Many of the pioneers of quantum mechanics – notably Planck, Einstein, Bohr, de Broglie, Heisenberg, Schrodinger, Born, Jordan, Landé, Wigner, and London – were seriously concerned with philosophical problems. In each case one can ask a question of psychological and historical interest: was it a philosophical penchant which drew the investigator towards a kind of physics research which is linked to philosophy, or was it rather that the conceptual difficulties of fundamental physics pulled him willynilly into the labyrinth of philosophy? I shall not undertake to discuss this question, but shall cite an opinion of Peter Bergmann, which I find congenial: he learned from Einstein that “the theoretical physicist is … a philosopher in workingman's clothes” ([1], q. v.). The problems with which I am preoccupied concern the philosophical implications of quantum mechanics – either epistemological, bearing on the extent, validity, and character of human knowledge; or metaphysical, bearing on the character of reality. Although quantum mechanics is not a system of philosophy, one can wonder whether it is susceptible to coherent incorporation in a philosophical system. I propose to examine the thought of three masters of quantum mechanics – Bohr, Heisenberg, and Schrödinger – not with a critical or historical intention, but in hope of finding some enlightenment concerning the problems posed by contemporary physics. I can say in advance that enlightenment will continue to elude us; nevertheless, the ideas of Bohr, Heisenberg and Schrödinger are rich and evocative for new studies.
Many of the pioneers of quantum mechanics – notably Planck, Einstein, Bohr, de Broglie, Heisenberg, Schrodinger, Born, Jordan, Landé, Wigner, and London – were seriously concerned with philosophical problems. In each case one can ask a question of psychological and historical interest: was it a philosophical penchant which drew the investigator towards a kind of physics research which is linked to philosophy, or was it rather that the conceptual difficulties of fundamental physics pulled him willynilly into the labyrinth of philosophy? I shall not undertake to discuss this question, but shall cite an opinion of Peter Bergmann, which I find congenial: he learned from Einstein that “the theoretical physicist is … a philosopher in workingman's clothes” ([1], q. v.). The problems with which I am preoccupied concern the philosophical implications of quantum mechanics – either epistemological, bearing on the extent, validity, and character of human knowledge; or metaphysical, bearing on the character of reality. Although quantum mechanics is not a system of philosophy, one can wonder whether it is susceptible to coherent incorporation in a philosophical system. I propose to examine the thought of three masters of quantum mechanics – Bohr, Heisenberg, and Schrödinger – not with a critical or historical intention, but in hope of finding some enlightenment concerning the problems posed by contemporary physics. I can say in advance that enlightenment will continue to elude us; nevertheless, the ideas of Bohr, Heisenberg and Schrödinger are rich and evocative for new studies.
The ideas in Heisenberg's paper on quantum mechanics in 1925 mark the beginning not only of a new phase in modern physics, but also of Heisenberg's own philosophical journey. This thesis examines that journey between 1925 and the late 1950s by situating Heisenberg's philosophy of quantum mechanics in the context of his encounters with his contemporaries as well as the context of various strands of thought in the German-speaking world at the time. Heisenberg's early philosophical critique of the 'classical' viewpoint between 1925 and 1927 bears the decisive influence of Einstein's theory of relativity, more specifically, the positivism he saw as underpinning Einstein's emancipation from Newtonian physics. The positivist influence on Heisenberg's early attitude to quantum mechanics is evident in three ways: (a) his invocation of an observability principle in 1925 to justify the renunciation of the concept of the electron orbit, (b) an instrumentalist conception of understanding, which characterised Heisenberg's response to Schrodinger's demand for classical visualisation in space and time in 1926-7, and (c) the introduction of an operational definition of concepts such as position and velocity in 1927, in an attempt to replace the concepts of classical physics. But after discussions with Bohr and Einstein in 1926-7, Heisenberg soon recognised what we might term his `empiricist' viewpoint was problematic. In 1927 Heisenberg's thought undergoes a shift away from the `empiricist' viewpoint that had underpinned his early philosophy of quantum mechanics. The nature and scope of this transformation, which forms the central theme in this thesis, has, up until now, been poorly understood and often completely neglected. Through his discussions with Bohr, Heisenberg came to the realisation that despite their limitations, classical concepts were conditions for the possibility of the description of all experience. This marked the abandonment of his earlier attempt to replace classical concepts with quantum concepts. The recognition of the primacy of classical language forms the point of departure for much of Heisenberg's later thought, which brought him into contact with the attempts in the German-speaking world in the 1920s to reconstruct Kantian epistemology. By the mid-1930s, Heisenberg advocated a 'pragmatic transformation' of Kantian philosophy, in which classical concepts were held to be a priori in the sense that they remained the conditions for the possibility of experience, but were no longer held to be necessary or universal in a strict Kantian sense. After 1940 Heisenberg saw the paradoxes of quantum mechanics under the aegis of what can be termed a 'transcendental conception of language', according to which language is not a mere tool, but actively shapes, gives form, and objectifies, our 'reality'. The limits of a classical 'description' in quantum mechanics therefore came to signify for Heisenberg, the limits of 'objective reality'. While Bohr exerted perhaps the most important philosophical influence on Heisenberg, their intellectual relationship was characterised by disagreement and misunderstanding. This is most strikingly displayed in their respective views on wave-particle duality and complementarily. While after 1927 Heisenberg accepted Bohr's basic insight that our knowledge of the quantum world is mediated through classical language, he did not share Rohr's interpretation of complementarity. While Heisenberg certainly used terms such as 'complementarity' and wave-particle duality' in his writings, a close reading reveals that these terms had very different meanings for the two physicists. This is particularly evident in the contrast between Heisenberg's notion of wave-particle equivalence and Bohr's idea of complementarity. In bringing to light these divergences between Bohr and Heisenberg, this thesis lends further weight to the view - already advocated by scholars such as John Hendry and Mara Beller - that the so-called Copenhagen interpretation of quantum mechanics should not be thought of as a unified philosophical position, but actually comprises a number of different strands.
