Abstract There is widespread agreement that quantum mechanics has something radical to teach us about causation. But opinions differ on what this is. Physicists have often taken the central lesson to be that many physical events occur spontaneously, so that a principle of causality is violated whenever an atom emits light, or a uranium nucleus decays, even though nothing that happened beforehand made this inevitable. Feynman urged philosophers to acknowledge that this implication of quantum mechanics undermines the view that causal determinism forms a precondition of scientific inquiry. But while some physicists have denied the implication, most philosophers since Reichenbach have accepted it with alacrity, and sought to develop accounts of causation equally applicable in an indeterministic or a deterministic world.
Abstract There is widespread agreement that quantum mechanics has something radical to teach us about causation. But opinions differ on what this is. Physicists have often taken the central lesson to be that many physical events occur spontaneously, so that a principle of causality is violated whenever an atom emits light, or a uranium nucleus decays, even though nothing that happened beforehand made this inevitable. Feynman urged philosophers to acknowledge that this implication of quantum mechanics undermines the view that causal determinism forms a precondition of scientific inquiry. But while some physicists have denied the implication, most philosophers since Reichenbach have accepted it with alacrity, and sought to develop accounts of causation equally applicable in an indeterministic or a deterministic world.
In the 19th century, determinism was often seen as a presupposition of scientific thinking, although its validity was not universally accepted among scientists and philosophers. There were different conceptions of causality, and the concept of cause was often linked to that of force or energy; but toward the end of the 19th century, the causal conception of forces was increasingly criticised and some argued that the concept of cause should be abandoned altogether. After the development of the modern theory of quantum mechanics in 1925, many physicists and philosophers claimed that this theory implied an overthrow of the classical conception of causality; however, only few authors abandoned causality altogether. The debate of the implications of quantum mechanics for causality soon became muddled because of the many different meanings attached to the term “causality”.
This chapter focuses specifically on the notion of causation. Unless a causal interpretation can be given of quantum mechanics, there is a fundamental discontinuity between the quantum realm and everything else in the world. The quantum-theoretical dualism of waves and particles makes the same entity appear both as matter and as force. There are of course challenges to other basic ontological categories that come from quantum physics, such as to notions of continuity and identity. Both in philosophy and physics, there is a frequent assumption that causation is the provider or vehicle of determinism, via causal necessitation. The indeterminism of quantum physics leaves people with a more practical concern about causation. A feature of causation that makes predictions fallible is that causal processes are extremely sensitive to a change in context. The genuine interaction of multiple factors should therefore be seen as a symptom of causation rather than as something that needs to be analysed away by theoretical abstraction.
Two mysteries -loosely connected?The Authors who have suggested a deep connection between the solution of the mind-body problem and the understanding of quantum mechanics are too numerous to list.But what exactly would be the alleged structural fit between these two deeply puzzling theoretical issues?Can a genuine philosophical issue -which by its very nature is conceptual in nature -ultimately be resolved by advances in the empirical sciences?The connection is all too often asserted to lie in the subjects confounding natures, but the mere fact that two problems are of equal impenetrability to the human mind does not imply that there is a profound interdependence between the two.There is however an obvious way in which quantum mechanics might have a bearing on the philosophical mind-body problem: if quantum mechanics implies that for some physical events there is no physical cause, then at least the very strong variants of the so-called "principle of causal closure" of the physical realm lose much of their corroborative underpinning in physics as practiced by real physicists as such, and are thus constrained to the speculative physicalism advanced by metaphysicians.However, unless one assumes a strong version of the principle of sufficient reason, which would exclude chance events in principle, even events supposedly not caused by some physical cause are not immediately thereby caused by some nonphysical cause.Moreover the fact is that not all of the interpretations of quantum mechanics imply indeterministic 'choices' of events: neither the many-worlds interpretation nor a Bohmian hidden variable interpretation assume indeterminism.Even in the most obvious theoretical nexus
We still lack any consensus about what one is actually talking about as one uses quantum mechanics. There is a gap between the abstract terms in which the theory is couched and the phenomena the theory enables each of us to account for so well. Because it has no practical consequences for how we each use quantum mechanics to deal with physical problems, this cognitive dissonance has managed to coexist with the quantum theory from the very beginning. The absence of conceptual clarity for almost a century suggests that the problem might lie in some implicit misconceptions about the nature of scientific explanation that are deeply held by virtually all physicists, but are rarely explicitly acknowledged. I describe here such unvoiced but widely shared assumptions. Rejecting them clarifies and unifies a range of obscure remarks about quantum mechanics made almost from the beginning by some of the giants of physics, many of whom are held to be in deep disagreement. This new view of physics requires physicists to think about science in an unfamiliar way. My primary purpose is to explain the new perspective and urge that it be taken seriously. My secondary aims are to explain why this perspective differs significantly from what Bohr, Heisenberg, and Pauli had been saying from the very beginning, and why it is not solipsism, as some have maintained. To emphasize that this is a general view of science, and not just of quantum mechanics, I apply it to a long-standing puzzle in classical physics: the apparent inability of physics to give any meaning to 'Now'-the present moment.
