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.
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.
What does not satisfy me in that theory [quantum mechanics], from the standpoint of principle, is its attitude towards that which appears to me to be the programmatic aim of all physics: the complete description of any (individual) real situation (as it supposedly exists irrespective of any act of observation or substantiation). Albert Einstein, Reply to Criticisms [This] implies the impossibility of any sharp separation between the behaviour of atomic objects and the interaction with the measuring instruments which serve to define the conditions under which the phenomena appear . In fact, the individuality of the typical quantum effects finds its proper expression in the circumstance that any attempt of subdividing the phenomena will demand a change in the experimental arrangement introducing new possibilities of interaction between objects and measuring instruments which in principle cannot be controlled. Consequently, evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomena exhausts the possible information about the objects. Indeed the finite interaction between object and measuring agencies conditioned by the very existence of the quantum of action entails – because of the impossibility of controlling the reaction of the object on the measuring instruments, if these are to serve their purpose – the necessity of a final renunciation of the classical idea of causality and a radical revision of our attitude towards the problem of physical reality. […]
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.
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.
Heelan argues that the philosophy of complementarity, although successful in providing physicists with a common language with which to describe quantum phenomena, also contains a theory about scientific method and about human knowing which is open to criticism. Heelan here criticises the following points arising out of the philosophy of complementarity: psycho-physical parallelism; the view that quantum mechanical properties are to be defined classically; and the perturbation theory of measurement. In the course of the criticism, he elaborates the distinction between two types of concepts with different logical structures; viz., operational or observational concepts which state a similarity between things judged on the basis of appearance or utility to us, and explanatory concepts which state a similarity between things judged on the basis of a self-defining set of different relations between things. Heelan thus shows how a physical concept is definable by any appropriate measuring-process. The description of the measuringprocess and, hence, the definition of the physical property involve the two classes of concepts described above, but in different ways. Thus one may regard the Indeterminacy Relations, not as stating limitations of our knowledge, but as describing more exactly the behaviour of individual systems.
At the beginning of the paper presented at Como in 1927 Bohr stated: ‘On the one hand, the definition of the state of a physical system, as ordinarily understood, claims the elimination of all external disturbances. But in that case, according to the quantum postulate, any observation will be impossible, and, above all, the concepts of space and time lose their immediate sense. On the other hand, if in order to make observation possible we permit certain interactions with suitable agencies of measurement, not belonging to the system, an unambiguous definition of the state of the system is naturally no longer possible, and there can be no question of causality in the ordinary sense of the word. The very nature of the quantum theory thus forces us to regard the space-time coordination and the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and definition respectively’. The contrast effectively illustrated between possibilities of definition and conditions of observation thus expresses the cognitive scope of quantum theory and summarizes the complementary and irreducible aspects of the description of objects belonging to the microworld. In Bohr's view, this contrast is the direct consequence of two general assumptions: the postulate regarding the discontinuity or individual nature of atomic processes, symbolically represented by the quantum of action, and an epistemologically binding judgement on the system of concepts whereby such processes may be described.
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.