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Causation

Peter J. Lewis

Published Aug 18, 2016• 0 citations
Publisher

Abstract

Abstract What are the consequences of quantum mechanics for the structure of causal explanation? It depends on the interpretation you choose. According to a hidden variable theory like Bohm’s, a measurement on one particle can have instantaneous effects on another, no matter how distant. This nonlocality is in prima facie conflict with special relativity. The prospects for constructing a local hidden variable theory are assessed, in particular by invoking backward-in-time causation. Many-worlds theories do not invoke nonlocal causation. However, they do entail that microcausal explanations are in terms of waves, not particles, and they inject consideration of the observer’s location among the worlds into causal explanations. Spontaneous collapse theories like GRW do invoke nonlocal causation; the prospects for a local spontaneous collapse theory are investigated. All of the major interpretations of quantum mechanics modify the structure of causal explanation from the received classical ideal.

DOI
10.1093/acprof:oso/9780190469825.003.0005

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Abstract What are the consequences of quantum mechanics for the structure of causal explanation? It depends on the interpretation you choose. According to a hidden variable theory like Bohm’s, a measurement on one particle can have instantaneous effects on another, no matter how distant. This nonlocality is in prima facie conflict with special relativity. The prospects for constructing a local hidden variable theory are assessed, in particular by invoking backward-in-time causation. Many-worlds theories do not invoke nonlocal causation. However, they do entail that microcausal explanations are in terms of waves, not particles, and they inject consideration of the observer’s location among the worlds into causal explanations. Spontaneous collapse theories like GRW do invoke nonlocal causation; the prospects for a local spontaneous collapse theory are investigated. All of the major interpretations of quantum mechanics modify the structure of causal explanation from the received classical ideal.
It has been claimed that quantum mechanics is a non-local theory, i.e., that the predictions of quantum mechanics are at odds with a conception of the world where all the interactions are mediated by local mechanisms. The cited reason is the violation of the Bell inequality in cases where measurements are performed on quantum systems of particles that have interacted in the past. Quantum mechanics predicts that the state of the particles in such a system are correlated, a result obtained from considering a thought-experiment often known as EPR. In this thesis, I will argue that the violation of the Bell inequality in EPR forces us to either accept that causes sometimes act both backwards and forwards in time, or give up some widely-held assumptions about causal relations or the role of causal explanations. First, I will present three principles regarding causality that I take be widely accepted. I will then present the background for the claim the quantum mechanics is a non-local theory, and discuss the implications of non-local causal influences in light of the Special Theory of Relativity. One argument against a causal explanation of the EPR-correlations will be discussed, and different models for the causal structure of EPR will be assessed. I will argue that a retrocausal model – models that include instances of retrocausation, where the direction of causation is the opposite of the direction of time – is the only viable option if we want to uphold the three principles presented in chapter 1. I will discuss the plausibility of rejecting one of these principles, and argue that at least one of the can be restricted in a way that still accounts for our commitment to it. Finally, I will assess the non-local models against the retrocausal models, and argue that a retrocausal model offers interpretational advantages over the non-local alternatives.
n their contribution to this book, McQueen and Vaidman argue that common sense requires that explanations in physics be not only causal1 but also local. Their main claim is that the Many Worlds Interpretation (MWI) of quantum mechanics (originally due to Everett) provides local explanations of the outcomes of experiments that in other interpretations of quantum mechanics seem to require (some sort of) nonlocality. Particles in the MWI leave traces both in the world where they exist but also in other worlds in which they do not exist. So the explanation is said to be local because spacetime splits together with the particles so that there is no influence at space-like separation within a world, but the causal influence literally travels across ‘parallel’ worlds. This seems to stretch the concept of locality beyond common sense, if not beyond breaking point, even if one understands causation weakly in terms of counterfactual dependence rather than by straightforward physical interaction.
A brief account of the world view of classical physics is given first. We then recapitulate as to why the Copenhagen interpretation of the quantum mechanics had to renounce most of the attractive features of the clasical world view such as a causal description, locality, scientific realism and introduce a fundamental distinction between system and apparatus. The crucial role is played in this by the Bohr's insistence on the wavefunction providing the most complete description possible for an even individual system. The alternative of introducing extra dynamical variables, called hidden variables, in addition to the wavefunction of the system so as to be able to retain at least some of the desirable features of classical physics, is then explored. The first such successful attempt was that of Bohm in 1952 who showed that a realistic interpretation of the quantum mechanics can be given which maintains a causal description as well as does not treat systems and measuring appeartus differently. We begin with the construction of the Bohm's theory. He introduces particle positions as the hidden variables. The particle positions play a special role in Bohm theory. The particle trajectories are guided by the wavefunction. The Bohm theory is deterministic. The probability enters through a special assumption, ``quantum equilibrium'' hypothesis, for the initial conditions on the ensemble of particle trajectories. The ``wave or particle'' dilemma is resolved by a ``wave and particle'' resolution. The measurements in Bohm theory can be described without mysticism. Bohm's theory is however nonlocal.
By moving to the context of relativistic space-time structure, this chapter completes the argument of Chapter 4 that we can use quantum theory locally to explain correlations that violate Bell inequalities with no instantaneous action at a distance. Chance here must be relativized not just to time but to a space-time point, so that an event may have more than one chance at the same time—it may even be certain relative to one space-time point but ‘at the same time’ completely uncertain relative to another. This renders Bell’s principle of Local Causality either inapplicable or intuitively unmotivated. Counterfactual dependence between the outcomes of measurements on systems assigned an entangled state is not causal since neither outcome is subject to intervention: but it may still be appealed to in a non-causal explanation of one in terms of the other.
Abstract Scientific explanations should have a character that matches their subject matter: if a fact is not apt for causal explanation, it should not be explained causally. In this chapter, the author first reviews some cases where causal explanation has been extended beyond its proper limits, identifying a pattern of error termed the problem of causal overreach. The discussion then relates these considerations to the debate over causal locality in Everettian (many-worlds) quantum mechanics (EQM), in the context of three metaphysical distinctions central to recent discussions of locality: event causation vs. worldshaping causation, local branching vs. global branching, and overlapping worlds vs. diverging worlds. The author argues that worlds in EQM are not properly regarded as themselves subject to causal explanation; instead, causation should be understood as a wholly in-world matter. This approach avoids any nonlocal causal explanation; quantum entanglement instead supports a new category of noncausal worldshaping explanations. The chapter concludes by considering the implications of worldbound causation for principles of causal inference, in particular for the Causal Markov condition, and recommends the use of quantum causal models as a foundation for in-world causal inference.
At the basis of the problem of explaining non-local quantum correlations lies the tension between two factors: on the one hand, the natural interpretation of correlations as the manifestation of a causal relation; on the other, the resistance on the part of the physics underlying said correlations to adjust to the most essential features of a pre-theoretic notion of causation. In this paper, I argue for the rejection of the first horn of the dilemma, i.e., the assumption that quantum correlations call for a causal explanation. The paper is divided into two parts. The first, destructive, part provides a critical overview of the enterprise of causally interpreting non-local quantum correlations, with the aim of warning against the temptation of an account of causation claiming to cover such correlations 'for free'. The second, constructive, part introduces the so-called structural explanation (a variety of non-causal explanation that shows how the explanandum is the manifestation of a fundamental structure of the world) and argues that quantum correlations might be explained structurally in the context of an information-theoretic approach to QT.