Now that my account of Bohr's philosophy of physics has been completed, it remains for me to give a general assessment of it. What value should we assign to Bohr's main ideas – the notions of correspondence and complementarity? How does his interpretation of quantum physics fare in comparison with some of its rivals? What should our assessment of the Bohr–Einstein debate be? I shall take up these questions in the reverse order. Einstein or Bohr? The final verdict The Bohr–Einstein debate is primarily a dispute about the philosophical foundations in terms of which the interpretation of quantum mechanics should be based: specifically it is a dispute about the intrinsic-properties theory, i.e. about the scope of a realist interpretation of quantum mechanics. Einstein wished to maintain the two realist theses, the intrinsic-properties theory and the objective-values theory, whereas Bohr wished to hold only the latter. My own philosophical leanings are in the direction of realism. To my mind the notion of a physical reality the existence and constitution of which are independent of human thought and perception is coherent and plausible: what physical objects there are, and what properties they have, is not determined by human thought or perception, or by human practical interests. I have sympathy, therefore, with Einstein's position concerning the indefinability thesis and the question of the completeness of quantum mechanics.
THE conceptual problems raised by the discovery of quantum effects have not been fully resolved after half a century. Part of the reason for this is undoubtedly to be found in the mutual ignorance which prevails between physicists and philosophers. In his book Heisenberg brings together a philosophically inclined temperament with an unrivalled knowledge of physics. The result is a book of very great interest, however much one might disagree with his conclusions. The collection of essays of which the other book is made up is intended to bring crucial conceptual problems in physics especially to the notice of philosophers. The design and intention of the collection is admirable for this is the proper way to overcome the departmentalization of interest that has left the deep conceptual problems still unresolved. Men like Heisenberg who combine philosophical temperament with scientic knowledge of great distinction are rare, and yet the difficulties can only be cleared up by the application of philosophical techniques of analysis to problems which are an intimate and difficult part of physics. A great deal of the interest of Heisenberg's book lies in the very comprehensive account he gives of the origin and growth of the Copenhagen school, and the way the new phenomena forced a largely unwelcome picture of nature upon Bohr and his friends. The most important chapter from a philosophical point of view is that in which Heisenberg defends this interpretation against the attacks to which it has been subjected. The essence of his argument for the interpretation can be summed up in two propositions. (i) The Copenhagen interpretation of physical theory, which depends upon the acceptance of the so-called Principle of Complementarity, is an inevitable and final interpretation. The reason for this is that there are only two possible general models for understanding given physical phenomena; a particle model or a wave model. These are exhaustive and exclusive because the former depends upon discontinuous and the latter upon continuous (using substance here in the traditional philosophical sense). If we find, as we do, that the two fundamental constituents of the universe, matter and radiation, each exhibit two classes of phenomena, one explainable only on a particle model and one on a wave model, then we are compelled to adopt a complementarity hypothesis and give up the idea of finding a single and uniform model for the understanding of phenomena. There are only two kinds of models; they are mutually exclusive; both are required; hence we either despair before an insoluble problem or adopt the Copenhagen interpretation. (2) Any serious criticism of this interpretation must offer an alternative
‘Sprache und Wirklichkeit in der modernen Physik’ is the title of a lecture Heisenberg gave in 1960 at the Bayerische Akademie der Schönen Künste. The polemics that had accompanied the establishment of quantum mechanics were over by then and the so-called problems of its foundations had lost much of their initial interest. Einstein had been dead for some years. Right up to the end he had expressed his profound dissatisfaction with a theory which, in his opinion, had something unreasonable about it. With the volume dedicated to him by the community of physicists and philosophers, the long debate between Bohr and Einstein had come to an end. It had been a disappointing conclusion, marked by the reaffirmation of the respective viewpoints of two scientists who by now found great difficulty even in defining a common code of communication. As though seeking to narrow the gap between them, Heisenberg took the opportunity to underline the existence of a common feature in their contributions to 20th century physics. Their discoveries had in fact made it possible to recognize that ‘even the fundamental and most elementary concepts of science, such as space, time, place, velocity, have become problematic and must be re-examined’. The conceptual and cognitive implications of relativity and of quantum mechanics were obviously different but Heisenberg maintained that both theories asked the same question: ‘Does the language we use when we speak of experiments correspond to the artificial language of mathematics which, as we know, describes real relationships correctly; or has it become separated from it so that we must be content with imprecise linguistic formulations and only return to the artificial language of mathematics when we are forced to express ourselves with precision?’.
