… in apprehension, how like a god! The beauty of the world … Shakespeare , Hamlet, Act 2 Scene 2 Open theories When it comes to the issues of reductionism, scientific methodology, and theory change, the views of Werner Heisenberg and Paul Dirac diverge in fundamental and interesting ways. They revisited their disagreements over these philosophical issues many times throughout their careers, and their disagreements can be most succinctly described as a debate over whether physical theories are “open” or “closed.” As we saw in the last chapter, Heisenberg's belief that classical and quantum mechanics are closed leads him to view these theories as perfectly accurate within their domains, inalterable, and correct for all time. Although Dirac never uses the term, his own views on classical and quantum mechanics can be fruitfully understood as a rival account of “open theories.” Dirac argues that even the most well-established parts of quantum theory are open to future revision; indeed he takes no part of physics to be a permanent achievement, correct for all time. Instead of viewing classical mechanics as a theory that had been replaced, he sees it as a theory that should continue to be developed, modified, and extended. Unlike Heisenberg, who views physics as a set of consistent axiomatic systems, Dirac sees physics as a discipline much closer to engineering.
… in apprehension, how like a god! The beauty of the world … Shakespeare , Hamlet, Act 2 Scene 2 Open theories When it comes to the issues of reductionism, scientific methodology, and theory change, the views of Werner Heisenberg and Paul Dirac diverge in fundamental and interesting ways. They revisited their disagreements over these philosophical issues many times throughout their careers, and their disagreements can be most succinctly described as a debate over whether physical theories are “open” or “closed.” As we saw in the last chapter, Heisenberg's belief that classical and quantum mechanics are closed leads him to view these theories as perfectly accurate within their domains, inalterable, and correct for all time. Although Dirac never uses the term, his own views on classical and quantum mechanics can be fruitfully understood as a rival account of “open theories.” Dirac argues that even the most well-established parts of quantum theory are open to future revision; indeed he takes no part of physics to be a permanent achievement, correct for all time. Instead of viewing classical mechanics as a theory that had been replaced, he sees it as a theory that should continue to be developed, modified, and extended. Unlike Heisenberg, who views physics as a set of consistent axiomatic systems, Dirac sees physics as a discipline much closer to engineering.
I could be bounded in a nutshell, and count myself a king of infinite space. Shakespeare , Hamlet, Act 2 Scene 2 Introduction As we saw in the last chapter, the received account of the relation between classical and quantum mechanics is a form of reductionism, where classical mechanics is supposed to emerge from quantum mechanics in the limit of some parameter. When we examine more closely the views of three of the key founders of quantum theory, however, we see that none of them took the relation between these theories to be adequately captured by such a reductionist limit. Indeed as we shall see presently, Werner Heisenberg's account of the relation between classical and quantum mechanics is actually a strong form of theoretical pluralism, quite similar to Nancy Cartwright's metaphysical nomological pluralism introduced in Section 1.3. Rather than viewing quantum mechanics as the fundamental theory that replaced classical mechanics, Heisenberg argues that both theories are required, each having its own proper domain of applicability and each being a perfectly accurate and final description of that domain. Throughout his career, Heisenberg held a highly original view in the philosophy of science, centered on his notion of a closed theory. Very briefly, a closed theory is a tightly knit system of axioms, definitions, and laws that provides a perfectly accurate and final description of a certain limited domain of phenomena. This notion has profound implications for Heisenberg's understanding of scientific methodology, theory change, intertheoretic relations, and realism.
Although Dirac rarely participated in the interpretational debates over quantum theory, it is traditionally assumed that his views were aligned with Heisenberg and Bohr in the so-called Copenhagen-Göttingen camp. However, an unpublished—and apparently unknown—lecture of Dirac's reveals that this view is mistaken; in the famous debate between Einstein and Bohr, Dirac sided with Einstein. Surprisingly, Dirac believed that quantum mechanics was not complete, that the uncertainty principle would not survive in the future physics, and that a deterministic description of the microworld would be recovered. In this paper I show how we can make sense of this unpublished lecture in the context of Dirac's broader philosophy of quantum mechanics, and how our present understanding of Dirac's philosophical views must be revised.
