Tim Maudlin, Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics (Aristotelian Society Series, Volume 13), Oxford UK & Cambridge USA:
Quantum Non-Locality and Relativity has the rare quality of being both a significant contribution to the philosophy of physics literature and a superb introduction for philosophers unacquainted with the field. Maudlin sets out to examine the implications for our understanding of the fundamental structure of space, time, and causation of what appears to be a mysterious action-at-a-distance observed in the quantum domain. Physicists and philosophers of physics have been grappling with these implications for the last half-century, and Maudlin does a remarkable job of elucidating central themes in the literature and clarifying conceptual confusions. Throughout the book, Maudlin draws out salient points from what can easily become a morass of scientific and philosophical technicalities. At the root of puzzles concerning relativity and quantum mechanics lie a few simple experimental facts. In an experimental setup in which quantum particles begin from a common source and travel in opposite directions, distant (spacelike separated) measurements of properties of the two particles are strangely correlated. These correlations have been observed, for example, in experiments which measure polarization properties of photons. A well-known theorem due to John S. (1964) expresses a constraint on the kind of correlations that measurement outcomes in this experimental setup (known as a Bell experiment) can display, in the absence of any communication between them. The surprise is that observed correlations violate Bell's theorem, meaning that the measurement events appear somehow to be influencing each other or to be otherwise connected. But there's the rub: in experiments, measurements are spacelike separated, which means they are too far apart for any force of nature travelling at or below the speed of light to traverse the distance between them in time. Faster-than-light influences seem to fly squarely (and rather quickly) in the face of everything Einstein taught us about the structure of space and time. As it is often understood, the special theory of relativity asserts that the speed of light in a vacuum is some sort of physical limit, and that matter, energy, signals, causal influence, and the like cannot travel faster than the speed of light. Do the
Quantum Non-Locality and Relativity has the rare quality of being both a significant contribution to the philosophy of physics literature and a superb introduction for philosophers unacquainted with the field. Maudlin sets out to examine the implications for our understanding of the fundamental structure of space, time, and causation of what appears to be a mysterious action-at-a-distance observed in the quantum domain. Physicists and philosophers of physics have been grappling with these implications for the last half-century, and Maudlin does a remarkable job of elucidating central themes in the literature and clarifying conceptual confusions. Throughout the book, Maudlin draws out salient points from what can easily become a morass of scientific and philosophical technicalities. At the root of puzzles concerning relativity and quantum mechanics lie a few simple experimental facts. In an experimental setup in which quantum particles begin from a common source and travel in opposite directions, distant (spacelike separated) measurements of properties of the two particles are strangely correlated. These correlations have been observed, for example, in experiments which measure polarization properties of photons. A well-known theorem due to John S. (1964) expresses a constraint on the kind of correlations that measurement outcomes in this experimental setup (known as a Bell experiment) can display, in the absence of any communication between them. The surprise is that observed correlations violate Bell's theorem, meaning that the measurement events appear somehow to be influencing each other or to be otherwise connected. But there's the rub: in experiments, measurements are spacelike separated, which means they are too far apart for any force of nature travelling at or below the speed of light to traverse the distance between them in time. Faster-than-light influences seem to fly squarely (and rather quickly) in the face of everything Einstein taught us about the structure of space and time. As it is often understood, the special theory of relativity asserts that the speed of light in a vacuum is some sort of physical limit, and that matter, energy, signals, causal influence, and the like cannot travel faster than the speed of light. Do the
