th century ‐ the theory of relativity and the quantum theory. Based on these developments, it is interesting for the future of physics that when examined in terms of their conceptual and mathematical bases, these theories are incompatible. On the other hand, each of these theories requires an incorporation of the other to proceed toward its completion. This is the dilemma we face in these early decades of the 21 st century: Which of the two theories should be abandoned and which should be maintained?[1] Albert Einstein was the leading proponent of the theory of relativity and its philosophy, Niels Bohr was the leading proponent of the quantum theory and its philosophy ‐ as fundamental theories of matter.[2] Because of the incompatibility of the mathematical and conceptual bases of these theories, in fundamental terms, as will be discussed in detail later on, they cannot peacefully coexist. But because aspects of each of these theories are necessary to complete the other, the form of the accepted theory must be generalized in such a way that it yields, as a mathematical approximation, the form of the theory whose concepts are abandoned. This is an appeal to the wellknown principle of correspondence. Bohr appealed to this principle in showing how the formal expression of quantum mechanics goes smoothly into the structure of classical mechanics as the fundamental unit of action ‐ Planck’s constant h ‐ approaches zero, compared with the values of classical action. Similarly, the principle of correspondence reveals how Einstein’s relativistic mechanics goes smoothly
th century ‐ the theory of relativity and the quantum theory. Based on these developments, it is interesting for the future of physics that when examined in terms of their conceptual and mathematical bases, these theories are incompatible. On the other hand, each of these theories requires an incorporation of the other to proceed toward its completion. This is the dilemma we face in these early decades of the 21 st century: Which of the two theories should be abandoned and which should be maintained?[1] Albert Einstein was the leading proponent of the theory of relativity and its philosophy, Niels Bohr was the leading proponent of the quantum theory and its philosophy ‐ as fundamental theories of matter.[2] Because of the incompatibility of the mathematical and conceptual bases of these theories, in fundamental terms, as will be discussed in detail later on, they cannot peacefully coexist. But because aspects of each of these theories are necessary to complete the other, the form of the accepted theory must be generalized in such a way that it yields, as a mathematical approximation, the form of the theory whose concepts are abandoned. This is an appeal to the wellknown principle of correspondence. Bohr appealed to this principle in showing how the formal expression of quantum mechanics goes smoothly into the structure of classical mechanics as the fundamental unit of action ‐ Planck’s constant h ‐ approaches zero, compared with the values of classical action. Similarly, the principle of correspondence reveals how Einstein’s relativistic mechanics goes smoothly
Quantum theory, the most successful physical theory of all time, provoked intense debate between the twentieth century's two greatest physicists, Niels Bohr and Albert Einstein. Quantum information theory has emerged from intensive study of the structure and interpretation of quantum theory to become one of the fastest growing areas of twenty-first century science. This second edition has been extensively revised and updated to cover recent developments, including the findings of papers published since the well-received first edition. A substantial new chapter is devoted to the development and structure of quantum information theory. Developments in the experimental and theoretical study of Bell's Theorem are also covered in detail, and the accounts of ongoing work have been brought up to date. A fascinating account of the development of quantum theory, this book will appeal to anyone with an interest in the fundamental questions of physics, its philosophy and its history.
This book gives a detailed study of the development and the interpretation given to Niels Bohr's Principle of Correspondence. It also describes the role that this principle played in guiding Bohr's research over the critical period from 1920 to 1927. Quantum mechanics, developed in the 1920s and 1930s by Bohr, Heisenberg, Born, Schrödinger and Dirac, represents one of the most profound turning points in science. This theory required a wholly new kind of physics in which many of the principles, concepts and models representing reality, that had formed the basis of classical physics since Galileo and Newton, had to be abandoned. This book re-examines the birth of quantum mechanics, in particular examining the development of crucial and original insights of Niels Bohr.
Introduction Quantum mechanics is based on the intuition that the essential difference between atomic physics and classical physics is the occurrence of discontinuities (see in particular [1,4,58–63]). Quantum mechanics should thus be considered a direct continuation of the quantum theory founded by Planck, Einstein and Bohr. Bohr in particular stressed repeatedly, already before the birth of quantum mechanics, that the discontinuities must lead to the introduction of new kinematical and mechanical concepts, so that indeed classical mechanics and its corresponding conceptual scheme should be abandoned [1,4]. Quantum mechanics tries to introduce the new concepts through a precise analysis of what is ‘observable in principle’. In fact, this does not mean setting up the principle that a sharp division between ‘observable’ and ‘unobservable’ quantities is possible and necessary. As soon as a conceptual scheme is given, one can infer from the observations to other facts that are actually not observable directly, and the boundary between ‘observable’ and ‘unobservable’ quantities becomes altogether indeterminate. But if the conceptual scheme itself is still unknown, it will be expedient to enquire only about the observations themselves, without drawing conclusions from them, because otherwise wrong concepts and prejudices taken over from before will block the way to recognising the physical relationships [Zusammenhänge].
Wonderful news from Copenhagen Heisenberg and Schrödinger invented mathematical formalisms that provided correct answers to all the various problems of (non-relativistic) quantum theory. It was Bohr who uncovered the underlying significance of the theory; in particular he showed how the profound conceptual difficulties encountered as the theory developed – the so-called wave–particle duality, the totally unclassical nature of the uncertainty principle – could be neutralised by a revision, or more accurately a generalisation, of our use of physical concepts. Such was the generally accepted view of things from soon after 1927, when Bohr first put forward his ideas on complementarity at an international meeting of physicists at Como, through the 1930s, when it was felt that Bohr destroyed Einstein's contrary opinions, and certainly up to the time of Bohr's death in 1962. Bohr's analysis is usually spoken of as the Copenhagen interpretation of quantum theory, though it is instructive to realise that such a term was frowned upon by those closest to Bohr, since it appears to suggest that the interpretation is just one among (conceivably) many. Let us examine, for example, the words of Léon Rosenfeld, Bohr's disciple and long-term collaborator, as expounded at a conference in 1957 [52]. Rosenfeld maintained that any idea of ‘interpreting a formalism’ was a ‘false problem’, that in a good theory, the ‘ordinary language (spiced with technical jargon for the sake of conciseness)’ in which it is described, is ‘ inseparably united … with whatever mathematical apparatus is necessary’, that ‘we are here not faced with a matter of choice between two possible languages or two possible interpretations, but with a rational language intimately connected with the formalism and adapted to it, on the one hand, and with rather wild, metaphysical speculations … on the other’.
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 debate between Bohr and Einstein on the consistency, completeness, and finality of quantum theory ranks as one of the most significant philosophical debates of the twentieth century. It was a philosophical debate not in the sense that it had a considerable impact on the relatively technical field of the philosophy of physics. It was philosophical because at its root there lay two rival conceptions of physical reality and of our capacity to comprehend physical reality. With the possible exception of the Leibniz-Clarke correspondence in which Leibniz and Clarke - with the latter acting as Newton's frontman - pressed their respective conceptions of space and time in the early part of the eighteenth century, the Einstein-Bohr dialogue has no parallel in the history of physics. Unlike the Leibniz-Clarke correspondence, the Bohr-Einstein debate contained no bitterness and was not based on any personal rivalry. The debate was continuous from about 1927 and lasted right up to Einstein's death in 1955. For the contemporary philosopher of physics the most important episodes took place in the years between 1927 and 1935. However, in one respect at least the differences between Bohr and Einstein predated quantum mechanics itself. One is tempted to view the disagreement between Bohr and Einstein as the opposition of Bohr the revolutionary and Einstein the conservative, and in his best and clearest account of it Bohr certainly writes as if that were so.