WERNER HEISENBERG AND THE SEMANTICS OF QUANTUM MECHANICS

Bohr's Influence on Heisenberg and Issues with Einstein

          Niels Bohr was one of the leading atomic physicists of the first half of the twentieth century.  He had studied in England under J.J. Thompson and Lord Rutherford, and rece­ived the Nobel Memorial Prize for Physics in 1922 for his theory of the structure of the atom.  He founded the Copenhagen Institute for Theoretical Physics in 1920, and as its director was actively recruiting talented staff members, when he accepted an invitation to deliver a series of lec­tures on atomic physics at the University of Gottingen in the summer of 1922.  In "Quantum Theory and its Interpretation" in Niels Bohr (1963) Heisenberg reports that he first met Bohr at the Gottingen lectures, which he attended with his teacher, Arnold Sommerfeld.  At the time Heisenberg was a twenty-two year old, fourth semester student at the University of Munich.  Heisenberg came to Bohr's attention, because in the discussions following one of the lectures, he dissented from Bohr's optimistic assessment of a theory developed by Kramers at Copenhagen.  Heisenberg relates that Bohr was sufficiently worried about the objection, that after the discussion he asked Heisenberg to take a walk with him for a conversation.  During the walk Bohr talked about the fundamental physical and philosophical problems of mod­ern atomic theory, and the encounter resulted in an invita­tion for Heisenberg to visit the Institute at Copenhagen for a few weeks, and later to hold a position.  Heisenberg des­cribes his impressions of Bohr as primarily a philosopher rather than a physicist, and he states that he found Bohr's philosophy to be fascinating, although he also states that he and Bohr had different views on the role of mathematics in physics.
          Bohr's philosophy of atomic physics is set forth in his Atomic Physics and the Description of Nature (1934), "Dis­cussions with Einstein" in Albert Einstein (ed. Schilpp, 1949), Atomic Physics and Human Knowledge (1958), and Essays 1958/1962 on Atomic Physics and Human Knowledge (1963).  Bohr's philosophical views may have been influenced by some casual reading of the philosophical literature, but he never references any philosopher in his writings.  His views seem largely to be the product of his own reflections on his research in atomic physics and on the work of the staff at Copenhagen.  In "Quantum Theory and Its Interpretation" Heisenberg states that Bohr had developed views on the seman­tics of language and scientific theory many years before he had met Bohr and before he developed his matrix mechanics.  Bohr's mature philosophy of science included two theses: Firstly that the mathematical formalisms of microphysics cannot describe the microphysical domain that lies beyond ordinary experience.  Secondly that the only language that is capable of a descriptive semantics is the language of ordinary discourse and its refinement in classical New­tonian physics.  Heisenberg did not accept the first thesis, and had a different concept about the abstract nature of mathematics.  But Bohr's second thesis had a lifelong influence on him, an influence that had a retarding effect on his development of his own philosophy.
          Bohr gives various reasons why in his view the mathe­matical formalisms of microphysics have no descriptive semantics and are only symbolic instruments for making calculations and predictions.  One reason given in "Discussions with Einstein" is the occurrence of a complex number in the formalism.  Apparently he believed that reality could be described only by equations having variables and parameters that admit only real numbers for values.  Another reason given in "The Solvay Meetings and the Development of Quantum Theory" (1962) in his Essays 1958/1962 is the interpretation of the statistical wave function in a configuration space of more than four dimensions.  Like Einstein, Bohr believed that real physical space-time has no more than four dimensions.  But the basic reason why Bohr interpreted the mathematical formalism of quantum theory instrumentally is his belief that only the language of everyday discourse and its refinement in classical physics can have a descriptive semantics.  He maintained that ordinary language and classical physics must be used to describe any experimental set up in physics, while at the same time he believed that classical physics is too limited to describe the microphysical domain beyond ordinary experience.  It is limited not only because Newtonian physics is inadequate as a microphysical theory, but also due to the inherent nature of human cognitive perception.  This is a philosophy of the semantics of language that is a variation on the naturalistic thesis.  Due to Bohr's philosophy of perception, Einstein as well as many philosophers of science were led to conclude that Bohr's philosophy of science is Positivist.
          If Bohr's philosophy of science is a Positivist philo­sophy, it is a peculiar one.  His statements of his philo­sophy that are most often referenced in this connection by philosophers of science are those in Atomic Physics and the Description of Nature.  In the opening "Introductory Survey (1929)" he states that both relativity theory and quantum theory are concerned with physical laws that lie beyond ordinary experience, and which therefore present difficul­ties to our "accustomed forms of perception".  In quantum theory the limitations of these forms of perception are revealed by the need for the complementary description, the inconsistent description of the quantum phenomenon as both a wave and a particle.   Both of these two forms based on classical physics are necessary for a complete description, even though they are inconsistent in classical physics.  Yet these "customary" forms of perception cannot be dispensed with, since all human cognitive experience must be expressed in terms of them.  The fundamental concepts of classical physics therefore will never become superfluous for the description of physical experience; they must be used to describe experiments and to relate the mathematical symbol­isms to the data of experience.
