Reflections on the Quantum Theory

...His theory of the atom, published in 1913, was the first to introduce Max Planck's notion of quanta (or discrete units) of energy into a systematic account of atomic structure...

...Bohr's discussion of these crucial features of quantum theory is intellectually most rewarding...

...The theory also does not formulate precise laws for the behavior of individual electrons, even when the latter are construed to be particles...

...Instead, it supplies only statistical for probabilistic) laws for the motions of those entities...

...Bohr recognizes explicitly that his proposed extensions of the principle of complementarity from physics to other domains is based at best on formal analogies...

...Nevertheless, the analogies that guide Bohr's attempt to enlarge the scope of the principle of complementarity seem too vague to yield conclusions that are other than forced and arbitrary...

...In nearly all these essays he tries to show that this principle is not restricted to physics, and that it throws fresh light on problems in biology, psychology and anthropology...

...Bohr therefore thinks that the very existence of life "must in biology be considered as an elementary fact," not further explicable in physicochemical terms...

...The following are examples Bohr cites from these areas: A mechanistic approach to biological subject-matter is complementary to a finalistic (or teleological) one...

...His ideas have produced revolutionary changes in our conceptions of the basic constitution of the material world...

...The Bohr model of the atom brought within the scope of a single set of ideas a vast array of previously unexplained facts...

...Thus, unlike the theories of classical physics, the quantum theory is not "deterministic" and is commonly regarded as disproving the universality of the principle of causality...

...He also has been the chief source of the general ideas that most physicists today accept as expressing the essential philosophical import of modern quantum mechanics...

...Bohr has been indefatigable in his attempts to remove these puzzles, and he has advanced what he has called "the principle of complementarity" as the key to them...

...Bohr has been a vigorous defender of modern quantum theory against its critics...

...As Bohr points out, in assigning traits to subatomic entities, it is quite necessary "to pay attention to the circumstances under which evidence is obtained...

...He therefore sees in the statistical determinism of quantum mechanics simply a more general form of the classical principal of causality...

...He shows that electrons, for example, do not inherently and simultaneously "possess" the incompatible properties of waves and particles, but that electrons are assigned wave-like characteristics under one class of experimental arrangements and particle-like characteristics under an exclusively different class of circumstances...

...Thus, he declares, in elaborating the consequences of the principle for biology, that "in every important experiment on living organisms there must remain some uncertainty as regards the physical conditions to which they are subjected, and the idea suggests itself that the minimal freedom we must allow the organism will be just large enough to permit it, so to say, to hide its ultimate secrets from us...

...He has not only made fundamental technical contributions to special areas of theoretical analysis...

...Although being wave-like and being particle-like would be incompatible if these traits could be assigned to electrons under identical experimental conditions, the conditions under which one trait can be ascribed are incongruous with the conditions under which the other trait can be predicated...

...The seven essays in this volume are concerned with these larger philosophical questions...

...Reasoning by analogy can indeed be fruitful, and such reasoning plays a capital role in physics as well as in other areas of thought...

...On the other hand, his attempt to make a more general use of the principle of complementarity, and to apply it to problems in biology, psychology and anthropology, is far less happy...

...Reflections on the Quantum Theory Atomic Physics and Human Knowledge...

...It stimulated and directed further research of great importance on a very wide front...

...It throws much light on the structure of the theory and on the meanings that can be legitimately associated with its basic notions...

...Although the original Bohr theory was eventually displaced by a better integrated one, Bohr's views have continued to dominate the theoretical outlook of current physics...

...He maintains that the "indeterminate" behavior ascribed to electrons is simply a consequence of the fact that in those contexts in which the "position" of an electron can be measured with arbitrary precision, an interaction takes place between the electron and the measuring instruments, so that the speed of the electron can then not be ascertained with equal precision...

...Bohr also makes evident that the indeterminacy of quantum theory cannot be construed as a basis for introducing mystical "spiritual" agents into the domain of physics...

...Accordingly, it is impossible to give an intuitively satisfactory interpretation of the mathematical formalism of the theory in terms of a visualizable model...

...Despite his great services and to the clarification of its foundations, his efforts at developing a theory of human knowledge on the basis of the notion of complementarity are not promising, and have in any case born no fruits that are intellectually nourishing...

...However, the burden of his analysis deals with the alleged "dual" nature of electrons and other subatomic entities...

...Different human cultures, experienced under mutually exclusive historical conditions, are complementary to each other...

...Reviewed by Ernest Nagel Professor of philosophy, Columbia University NIELS BOHR is one of the great architects of modern physics...

...Bohr sometimes advances claims that are far from trivial...

...This interaction is said to be "uncontrollable," because physical energy occurs in discrete quanta, so that it is not possible to subdivide without limit the energies involved in the process of measurement...

...As the guiding spirit of the "Copenhagen School" of physics, he has been a primary influence upon the newer developments in subatomic theory...

...All of them are non-technical, except for the longest and most valuable one, in which Bohr reports quite dramatically his many discussions with Albert Einstein on aspects of the quantum theory which the latter regarded as unsatisfactory...

...He sometimes writes as if all future theories of physics dealing with subatomic processes must inevitably assume an indeterministic form...

...In order to explain undisputed experimental facts, the theory assumes the existence of various subatomic entities (such as electrons) to which it ascribes a "dual" nature: They possess the seemingly incompatible properties of waves as well as of particles...

...These traits are therefore not incompatible, but "complementary...

...Three of the essays are published here for the first time...

...But perhaps this is not entirely just...

...Though they are stylistically undistinguished and suffer from repetition, the essays convey to the general reader the substance of Bohr's philosophical reflections on the quantum theory and its overall significance for an adequate view of human knowledge...

...These examples are typical, and they make it difficult to escape the conviction that, whatever may be the merits of the principle of complementarity in clarifying the obscurities of quantum physics, elsewhere the principle yields only trivialities...

...In both these features quantum theory marks a departure from traditional forms of theory, and its assumption of a radical "indeterminism" for subatomic processes has made even men like Einstein doubt its adequacy...

...Despite the admitted and unprecedented successes of quantum mechanics in explaining and predicting phenomena associated with the radiation of energy, the theory has features that have puzzled experts as well as laymen...

...Bohr, of course, may be correct in his contention, but certainly the argument on which he bases it is worthless...

...The impossibility of distinguishing introspective experience between the phenomena apprehended and our conscious perception of them, shows the complementary character of introspective phenomena and our awareness of them...

...Two features of the quantum theory have been the occasion for much intellectual discomfort...