Abstract World War II drove several of the founders to Britain or the US. Foundational questions returned with renewed energy in the early postwar years when significant anniversaries came due. The usual suspects rejoined the debate. In the volume celebrating his 70th birthday, Einstein returned to the EPR paper. To its main point, which had been ‘buried alive, so to speak, by erudition’, he now brought exceptional clarity. If the two entangled particles are considered separable and ‘locally real’, then they must communicate via some peculiar mechanism of action at a distance. If they cannot be considered as separable a gaping explanatory hole opens. Either way, quantum theory is not up to the task. But the world of physics, now dominated by America, had moved on. Physicists ‘had no interest in understanding Bohr, and thought that Einstein’s distaste for [quantum mechanics] was just silly . . . It was a very unphilosophical time’.
Abstract World War II drove several of the founders to Britain or the US. Foundational questions returned with renewed energy in the early postwar years when significant anniversaries came due. The usual suspects rejoined the debate. In the volume celebrating his 70th birthday, Einstein returned to the EPR paper. To its main point, which had been ‘buried alive, so to speak, by erudition’, he now brought exceptional clarity. If the two entangled particles are considered separable and ‘locally real’, then they must communicate via some peculiar mechanism of action at a distance. If they cannot be considered as separable a gaping explanatory hole opens. Either way, quantum theory is not up to the task. But the world of physics, now dominated by America, had moved on. Physicists ‘had no interest in understanding Bohr, and thought that Einstein’s distaste for [quantum mechanics] was just silly . . . It was a very unphilosophical time’.
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 difficulties of the old quantum theory were largely dispelled by the development of quantum mechanics in the 1920s. First Heisenberg (1925) developed the rather abstract matrix mechanics and soon after Schro¨dinger (1926; see also Eckart, 1926) developed the physically more appealing wave mechanics. Subsequently he showed them to be mathematically equivalent. The physicists were greatly relieved; now they had once again a mathematical formalism that enabled the results of experiments to be calculated. It was possible to move forward. It is certainly a very elegant and indeed beautiful formalism, and does enable the results of many physical measurements to be calculated with great accuracy. It is used daily by atomic and nuclear physicists; there is no other way to make calculations. From the earliest days there has been intense debate about the interpretation of quantum mechanics, and it shows no signs of being settled; if anything it is becoming more intense over the years. As Penrose has remarked, no one understands quantum mechanics. Feynman (1967; see also Sykes, 1994) also admitted that ‘nobody understands quantum mechanics’, and Einstein once remarked: ‘I have thought a hundred times as much about the quantum problems as I have about general relativity theory’ (Pais, 1994, p. 57). It should therefore be evident that the most that can be done here is to try to show where the real difficulties lie, and how the failure to recognize that quantum mechanics is an essentially statistical theory has led to a series of confusing quantum paradoxes. It is just these paradoxes that have often been used to support theological speculations. It is useful to ask what ‘understanding quantum mechanics’ actually means. Certainly it means being able to calculate the results of many measurements. The same applies to Newton’s theory of gravitation, which we understand in the sense that we can use it as a basis of calculation, although we do not really understand the gravitational force itself. Very often, especially in modern physics, the mathematical formalism does not give us the sort of intuitive physical feeling for what is going on that we often obtain from classical mechanics. Perhaps we can never achieve more than this for quantum mechanics. In classical mechanics we can assume that a property that is measured objectively exists prior to the interaction of the measuring apparatus with the observed system. Quantum mechanics, however, ‘is incompatible with the proposition that measurement discovers some unknown but pre-existing property’ (Peres, 1997, p. 14). There is an important distinction between mysteries and absurdities. We may be forced to live with mysteries, but there is no reason why we should put up with absurdities such as the wave-particle duality, tunnelling through potential barriers, acausal events and the collapse of the wave function. These are the quantum paradoxes that will be discussed later on. They arise within the Copenhagen interpretation of quantum mechanics and are solved by other interpretations. These alternative interpretations enable us to make conjectures about what is going on, even though we cannot understand it in detail. This is acceptable and a stimulus to further research. It is certainly difficult enough to understand quantum mechanics, but there is no need to try to visualize logical absurdities in the effort to do so. Quantum mechanics is a mathematical formalism that makes it possible to calculate the results of some experiments. Anything more than this, such as questions about the interpretation of what is measured and whether it tells us what is actually going on in the system being studied, belongs to philosophy. Thus we are concerned here with what is essentially a philosophical debate; if it were just a matter of physics the debate would have been settled long ago. One of the difficulties is that it invokes the results of experiments and uses the language of quantum mechanics. Inevitably this language uses philosophical terms and then it is easy to give the impression that the philosophical propositions are in some way entailed by the physical results. These in turn can influence theological thinking. In order to be quite clear which arguments are justified and which are not, it is essential to separate the three levels of discourse: physical, philosophical and theological. Our task is to relate these together, and this immediately raises the traditional problems of the philosophy of science, including the relation of experiment to theory, and of theory to explanation. We begin by discussing experiments, distinguishing between those that are real and those that are imaginary.
