This Ph.D. thesis is about causality in Quantum Mechanics, namely about the possibility to provide a causal explanation for quantum Einstein – Podolsky – Rosen (EPR) correlations.The well-known interpretation of the Bell’s Theorem by Bastian Van Fraassen, who inferred that quantum correlations cannot be embedded in any causal model, compels us to suppose that microscopic nature cannot have a causal structure and that causality is an anthropological concept.However, this interpretation is not commonly accepted and this doctoral thesis is the first general overview of some recent works about causal explanation for EPR correlations. Particularly, it analyses the possible causal explanations for the EPR/Bohm experiments by means of the Hans Reichenbach’s model, by means of backward causation models, and by means of models that imply a direct causal link. In order to do it, it pays particular attention to the Relativistic space-time features and it considers the studies in the History of Philosophy about the link between causal arrow and temporal arrow. Moreover, this work aims to endorse the idea that the best way to explain EPR/Bohm correlations causally is to adopt an indeterministic non-Reichenbachian common cause model, particularly Cartwright’sgeneral forksmodel.My conclusion is that causal models for quantum correlations remain viable, and that there is no contrast between genuinely indeterministic interpretation of Quantum Theory and causal explanation, with the philosophical implication that the Causal Realism is still acceptable.
This Ph.D. thesis is about causality in Quantum Mechanics, namely about the possibility to provide a causal explanation for quantum Einstein – Podolsky – Rosen (EPR) correlations.The well-known interpretation of the Bell’s Theorem by Bastian Van Fraassen, who inferred that quantum correlations cannot be embedded in any causal model, compels us to suppose that microscopic nature cannot have a causal structure and that causality is an anthropological concept.However, this interpretation is not commonly accepted and this doctoral thesis is the first general overview of some recent works about causal explanation for EPR correlations. Particularly, it analyses the possible causal explanations for the EPR/Bohm experiments by means of the Hans Reichenbach’s model, by means of backward causation models, and by means of models that imply a direct causal link. In order to do it, it pays particular attention to the Relativistic space-time features and it considers the studies in the History of Philosophy about the link between causal arrow and temporal arrow. Moreover, this work aims to endorse the idea that the best way to explain EPR/Bohm correlations causally is to adopt an indeterministic non-Reichenbachian common cause model, particularly Cartwright’sgeneral forksmodel.My conclusion is that causal models for quantum correlations remain viable, and that there is no contrast between genuinely indeterministic interpretation of Quantum Theory and causal explanation, with the philosophical implication that the Causal Realism is still acceptable.
There has been an intense discussion, albeit largely an implicit one, concerning the inference of causal hypotheses from statistical correlations in quantum mechanics ever since John Bell’s first statement of his notorious theorem in 1966. As is well known, its focus has mainly been the so-called Einstein-Podolsky-Rosen (“EPR”) thought experiment, and the ensuing observed correlations in real EPR like experiments. But although implicitly the discussion goes as far back as Bell’s work, it is only in the last two decades that it has become recognizably and explicitly a debate about causal inference in the quantum realm. The bulk of this paper is devoted to a review of three influential arguments in the philosophical literature that aim to show that causal models for the EPR correlations are impossible, due to Bas Van Fraassen, Daniel Hausman and Huw Price. I contend that all these arguments are inconclusive since they contain premises or presuppositions that are false, unwarranted, or at least controversial. Five different common cause models are outlined that seem perfectly viable for the EPR correlations. These models are then employed to illustrate various difficulties with the premises and presuppositions underlying Van Fraassen’s, Hausman’s and Price’s arguments. In all these cases it is argued that the difficulties cut deep against these authors’ own theories of causation and causal inference. My conclusions are that causal models for the EPR correlations remain viable, that philosophical work is still required to assess their relative virtues, and that in any case the mere theoretical conceivability and empirical possibility of these models sheds doubts over Van Fraassen’s, Hausman’s and (important elements in) Price’s theories of causation and causal inference.
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.
In this doctoral thesis, starting from an analysis of Bertrand Russell's famous critique of the notion of causality, I argue for the outline of a causal model applicable to the indeterministic domain of quantum physics. To support this model, I propose a generalization of Hans Reichenbach's Principle of Common Cause (PCC) and a reformulation of his Mark Transmission Criterion (CTM), along with a reformulation of Wesley Salmon's Interactive Causal Fork (ICF) and his theory of causal processes. Reichenbach's Conjunctive Causal Fork (CCF) will be maintained to explain classical phenomena. The difference between the two kinds of causal fork will be considered one of the keys to understanding the distinction between probability and causality in classical and quantum physics. I also provide an introduction to classical statistical and classical causal modeling and argue for the importance of a more precise definition of the concept of determinism, as well as its non-equivalence with the notion of causality.
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.
Quantum theory describes our universe incredibly successfully. To our classically-inclined brains, however, it is a bizarre description that requires a reimagining of what fundamental reality, or “ontology”, could look like. This thesis examines different ontological features in light of the success of quantum theory, what it requires, and what it rules out. While these investigations are primarily foundational, they also have relevance to quantum information, quantum communication, and experiments on quantum systems. The way that quantum theory describes the state of a system is one of its most unintuitive features. It is natural, therefore, to ask whether a similarly strange description of states is required on an ontological level. This thesis proves that almost all quantum superposition states for d > 3 dimensions must be real—that is, present in the ontology in a well-defined sense. This is a strong requirement which prevents intuitive explanations of the many quantum phenomena which are based on superpositions. A new theorem is also presented showing that quantum theory is incompatible with macro-realist ontologies, where certain physical quantities must always have definite values. This improves on the Leggett-Garg argument, which also aims to prove incompatibility with macro-realism but contains loopholes. Variations on both of these results that are error-tolerant (and therefore amenable to experimentation) are presented, as well as numerous related theorems showing that the ontology of quantum states must be somewhat similar to the quantum states themselves in various specific ways. Extending these same methods to quantum communication, a simple proof is found showing that an exponential number of classical bits are required to communicate a linear number of qubits. That is, classical systems are exponentially bad at storing quantum data. Causal influences are another part of ontology where quantum theory demands a revision of our classical notions. This follows from the outcomes of Bell experiments, as rigorously shown in recent analyses. Here, the task of constructing a native quantum framework for reasoning about causal influences is tackled. This is done by first analysing the simple example of a common cause, from which a quantum version of Reichenbach’s principle is identified. This quantum principle relies on an identification of quantum conditional independence which can be defined in four ways, each naturally generalising a corresponding definition for classical conditional independence. Not only does this allow one to reason about common causes in a quantum experiments, but it can also be generalised to a full framework of quantum causal models (mirroring how classical causal models generalise Reichenbach’s principle). This new definition of quantum causal models is illustrated by examples and strengthened by it’s foundation on a robust quantum Reichenbach’s principle. An unusual, but surprisingly fruitful, setting for considering quantum ontology is found by considering time travel to the past. This provides a testbed for different ontological concepts in quantum theory and new ways to compare classical and quantum frameworks. It is especially useful for comparing computational properties. In particular, time travel introduces non-linearity to quantum theory, which brings (sometimes implicit) ontological assumptions to the fore while introducing strange new abilities. Here, a model for quantum time travel is presented which arguably has fewer objectionable features than previous attempts, while remaining similarly well-motivated. This model is discussed and compared with previous quantum models, as well as with the classical case. Together, these threads of investigation develop a better understanding of how quantum theory affects possible ontologies and how ontological prejudices influence quantum theory.