The philosophy of physics is continuous with physics itself. Just as certain issues in the Foundations of Mathematics have been discussed by both mathematicians and by philosophers of mathematics, so certain issues in the philosophy of physics have been discussed by both physicists and by philosophers of physics. And just as there are issues of a more epistemological kind that tend to concern philosophers of mathematics more than they do working mathematicians, so there are issues that concern philosophers of physics more than they do working physicists. In this brief report I shall try to give an account of the present state of the discussion in America of both kinds of issues, starting with the problems of quantum mechanics, which concern both physicists and philosophers, and ending with general questions about necessary truth and the analytic-synthetic distinction which concern only philosophers. The problem of ‘measurement’ in quantum mechanics Quantum mechanics asserts that if A and B are any two possible ‘states’ of a physical system, then there exists at least one state (and in fact a continuous infinity of states) which can be described as ‘superpositions of A and B ’. If A and B are the sorts of states talked about in classical physics – definite states of position, or momentum, or kinetic energy, etc. – then their superpositions may not correspond to classically thinkable states.
In PHYSICS AND PHILOSOPHY: The Revolution Modern Science, (1) Werner Heisenberg aims, in not too technical language, to clarify change concept of reality manifesting itself quantum theory. He means to contribute to an articulation of the (not only epistemology) that underlies quantum theory's modified logical patterns. (2) To highly unusual degree, Heisenberg is gripped by sense of responsibility of modern quantum physicist to convey, language immediately intelligible to thinking public, his most serious and far-reaching reflections on metaphysical implications of his scientific work. One may go so far as to say that there is substantial evidence, partly within Physics and Philosophy, that Heisenberg regards such reflections and their public communication as his most important task. Heisenberg is especially apt for this intensely self-conscious contribution to humanity's historical-cultural development because his stature as principal originating architect of quantum at its most fundamental level is complemented by his deployment of an unusually broad and deep erudition and lifelong preoccupation with philosophic and artistic ontology from Anaximander through Kant and Goethe to Heidegger. No thinker has yet appeared who possesses such an authoritative combination of decisively necessary learning quantum physics and historical development of philosophic ontology. Despite flurry of renewed scholarly interest development of Heisenberg's scientific work, (3) and his complex relation to dramatic unfolding of German cultural his time, (4) there has yet to be executed sustained and philosophically critical interpretative commentary on this book that is his crucial philosophical-ontological legacy. Given profound ontological puzzles that continue to attend quantum physics and its implications for humanity's past as well as present and future conception of reality, such critical exegesis of this text is overdue. In what follows I try to extricate central nerve of Heisenberg's sinuously unfolding, dialectical exposition, and process to elucidate its strengths but also its deep ambiguities and perplexities--which express, I believe, fundamental dilemmas that pervade contemporary ontology. I Heisenberg makes inviolability of principle of noncontradiction cornerstone of his discussion. Unlike Aristotle (5) Heisenberg does not wrestle with challenge posed by possible denial of principle. He understands nonmathematical articulation of quantum to escape what would otherwise be devastating contradictions--most obviously contradiction wave/particle character of basic entities described by theory--through Bohr's famous principle of complementarity (which Heisenberg closely associates with von Weizsacker's notion of coexistent states), combined with Heisenberg's own principle of potentia, potentiality, potentialities. (6) II After brief introduction, Heisenberg his second chapter sketches history of quantum theory and process begins to explain what he means by potentia. In 1924 Bohr and his colleagues tried to solve apparent contradiction between picture and particle picture by concept of wave. The electromagnetic were treated not as 'real' waves but as mathematical descriptions of probabilities, every point relevant space, for the absorption (or induced emission) of light quantum by an atom at this point. Later this idea of was applied more effectively to much more abstract mathematical in many-dimensional configuration space that was discovered through researches of Schrodinger. Heisenberg reinterprets this mathematical probability wave ontological tenus, as signaling tendency [Tendenz] for a definite happening [einem bestimmten Geschehen], thus introducing remarkable kind of physical reality, standing middle between possibility and actuality: a quantitative version of old concept of 'dunamis' or 'potentia' philosophy of Aristotle. …