There are many important questions that do not fall neatly into any one discipline; rather, their full investigation requires the integration of two or more distinct fields. This book is about just such a question – one that arises at the intersection of physics, philosophy, and history. The question can be simply stated as “What is the relation between classical and quantum mechanics?” The simplicity of the question, however, belies the complexity of the answer. Classical mechanics and quantum mechanics are two of the most successful scientific theories ever developed, and yet how these two very different theories can successfully describe one and the same world – the world we live in – is far from clear. One theory is deterministic, the other indeterministic; one theory describes a world in which chaotic behavior is pervasive, and the other a world in which it is almost entirely absent. Did quantum mechanics simply replace classical mechanics as the new universal theory? Do they each describe their own distinct domains of phenomena? Or is one theory really just a continuation of the other? In the philosophy literature, this sort of issue is known as the problem of intertheoretic relations. Currently, there are two accepted philosophical frameworks for thinking about intertheoric relations: the first is reductionism, and the second, pluralism. As we shall see, these labels each actually describe a family of related views.
Today there is no agreement on which, if any, of the several known realist interpretations of quantum mechanics is the correct one, and disagreement on this matter is not merely verbal, but substantial. However, I will show that the interpretations share a common aim: to recover the classical world given that it is quantum. This aim responds to implicit and well-entrenched philosophical intuitions that can be phrased in terms of intertheory relations, traditionally involving theory reduction, in philosophy, and the quantum-classical limit, in the foundations of physics. However, not one of those notions is free from controversies, and many contest that there is a smooth transition from the quantum to the classical. Hence, the account of the relationship between quantum mechanics and classical mechanics is an unresolved problem, and the philosophical character of the underlying framework can be contested. This thesis will offer a critical analysis of current well-known realist interpretations and will also put forward an alternative framework. I will critically examine traditional views on intertheory relations and the recent view of interstructuralism. My main claim will be that the role of intertheory relations is overrated, because a more basic question has to be answered first: 'what is a quantum system?'. That will motivate my novel view. I will critically evaluate and defend an alternative view based on the philosophy of Werner Heisenberg. The view proposes that physical theories should be regarded as "closed" systems. This has immediate implications for how we should understand intertheory relations in general, as well as scientific realism in particular. My view will appear radical in comparison to traditional views of quantum mechanics. Yet, I will examine fruitful comparison with forms of realism such as perspectivism and metaphysical pluralism. I shall conclude with indications for future work.
Abstract In the classical framework, not only can systems be identified by their space-time location, but they also possess mutually independent existences: they are individuals in the philosophical meaning of the concept. In the quantum domain, by contrast, matters are no so easy. Quantum systems cannot be labelled. Nor can they be assigned precise trajectories. Moreover, after interacting, quantum systems become interrelated in a way that does not diminish with either time or space: quantum correlations challenge the independent existence of the subsystems of a composite quantum system. Therefore, in the quantum framework the identification of open systems is a subtle issue. In this chapter I will emphasise the relative nature of open quantum systems in standard quantum mechanics by pointing out how this relativity is manifested under different forms: the relativity of entanglement, the identification of systems in decoherence, and the many ways in which a quantum system can be decomposed. Therefore, any view that considers open quantum systems as fundamental must take this relativity into account. The general purpose is to argue that the closed systems view and the open-system view are complementary approaches that need not be considered rival.
Abstract The steady development of the quantum theory that has taken place during the present century was made possible only by continual reference to the Correspondence Principle of Bohr, according to which, classical theory can give valuable information about quantum phenomena in spite of the essential differences in the fundamental ideas of the two theories. A masterful advance was made by Heisenberg in 1925, who showed how equations of classical physics could be taken over in a formal way and made to apply to quantities of importance in quantum theory, thereby establishing the Correspondence Principle on a quantitative basis and laying the foundations of the new Quantum Mechanics. Heisenberg’s scheme was found to fit wonderfully well with the Hamiltonian theory of classical mechanics and enabled one to apply to quantum theory all the information that classical theory supplies, in so far as this information is consistent with the Hamiltonian form. Thus one was able to build up a satisfactory quantum mechanics for dealing with any dynamical system composed of interacting particles, provided the interaction could be expressed by means of an energy term to be added to the Hamiltonian function. This does not exhaust the sphere of usefulness of the classical theory. Classical electrodynamics, in its accurate (restricted) relativistic form, teaches us that the idea of an interaction energy between particles is only an approximation and should be replaced by the idea of each particle emitting waves which travel outward with a finite velocity and influence the other particles in passing over them. We must find a way of taking over this new information into the quantum theory and must set up a relativistic quantum mechanics, before we can dispense with the Correspondence Principle.