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Quantum Drama

Jim Baggott, John L. Heilbron

Published Apr 25, 2024• 36 citations

Abstract

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.

DOI: 10.1093/oso/9780192846105.001.0001

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Abstract The confrontation between Einstein and Bohr over the interpretation of quantum theory has been painted in the past as a direct conflict between realism and positivism. However, more recent attempts to uncover the philosophical positions of both Einstein and Bohr have revealed that these were not so black and white. Einstein’s realism was conditioned by the strong empiricist influence in his own early experiences as a theorist and the growing wariness that led him eventually to reject Machian positivism. His was a realism born of a sense of necessity: a belief that a realist stance is the only one that leads to productive theorizing about the physical world. Bohr’s philosophy went through some quite marked changes, principally in response to Einstein’s challenges. The father of modem atomic theory could hardly subscribe to an uncompromising positivist position that rejected the reality of atoms. But the reality of the inner workings of those atoms was, Bohr contended, beyond the reach of a knowledge founded on complementary classical concepts of waves and particles. As Einstein’s challenges became deeper and more subtle, so Bohr’s position shifted to that of a distinctive anti-realism. Add to this a rhetoric in Bohr’s speeches and published writings that insisted on the logical inevitability of the Copenhagen interpretation, that insisted on its status as the only feasible interpretation, and that countenanced no other alternative, and you have dogma in the making. Heisenberg called it ‘Kopen hagener Geist der Quantentheorie’, the Copenhagen spirit of quantum theory which, he declared in 1930, ‘has directed the entire development of modem atomic physics’. Einstein frequently referred to the Copenhagen doctrine as ‘Talmudic’.

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•Apr 25, 2024•0 citations

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.

For over nine decades, quantum mechanics has profoundly shaped our understanding of nature, offering explanations for the stability of atoms, the formation of mole cules, the properties of materials, and phenomena such as light or radioactivity. It is essential to modern technologies—from microchips to lasers—and underpins the physics of elementary particles. Yet, despite its successes, it remains a deeply mysterious theory, confronting a central enigma: why does the quantum world behave so differently from everyday reality? Elements such as superposition and quantum entanglement—which allow particles to exist simultaneously in multiple states or remain interconnected regardless of distance—contradict classical world intuitions. This gap between microscopic and macroscopic behavior has given rise to what is known as the “measurement problem,” raising fundamental questions about the nature of reality. Diverging interpretations have led to two major schools of thought: anti-realism, promoted by Niels Bohr, which holds that reality depends on the observer and their interaction with measurement instruments; and realism, supported by Albert Einstein, which argues that quantum mechanics is incomplete and that eality exists independently of observation. Bohr’s complementary philosophy and Heisenberg’s uncertainty principle were central to the formulation of the “Copenhagen interpretation,” which became dominant in the 20th century. However, the persistent opposition of scientists like Einstein and Schrödinger inspired generations of physicists to seek a unification between quantum mechanics and relativity, culminating in new theories such as loop quantum gravity and string theory. Thus, the debate between realism and anti-realism remains alive, and philosophy continues to play a crucial role in exploring the new foundations of physics.

Abstract The confrontation between Einstein and Bohr over the interpretation of quantum theory has been painted in the past as a direct conflict between realism and positivism. However, more recent attempts to uncover the philosophical positions of both Einstein and Bohr have revealed that these were not so black and white. Einstein’s realism was conditioned by the strong empiricist influence in his own early experiences as a theorist and the growing wariness that led him eventually to reject Machian positivism. His was a realism born of a sense of necessity: a belief that a realist stance is the only one that leads to productive theorizing about the physical world. Bohr’s philosophy went through some quite marked changes, principally in response to Einstein’s challenges. The father of modem atomic theory could hardly subscribe to an uncompromising positivist position that rejected the reality of atoms. But the reality of the inner workings of those atoms was, Bohr contended, beyond the reach of a knowledge founded on complementary classical concepts of waves and particles. As Einstein’s challenges became deeper and more subtle, so Bohr’s position shifted to that of a distinctive anti-realism. Add to this a rhetoric in Bohr’s speeches and published writings that insisted on the logical inevitability of the Copenhagen interpretation, that insisted on its status as the only feasible interpretation, and that countenanced no other alternative, and you have dogma in the making. Heisenberg called it ‘Kopen hagener Geist der Quantentheorie’, the Copenhagen spirit of quantum theory which, he declared in 1930, ‘has directed the entire development of modem atomic physics’. Einstein frequently referred to the Copenhagen doctrine as ‘Talmudic’.

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.

Abstract Our drama deals with the distinctly human compulsion to divide the world into domains of experience, like religion and physics, defined by rules and expectations. Fundamentalists pursue the illusory goal of a final theory of everything belonging to a domain. Their aim is closure – to declare that no plausible alternatives are possible. Neils Bohr sought to close the domain of quantum mechanics within a few years of its discovery. Albert Einstein objected to this hasty surrender of the possibility of alternatives more in keeping with classical intuition. Bohr’s efforts to achieve closure in the face of Einstein’s attempts to prise it open framed one of the most dramatic debates of twentieth-century science, and led to experimental discoveries that deepened the mystery and may yet profoundly impact our lives.

Abstract The nature of the Bohr–Einstein debate was framed long before our protagonists became engaged in it. Both were reformers, ready to embrace controversy and even absurdity in their theories. But where Einstein held fast to classical principles he considered sacrosanct, Bohr would cheerfully embrace their violent reworking or replacement. Einstein’s 1921 Nobel prize was awarded in 1922. He had entertained the absurd notion that light consists of ‘quanta’, endowing particle-like properties governed by a characteristic wave frequency. Bohr’s 1922 Nobel prize involved entertaining the absurd notion of quantum ‘jumps’ between atomic orbits driven entirely by chance. He defended the inconsistencies with vague wordplay, declaring the quantities defining the orbits as ‘symbolic’, and introduced a fuzzy demand called the ‘correspondence principle’. At appropriate limits, the value of a quantity should be the same whether calculated using classical or quantum rules. Few quantum physicists could make it work.