The nature of emergence of complexity out of the underlying physics is a key issue in understanding the world around us (1). Genuine emergence can be claimed (2⇓–4) to depend on top-down causation, which enables higher emergent levels to direct the outcomes of causation at lower levels to fulfill higher-level causal requirements; for example, the needs of heart physiology at the systems level determine gene expression at the cellular level via gene regulatory networks (5) (Fig. 1). However, the idea of top-down causation has been denied by a number of commentators (e.g., refs. 6 and 7). The paper by Aharonov et al. (8) makes a strong contribution to this debate by giving quantum physics examples where top-down causation manifestly occurs. This physics result has strong implications for the philosophical debate about whether strong emergence is possible. Indeed, it gives specific examples where it occurs in a remarkably strong form.
Fig. 1.
Bottom-up and top-down causation from the quantum level up to the organism level and down again. Adapted with permission of Royal Society, from ref. 5; permission conveyed through Copyright Clearance Center, Inc.
Now, the word “causation” is regarded with suspicion by many philosophers of science, so to characterize what is happening one can perhaps rather use a number of different descriptions such as “whole–part constraint” or “top-down realization.” The key point remains the same, that higher levels can influence lower-level outcomes in many ways, and hence explain how strong emergence is possible. This occurs across science in general, and in physics in particular (3, 4). The latter point …
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Quantum mechanics has achieved unparalleled success as an operational theory, describing a wide range of experiments to remarkable accuracy. However, the physical foundations on which it rests remain as puzzling as they were a century ago, and a concise statement of the physical principles that underlie quantum mechanics is still outstanding. One promising approach holds that these principles should be formulated in terms of information, since many of the counter-intuitive effects in quantum theory concern questions such as what one can know about a quantum system and how the information encoded therein can be processed and distributed between parties. Another challenging feature of quantum theory is its incompatibility with the other cornerstone of modern physics, general relativity. In order to reconcile the two, one must identify and retain only the essential concepts and principles of each theory, and in the case of general relativity, causality has been identified as such a concept. This raises the question of how our classical understanding of causality must change when quantum theory is taken into account. In order to address these questions about information, knowledge and causality, I turn to the framework of causal models. In classical statistics, causal models explain the relations among a set of variables in terms of causal influences, which makes them a powerful tool for structuring our knowledge about complex systems and developing strategies for interacting with them. More importantly, the framework provides the conceptual underpinnings and mathematical methods for addressing questions about causation, information and knowledge in a rigorous manner. In this thesis, I develop a version of the classical causal models framework that is compatible with quantum theory and explore its physical implications. \n \n \nA first step is to define quantum versions of fundamental elements such as variables, conditionals and belief propagation rules. This allows one to consider the question of what one can come to know about a quantum variable from the point of view of causal modelling. The conditionals relating quantum variables are found to have a richer structure than their classical counterparts, which can be exploited for the task of discerning causal relations given limited data -- a central problem in classical causal modelling. The mathematical properties of quantum conditionals also establish a correspondence between various classes of two-party correlations, such as bound and distillable entanglement, and the types of causal structures that can give rise to them, which may become a useful tool for entanglement theory and quantum information processing. In the context of open quantum systems dynamics, quantum causal models provide a clear physical explanation of not completely positive maps and, more broadly, of non-Markovian quantum dynamics. Finally, I consider the possibility of non-classical effects in the way that different causal mechanisms are combined -- that is, non-classical causal structures. For the simple case of two causally ordered variables, I propose indicators that witness different classes of combinations of causal mechanisms, including a non-classical mixture, and describe an experiment realizing examples of the different classes. \n \n \nAs these results illustrate, the framework of quantum causal models provides both greater conceptual clarity and a comprehensive mathematical formalism for studying a diverse set of problems, ranging from foundational questions to applications in open systems dynamics and the exploration of non-classical causal structures, which are likely to feature in a future theory of quantum gravity. The material presented in this thesis is intended as a foundation and inspiration for further applications of quantum causal models.