In this last chapter I shall attempt a very brief assessment of the Bohr–Einstein debate. Who won it in the view of the audience? Who actually had the better case? I will also ask two rather more specific questions. The first is ‘Have the contributions of John Bell favoured the position of either of the protagonists?’ The second is ‘Has the dramatic arrival and progress of quantum information theory anything to tell us about the Bohr–Einstein debate?’ To help to answer these questions, let me first re-assemble a few points, putting together in turn an argument for each of the protagonists against a rather hostile critic. First Bohr, and I take as his critic John Bell himself. Bell was prepared, in fact, to give Bohr great credit for what Bell called the pragmatic approach to quantum theory, which I discussed earlier. This approach says that the world must be divided into ‘classical’ and ‘quantum’ parts, with an arbitrarily placed cut between them; that the macroscopic measuring device must be described in classical terms, but that we should not expect to picture the ‘quantum’ system in physical terms, and should be content just to possess rules of calculation that work well. But Bell regarded Bohr's philosophy of what lay behind pragmatism, complementarity , as ill-defined, unsatisfactory and bizarre.
In this last chapter I shall attempt a very brief assessment of the Bohr–Einstein debate. Who won it in the view of the audience? Who actually had the better case? I will also ask two rather more specific questions. The first is ‘Have the contributions of John Bell favoured the position of either of the protagonists?’ The second is ‘Has the dramatic arrival and progress of quantum information theory anything to tell us about the Bohr–Einstein debate?’ To help to answer these questions, let me first re-assemble a few points, putting together in turn an argument for each of the protagonists against a rather hostile critic. First Bohr, and I take as his critic John Bell himself. Bell was prepared, in fact, to give Bohr great credit for what Bell called the pragmatic approach to quantum theory, which I discussed earlier. This approach says that the world must be divided into ‘classical’ and ‘quantum’ parts, with an arbitrarily placed cut between them; that the macroscopic measuring device must be described in classical terms, but that we should not expect to picture the ‘quantum’ system in physical terms, and should be content just to possess rules of calculation that work well. But Bell regarded Bohr's philosophy of what lay behind pragmatism, complementarity , as ill-defined, unsatisfactory and bizarre.
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 EPR paper and Bohr's reply to it brought to an end the public debate between Einstein and Bohr. In the aftermath, Bohr's construal of quantum mechanics established itself as the basis of the orthodox interpretation – the ‘Copenhagen interpretation’ as it came to be called. The vast majority of physicists regarded Einstein's criticism as reactionary, and dismissed it with disdain. One of the main causes of this disdainful attitude was the general opinion that Einstein's criticism committed him to the existence of hidden states, and the existence of these, it was thought, had been conclusively ruled out by the work of von Neumann and others. What light does the subsequent work on the question of hidden states, and in particular the work of Bell, throw on the Bohr–Einstein dispute? Completeness and hidden states There are three distinct questions concerning hidden states. First, might there be such states, unbeknown to quantum mechanics? Second, can quantum mechanics, as it stands, take account of such states? Third, could some theory containing descriptions of such states reduplicate the predictions of quantum mechanics? Concerning the first of these questions, Einstein's view was that it is very likely that such states exist, and since quantum mechanics takes no account of them, it is incomplete and best regarded as a statistical theory analogous in some respects to classical statistical mechanics.
Abstract It was not a question of whether light should be pictured as waves or particles, or atoms as solar systems, but of whether the microworld could be pictured at all. The famous debate between Albert Einstein and Niels Bohr reached beyond quantum physics to the deepest foundations of science. Faced with the ever more perplexing ‘irrationalities’, Bohr, Einstein, and their followers fell into two broadly opposed camps. Bohr’s approach of ‘complementarity’ accommodated the apparent absurdities of the microworld, declared quantum concepts to be purely ‘symbolic’, and insisted that the manifestation of quantum phenomena in experiment be discussed only in the language of classical physics. He asserted that quantum theory was mathematically coherent and complete. Einstein could not accept Bohr’s ‘tranquilizing philosophy’, which dealt in probabilities rather than certainties. He insisted that physicists look more deeply for the causes of entanglement and rejected the spooky actions at a distance that this implies. Their discussion, respectful and profound, reached stasis in the mid-1930s with the apparent victory of Bohr, while attention shifted to nuclear and particle physics. But their arguments continued to reverberate, and later physicists identified testable consequences of the theory. Ingenious experiments have now confirmed its radical strangeness. Vividly capturing the personalities and interactions of a remarkable cast of characters, the prevailing socio-political circumstances, and threaded through with explanations of theory and experiment, this erudite and carefully crafted telling by Jim Baggott and John Heilbron may prove to be the definitive account of the Bohr–Einstein debate and its legacy.
The rapid rise of quantum information For the last four chapters I have been describing what may be called the ‘fundamental’ or ‘foundational’ aspects of quantum theory. I have been explaining what happens when you penetrate deeper into the ideas of quantum theory than when you merely make use of it, when you try to understand its basic meaning and what, if true, it tells us about the physical nature of the Universe. As I said in Chapter 5, once Bohr had given his approach to these matters in his Como paper of 1927, general opinion was that his views were sacrosanct, and that no self-respecting physicist needed to return to discuss them further. It was Bell's great achievement to dent this complacency at least a little, and from the very end of the 1960s for the next quarter of a century or so, there were a few experimentalists keen to test the Bell inequalities with increasing stringency, and a few theoreticians with fresh ideas about how quantum theory should be interpreted. While it was generally admitted that Bell's work was a genuine and important contribution to our understanding of quantum theory, and eventually the ideas actually reached some of the textbooks, in the 1970s and 1980s the field was still very much a minority interest, and even those who found it interesting would scarcely have felt that it had, or was likely to have in the foreseeable future, practical application.
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.
The famous debate between Einstein and Bohr began at the Solvay Council in 1927. The debate was about the interpretation of quantum mechanics, but also addressed the fundamental question of what the purpose and aim of a physical theory should be. Their conflicting positions were based on two diametrically opposed philosophical approaches to the fundamental problems of physics. The many books popularising quantum mechanics quite rightly place the emphasis on the problem of interpretation: they discuss the opposing positions of Einstein’s “realism” and the “Copenhagen interpretation” of which Bohr is seen as the leading protagonist.