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Causality and Complementarity : Supplementary Papers
Additional essays to complete Bohr's philosophical perspective . Collected by Jan Faye and Henry Folse. These expand on the epistemological issues faced by 20th century physics.
191 pages, Hardcover
First published January 1, 1999
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About the author
Niels Bohr
106 books230 followersNiels Henrik David Bohr (Danish pronunciation: [ni:ls ˈboɐ̯ˀ]) was a physicist who made foundational contributions to understanding atomic structure and quantum mechanic. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was part of a team of physicists working on the Manhattan Project. Bohr has been described as one of the most influential scientists of the 20th century.
In 1922 Niels Bohr was awarded the Nobel prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them".
Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, grew up to be an important physicist who in 1975 also received the Nobel Prize.
In 1922 Niels Bohr was awarded the Nobel prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them".
Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, grew up to be an important physicist who in 1975 also received the Nobel Prize.
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November 19, 2020
This final volume collects miscellaneous writings of Bohr’s through the years which, for whatever reason, he declined to include in the publications he authorized during his lifetime (vols. I-III in the present series). A few are admittedly insubstantial and ephemeral, but the greater portion supplement nicely the themes on which he has dilated elsewhere and for which he is known. The volume is prefaced with a twenty-page introduction by the editors which sketches the whole development of Bohr’s thought and situates the contributions reprinted here within that development. The book would be worth its purchase price for this introduction along with the appended bibliography of the pertinent secondary literature alone.
The major supplementary papers in this volume circle around three topics: the question of completeness of the quantum-mechanical description of nature, causality and complementarity and, lastly, biology. As to the first question, we get a paper from 1935 spelling out very clearly Bohr’s considered response to the recent paper by his opponents Einstein, Podolsky and Rosen (universally referred to as EPR). EPR present a thought-experiment that, they claim, enables one to perform an exact measurement of all four of two pairs of canonically conjugate variables and thereby to defeat Heisenberg’s uncertainty principle. If this were possible, it would mean that quantum mechanics is incomplete, as there would have to exist some ‘element of physical reality’, as EPR dub it, that would determine the outcome of the experiment and yet which cannot be incorporated into the conventional formalism of quantum mechanics, as currently known (by the way, EPR’s version of this thought-experiment differs from the simplified version under which it is usually discussed these days, due to Bohm some twenty years later). Bohr’s rejoinder is lucid and well reasoned, in what may come as a surprise given his reputation for obscurity (moreover, Bohr’s reprise of EPR’s argument itself is even clearer than what they give their original paper!). The objection Bohr raises is that the requisite measuring apparatus could not actually be built; EPR are guilty of applying the abstract postulates of quantum mechanics all-too formally, without proper consideration of the physical context from which the postulates were abstracted in the first place. The point is a subtle one (otherwise, EPR couldn’t have missed it), but, to this reviewer, persuasive—Bohr changed his mind!
The cycle of papers on causality and complementarity expands on this theme in a number of iterations. Bohr spells out very carefully what the unavoidable influence of the observer means. Roughly speaking, his manner of framing the issue is this: any apparatus we may employ with which to carry out a measurement is going to be described by us only in terms of classical macroscopic properties. Hence, it will have innumerable microstates, any one of which is consistent with its macroscopic description, so that when it is brought into interaction with the system we wish to study, there will always remain degrees of freedom not entirely under our control and from this it follows that there must be some degree of unpredictability in the outcome of the measurement, which is quantified by Heisenberg’s uncertainty relation. Thus, in view of the fact that a macroscopically different apparatus is called for to measure each of two canonically conjugate variables, we cannot envision subjecting both to exact measurement at the same time and hence it makes no sense to speak as if simultaneously existing definite values could be attributed to the two quantities. So far, so good. Bohr was always sensitive to the context in which measurement takes place. If one ponders his comments long enough, one may come around to agreeing with Bohr when he reaches the conclusion that only the macroscopic classical world can exist for us and that it is useless to try to picture the microscopic quantum world in terms of inappropriate classical concepts based upon macroscopic experience, having no instrumental meaning at the purely quantum level. Bohr’s philosophy of the quantum world, conventionally designated under the heading of the so-called ‘Copenhagen interpretation’ (the term is due to Heisenberg, not to Bohr himself), is more careful and thorough than it gets credit for among the revisionists of today, who pretend as if the quantum revolution never happened and we can blithely get by with naïve classical concepts outside their known range of applicability. One wishes merely that he could have elaborated it more completely and supported it with a quantitative mathematical framework.
Lastly, a couple of papers on biology and the quantum. Bohr’s basic point in connection with the possibility of a scientific description of living organisms is just that they are too complex ever to be submitted to exhaustive experimental observation. In principle according to the abstract postulates of quantum mechanics, one ought to be able simultaneously to measure all the position degrees of freedom of the particles making up the body of an organism, since they commute among themselves, but in practice no physical apparatus exists that would correspond to the hypothesized measurement. Thus, Bohr keeps an open mind about the phenomenon of life and considers that the physical principles we already have at hand might not be sufficient to explain it:
Although, thus, we have no reason to expect any inherent limitation of the application of elementary physical and chemical concepts to the analysis of biological phenomena, the peculiar properties of living organisms, which have resulted from the whole history of organic evolution, reveal potentialities of immensely complicated material systems, which have no parallel in the comparatively simple problems with which we are concerned in ordinary physics and chemistry. (pp. 184-185)
Bohr’s measured tone in this concluding statement compares favorably with the stridently dogmatic stance of most every atheist, who groundlessly supposes that our existing mechanical concepts must extend beyond their demonstrated domain of validity simply because he finds himself unable to imagine otherwise. He forgets the celebrated aphorism attributed to Heisenberg: ‘Not only is the universe stranger than we think, it is stranger than we can think’ [Das Universum ist nicht nur seltsamer als wir denken, es ist auch seltsamer als wir denken können]!
