantum physics. The very aim of experimental investigation is to perform observations in those particular ways which make possible the communication of information sufficient for the reproduction of the observed phenomena. Concordantly the elementary physical variables, which are indispensable to any systematic analysis in natural science, are defined through reference to measuring arrangements constructed so as to be unambiguously describable in ordinary language. Thus no scientific formula, law, or result of measurement can be given objective significance independent of ordinary language. Through every hierarchy of descriptions in natural science ordinary language penetrates as the unambiguity of propositions.
“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.
A short exposition is given of the foundation of the causal description in classical physics and the failure of the principle of causality in coping with atomic phenomena. It is emphasized that the individuality of the quantum processes excludes a separation between a behaviour of the atomic objects and their interaction with the measuring instruments denning the conditions under which the phenomena appear. This circumstance forces us to recognize a novel relationship, conveniently termed complementarity, between empirical evidence obtained under different experimental conditions. An appropriate tool for a complementary mode of description is provided by the quantum-mechanical formalism which allows us to account for regularities of definite or statistical character beyond the grasp of classical physical explanation. – N. B.
‘Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One’. These views were contained in a letter written by Albert Einstein to Max Born in December 1926. Though the following year would see the completion of work on the theoretical foundation of the modern physics of atoms and particles, Einstein was never to change his judgement and remained firmly convinced that ‘this business […] contains some unreasonableness’. He was unwilling to sacrifice his own ideas as to the cognitive scope of science even in the face of the important results being achieved by the new theory. To the undeterministic findings of quantum mechanics he opposed his belief in the ‘possibility of giving a model of reality, a theory, that is to say, which shall represent events themselves and not merely the probability of their occurrence’. For this reason he chose to live with the isolation of his scepticism and dissent, ironically accepting the reputation as a obstinate heretic that he had won over the years among his colleagues, and worked to the end on his own research programme to develop a unified field theory of rigorously causal nature. As is generally known, Einstein carried on a strong scientific and philosophical dispute with the defenders of the so-called official interpretation of quantum mechanics.
A quarter of a century has elapsed since quantum mechanics was discovered. Perhaps it is not too much to say, in retrospect, that the time was ripe for this particular development. This is attested, not only by the speed with which the edifice of the theory was completed immediately following the basic discoveries of Heisenberg and Schrödinger, but also by the rapidity, well-nigh unprecedented in the history of science with which the new results were applied to almost every branch of physics and chemistry. A mental readiness on the part of the scientists involved might have had much more to do with this than the facilities of modern communication systems.