          In Einstein's attack on Bohr's philosophy of quantum theory the central issue is the ontology of the Copenhagen interpretation, which Einstein critiqued with his program­matic aim of all physics.  The explicit criterion set forth in the programmatic aim of science is the "complete" description of any individual situation, as it supposedly exists irrespective of any act of observation or substantia­tion.  Accordingly he characterized the Copenhagen interpretation as a version of Bishop Berkeley's idealist thesis "esse est percipi", a characterization that is not accurate, because Bohr did not maintain that the atomic phenomenon is produced by a cognitive process but rather by the physical processes of measurement in the experimental set up.  In this matter Einstein seems to have confused an epistemologi­cal issue with a physical one.  But Bohr is not blameless for the confusion.  For example in "Introductory Survey (1929)" he opens with statements emphasizing the subjectiv­ity of all experience and the difficulties in distinguishing between phenomena and their observation; and he concludes the chapter with the statement that "to be" and "to know" lose their unambiguous meanings.   From an epistemological viewpoint some of Bohr's statements are ambiguous as to whether he is advancing a realist or an idealist philosophy.  Some of Heisenberg's earlier statements are also suggestive of an idealist position.  For example he writes in the opening chapter of The Physicist's Conception of Nature, that since we can no longer speak of the behavior of the particle independently of the process of observation, the natural laws formulated in the quantum theory no longer deal with the elementary particles themselves, but only with our knowledge of them.  But later Heisenberg is very clear about avoiding any metaphysical idealism. In "The Copenhagen Interpretation of Quantum Theory" in Physics and Philosophy he states explicitly that quantum theory does not contain genu­inely subjective features, since it does not introduce the mind of the physicist as part of the atomic event, and that the transition from possible to actual in the act of obser­vation is in the physical and not the psychical act of observation.
          This metaphysical idealist/realist confusion notwith­standing, however, Einstein's central ontological thesis is that the statistical quantum theory is incomplete in the sense that further theoretical research is necessary, in order to develop a complete theory that would give Heisenberg’s uncertainty relations a status in future physics, which he thought should be analogous to the status had by statistical mechanics.  What is most noteworthy is that Einstein admits that the indeterminacy principle is not empirically incor­rect, even as he rejects the Copenhagen ontology because it does not conform to his explicit ontological criterion.  In the 1949 "Reply to Criticisms" Einstein conceded that his incompleteness thesis is the minority view among physicists; contemporary philosophers as well as physicists have accep­ted the indeterminacy thesis of the Copenhagen interpreta­tion of the statistical quantum theory, and have rejected the deterministic ontology advocated by Einstein.  When confronted with the dilemma of having to choose between an established ontological criterion and a new but empirically adequate quantum theory, both the contemporary physicists and the contemporary Pragmatist philosophers of science have opted for the latter, contrary to Einstein's arguments for the former.
          In addition to the ontological issue between Bohr and Einstein about what is physically real, there is also a related epistemological issue about the relation between sense perception and intellectual concepts, which is also a semantical issue about what the Positivists called the relation between observation lan­guage and theory language.  Einstein had portrayed Bohr as a Positivist due to Bohr's views about perception and the semantics of language.  This portrait is debatable, because Positivists do not usually speak of forms of perception, and particularly about the limitations of such forms of perception for physics.  But in his 1934 book Bohr writes of the necessity of these forms of perception for science to reduce our "sense impressions" to order.  Even though Einstein him­self uses the phrase "sense impressions" in his statement of the aim of science in "Physics and Reality" in 1936, he seems to have taken Bohr's discussion referencing sense im­pressions to mean that these are no concepts or categories in perception.  Einstein opposed this view, and states in 1949 in his "Reply to Criticisms" that thinking without positing categories and concepts is as impossible as breath­ing in a vacuum.  He furthermore states that his philosophy differs from Kant's only by the fact that he does not view categories as unalterable and as conditioned by the understanding, but rather views them as "free conventions".  The philosopher of science may ask whether Einstein's neo-Kantian views without Kant's idealism and a priorism is still recognizably Kantian.  But the point to be emphasized is that Einstein's thesis that concepts are necessary for perception and that they are free conventions amounts to a restatement of what he told Heisenberg in 1926, when he said that it is the theory that decides what we can observe.  In this earlier statement Einstein might consistently have told Heisenberg that observation without theory is as impossible as breathing in a vacuum.  Perhaps it was in response to Einstein's criticisms in these matters that Bohr refrains in his later writings from using the phrase "sense impressions".  Instead Bohr merely describes the concepts of clas­sical physics as a refinement of the concepts of ordinary discourse, so that he is no longer mistakenly taken as saying that perception occurs without any concepts or forms.