The famously controversial 1935 paper by Einstein, Podolsky, and Rosen (EPR) took aim at the heart of the flourishing field of quantum mechanics. The paper provoked responses from the leading theoretical physicists of the day, and brought entanglement and nonlocality to the forefront of discussion. This book looks back at the seminal year in which the EPR paper was published and explores the intense debate it unleashed. These conversations in print and in private correspondence offer significant insight into the minds of pioneering quantum physicists including Niels Bohr, Erwin Schrödinger and Albert Einstein himself. Offering the most complete collection of sources to date – many published or translated here for the first time – this text brings a rich new context to this pivotal moment in physics history. Both researchers and students in the history and philosophy of science, and enthusiasts alike, will find this book illuminating.
Abstract In 1957, Bohm and Yakir Aharonov alighted on experiments on correlated photons performed some years earlier. Although the experimenters had not looked for EPR-type correlations, Bohm and Aharonov found no contradictions with quantum theory, concluding that entanglement is a real property of matter. Inspired by Bohm, in 1964 John Bell first dispatched von Neumann’s impossibility proof, readmitting the possibility of local hidden variables, and showed how experiment could decide if they existed. Many physicists later misunderstood this as a challenge to Copenhagen. But, as Bohm and Aharonov explained: ‘ . . . there is no inconsistency in the quantum-mechanical conclusion that such correlations exist, but there is, in [Bohr’s] point of view, no way even to raise the question of what is their origin’. If the particles cannot be considered as separable, Einstein’s gaping explanatory hole returns. Interested experimenters had first to disinter the question, which Bell had buried in an obscure journal.
‘Quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal but hardly brings us closer to the secret of the Old One’. These views were contained in a letter written by Albert Einstein to Max Born in December 1926. Though the following year would see the completion of work on the theoretical foundation of the modern physics of atoms and particles, Einstein was never to change his judgement and remained firmly convinced that ‘this business […] contains some unreasonableness’. He was unwilling to sacrifice his own ideas as to the cognitive scope of science even in the face of the important results being achieved by the new theory. To the undeterministic findings of quantum mechanics he opposed his belief in the ‘possibility of giving a model of reality, a theory, that is to say, which shall represent events themselves and not merely the probability of their occurrence’. For this reason he chose to live with the isolation of his scepticism and dissent, ironically accepting the reputation as a obstinate heretic that he had won over the years among his colleagues, and worked to the end on his own research programme to develop a unified field theory of rigorously causal nature. As is generally known, Einstein carried on a strong scientific and philosophical dispute with the defenders of the so-called official interpretation of quantum mechanics.
Now that we have reached the end of our brief tour through the early history of quantum theory, we need to stop and consider some important themes. From a historical point of view, a distinction can be drawn between ‘old quantum theory’ and the full quantum theory that we now use. Old quantum theory was in essence a patchwork cobbled together from classical physics with a few quantum ideas thrown in. e works of Planck, Einstein, Bohr, and Sommerfeld can be classed as old quantum theory. is is not meant to undermine the tremendous achievements that old quantum theory represents, but simply to underline its standing as a stage on the way to a fully formed quantum theory, which slowly emerged from the works of Heisenberg, Schrödinger, Born, and Dirac.