Not as easy a read as vol. I (191 pages in smaller print), but rewarding and thought-provoking all the same. Highly recommended for the serious physicist!
The major supplementary papers in this volume circle around three topics: the question of completeness of the quantum-mechanical description of nature, causality and complementarity and, lastly, biology. As to the first question, we get a paper from 1935 spelling out very clearly Bohr’s considered response to the recent paper by his opponents Einstein, Podolsky and Rosen (universally referred to as EPR). EPR present a thought-experiment that, they claim, enables one to perform an exact measurement of all four of two pairs of canonically conjugate variables and thereby to defeat Heisenberg’s uncertainty principle. If this were possible, it would mean that quantum mechanics is incomplete, as there would have to exist some ‘element of physical reality’, as EPR dub it, that would determine the outcome of the experiment and yet which cannot be incorporated into the conventional formalism of quantum mechanics, as currently known (by the way, EPR’s version of this thought-experiment differs from the simplified version under which it is usually discussed these days, due to Bohm some twenty years later). Bohr’s rejoinder is lucid and well reasoned, in what may come as a surprise given his reputation for obscurity (moreover, Bohr’s reprise of EPR’s argument itself is even clearer than what they give their original paper!). The objection Bohr raises is that the requisite measuring apparatus could not actually be built; EPR are guilty of applying the abstract postulates of quantum mechanics all-too formally, without proper consideration of the physical context from which the postulates were abstracted in the first place. The point is a subtle one (otherwise, EPR couldn’t have missed it), but, to this reviewer, persuasive—Bohr changed his mind!
The cycle of papers on causality and complementarity expands on this theme in a number of iterations. Bohr spells out very carefully what the unavoidable influence of the observer means. Roughly speaking, his manner of framing the issue is this: any apparatus we may employ with which to carry out a measurement is going to be described by us only in terms of classical macroscopic properties. Hence, it will have innumerable microstates, any one of which is consistent with its macroscopic description, so that when it is brought into interaction with the system we wish to study, there will always remain degrees of freedom not entirely under our control and from this it follows that there must be some degree of unpredictability in the outcome of the measurement, which is quantified by Heisenberg’s uncertainty relation. Thus, in view of the fact that a macroscopically different apparatus is called for to measure each of two canonically conjugate variables, we cannot envision subjecting both to exact measurement at the same time and hence it makes no sense to speak as if simultaneously existing definite values could be attributed to the two quantities. So far, so good. Bohr was always sensitive to the context in which measurement takes place. If one ponders his comments long enough, one may come around to agreeing with Bohr when he reaches the conclusion that only the macroscopic classical world can exist for us and that it is useless to try to picture the microscopic quantum world in terms of inappropriate classical concepts based upon macroscopic experience, having no instrumental meaning at the purely quantum level. Bohr’s philosophy of the quantum world, conventionally designated under the heading of the so-called ‘Copenhagen interpretation’ (the term is due to Heisenberg, not to Bohr himself), is more careful and thorough than it gets credit for among the revisionists of today, who pretend as if the quantum revolution never happened and we can blithely get by with naïve classical concepts outside their known range of applicability. One wishes merely that he could have elaborated it more completely and supported it with a quantitative mathematical framework.
Lastly, a couple of papers on biology and the quantum. Bohr’s basic point in connection with the possibility of a scientific description of living organisms is just that they are too complex ever to be submitted to exhaustive experimental observation. In principle according to the abstract postulates of quantum mechanics, one ought to be able simultaneously to measure all the position degrees of freedom of the particles making up the body of an organism, since they commute among themselves, but in practice no physical apparatus exists that would correspond to the hypothesized measurement. Thus, Bohr keeps an open mind about the phenomenon of life and considers that the physical principles we already have at hand might not be sufficient to explain it:
Although, thus, we have no reason to expect any inherent limitation of the application of elementary physical and chemical concepts to the analysis of biological phenomena, the peculiar properties of living organisms, which have resulted from the whole history of organic evolution, reveal potentialities of immensely complicated material systems, which have no parallel in the comparatively simple problems with which we are concerned in ordinary physics and chemistry. (pp. 184-185)
Bohr’s measured tone in this concluding statement compares favorably with the stridently dogmatic stance of most every atheist, who groundlessly supposes that our existing mechanical concepts must extend beyond their demonstrated domain of validity simply because he finds himself unable to imagine otherwise. He forgets the celebrated aphorism attributed to Heisenberg: ‘Not only is the universe stranger than we think, it is stranger than we can think’ [Das Universum ist nicht nur seltsamer als wir denken, es ist auch seltsamer als wir denken können]!
Not as easy a read as vol. I (191 pages in smaller print), but rewarding and thought-provoking all the same. Highly recommended for the serious physicist!
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