          Nonetheless there is still a fundamental difference between the semantical views of Bohr and Einstein.  Eins­tein's thesis that concepts are free conventions is intended to mean that there are none of the inherent limitations in observation or in language that Bohr had maintained.  In Bohr's phrase "customary forms of perception", the term "customary" does not mean the same thing as the term "con­vention" in Einstein's phrase "free conventions".  The limi­tations that Bohr said these customary forms of perception impose on descriptive language are not temporary limita­tions, which will be removed with the change in language customs resulting from the further development of theory.  Rather these limitations are inherent in the nature of the human cognitive processes of perception and consequently in the semantics of descriptive language.  They are therefore permanent.  There is no such permanence according to Ein­stein's view; the free conventions of human thought, in the concepts and categories in language and scientific theory, are not only conventions that are free to change, but are destined to change with the advancement and further develop­ment of scientific theory.  The difference between Bohr's and Einstein's semantical views is the difference between the naturalistic and the artifactual philosophies of the semantics of language.
          A few more comments about the relation of the seman­tical issue to contemporary philosophy of science: Ein­stein's semantical views anticipated those of the contem­porary Pragmatist philosophers of science in several respects, and his arguments against Positivism undoubtedly had an influence on the Pragmatists, even though he is seldom referenced in the philosophical literature.  Einstein rejected the Positi­vist thesis that each individual concept in a theory re­quires specific justification of its meaningfulness, when the concept is indispensable for the theory, and when the theory in its entirety has been empirically validated.  This is a rejection of the Logical Positivist problem of theo­retical terms.  He also rejected Bridgman's operationalist thesis, and its requirement that each of a theory's asser­tions must be independently interpreted and tested, because this procedure has never yet been accomplished for any sci­entific theory, and furthermore in Einstein's view, it can­not be accomplished.  On Einstein's thesis a physical theory need only imply some empirically testable assertions; there exists no logical path from the empirically given to the conceptual world.  Both the individual concept and the individual assertion in a theory confront the empirically given in connection with the entire system of assertions, because there is an element of arbitrary choice between the empirical and the conceptual world, that result in what Ein­stein calls an "embarrassment of riches" for the theorist. This element of the arbitrary in the relation between the empirical and the conceptual is the basis for the con­temporary Pragmatist philosophers' thesis that the semantics of language is not predetermined by nature, as Bohr and the Positivists had maintained, but rather is a cultural artifact.  Thus the meanings of individual terms and asser­tions are not determined by their relation to the empirical world individually, but by their relation to one another in the larger context of a discourse, such as a scientific theory.  This Pragmatist thesis is at least consistent with Einstein's views.  But many contemporary Pragmatists take a step that probably Einstein cannot be associated with.  They equate the dependence of meanings upon context with a wholistic view of the semantics of language.  But one cannot be certain about what Einstein might have said.  While Einstein affirmed an artifactual theory of the semantics of language, he did not develop a theory of meaning description.
          On the other hand Einstein took his views a step in another direction than the Pragmatists, when he advanced his explicit ontological criterion of logical simplicity for the whole of physics.  This is a nonempirical criterion for sci­entific criticism, which Einstein used to argue that the concepts that are successful in field theory must also be used in quantum theory.  It is this requirement that macro­physical and microphysical theories use the same ontological categories that led Einstein to reject the ontology of the Copenhagen interpretation of the statistical quantum theory.  Here the contemporary Pragmatist philosophers of science depart from Einstein's views.  The element of arbitrariness that both they and Einstein admit in the relation between the empirical and the conceptual, leads the Pragmatists to admit pluralism in empirical science that both Einstein and most Positivist philosophers found scandalous.  This pluralism is opposed to Einstein's explicit ontological criterion of simplicity for all of physics.  The Pragmatists do not find the theorists' embarrassment of riches permitted by the artifactual character of the semantics of language embarrassing as Einstein's nonempirical criterion for scientific criticism, which seeks to constrain the develop­ment of scientific theory by imposing a uniform monolithic ontology.  On the Pragmatist view pluralism is characteristic of the development of science, and some Pragmatist philosophers maintain that it is a condition for its advancement.

Pages [1] [2] [3] [4] [5] [6]
NOTE: Pages do not corresponds with the actual pages from the book