Introduction Fifty years after the publication of Bell's theorem, there remains some controversy regarding what the theorem is telling us about quantum mechanics, and what the experimental violations of Bell inequalities are telling us about the world. This chapter represents my best attempt to be clear about what I think the lessons are. In brief: There is some sort of nonlocality inherent in any quantum theory, and, moreover, in any theory that reproduces, even approximately, the quantum probabilities for the outcomes of experiments. But not all forms of nonlocality are the same; there is a distinction to be made between action at a distance and other forms of nonlocality, and I will argue that the nonlocality needed to violate the Bell inequalities need not involve action at a distance. Furthermore, the distinction between forms of nonlocality makes a difference when it comes to compatibility with relativistic causal structure. The Bell locality condition is a condition which, if satisfied, renders possible a completely local account of the correlations between outcomes of spatially separated experiments. Bell's theorem tells us that the probabilities that we compute from quantum mechanics do not admit of such an account. Modulo auxiliary assumptions that, in my opinion, should be noncontroversial, this yields the conclusion that there must be something nonlocal about any theory that yields the quantum mechanical probabilities or anything close to them. One way to violate the Bell locality condition – and this is the way that any deterministic theory must do it – is via straightforward action at a distance. However, we should also consider the possibility of a stochastic theorywith probabilistic laws that involve irreducible correlations. The sort of nonlocality inherent in such a theory involves relations between events that are significantly different from causal relations as usually conceived. Perhaps surprisingly, such a theory, unlike its deterministic cousins, need not invoke a distinguished relation of distant simultaneity. This makes a difference for the situation of the two sorts of theories vis-a-vis relativity. Though a deterministic theory can achieve the appearance of being relativistic, at the level of observable phenomena, it can do so only by introducing a preferred foliation and with it a causal structure at odds with relativistic causal structures. Things are otherwise with dynamical collapse theories.
Between 1964 and 1990, the notion of nonlocality in Bell's papers underwent a profound change as his nonlocality theorem gradually became detached from quantum mechanics, and referred to wider probabilistic theories involving correlations between separated beables. The proposition that standard quantum mechanics is itself nonlocal (more precisely, that it violates ‘local causality’) became divorced from the Bell theorem per se from 1976 on, although this important point is widely overlooked in the literature. In 1990, the year of his death, Bell would express serious misgivings about themathematical form of the local causality condition and leave ill-defined the issue of the consistency between special relativity and violation of the Bell-type inequality. In our view, the significance of the Bell theorem, in both its deterministic and stochastic forms, can only be fully understood by taking into account the fact that a fully Lorentz covariant version of quantum theory, free of action at a distance, can be articulated in the Everett interpretation. Introduction John S. Bell's last word on his celebrated nonlocality theorem and its interpretation appeared in his 1990 paper ‘La nouvelle cuisine’, first published in the year of his untimely death. Bell was careful here to distinguish between the issue of ‘no superluminal signalling’ in quantum theory (both quantum field theory and quantum mechanics) and a principle he first introduced explicitly in 1976 and called ‘local causality’ [1]. In relation to the former, Bell expressed concerns that amplify doubts he had already expressed in 1976. These concerns touch on what is now widely known as the no-signalling theorem in quantum mechanics, and ultimately have to do with Bell's distaste for what he saw as an anthropocentric element in orthodox quantum thinking. In relation to local causality, Bell emphasised that his famous factorizability (no-correlations) condition is not to be seen ‘as the formulation of local causality, but as a consequence thereof’ and stressed how difficult he found it to articulate this consequence. He left the question of any strict inconsistency between violation of factorizability and special relativity theory unresolved, a not insignificant shift from his thinking up to the early 1980s.
Abstract Quantum mechanics and relativity are two of the most successful theoretical frameworks in modern physics, yet their conceptual foundations remain difficult to reconcile at the deepest level. Quantum theory describes physical systems through states, observables, probabilities, measurement contexts, superposition, entanglement, and nonclassical correlations. Relativity describes physical processes through space-time structure, causal order, invariant physical law, observer-dependent decomposition of time and space, and constraints on signal propagation. This article examines the conceptual relationship between quantum measurement, relativistic causality, and space-time description. The purpose is not to present a mathematical unification but to analyze how the two frameworks organize physical explanation differently and where their interpretive tensions arise. Special attention is given to the measurement problem, the status of quantum states, the role of observers and reference frames, the meaning of locality, the distinction between correlation and causal influence, and the challenge of describing quantum processes without privileging a single global time. The article argues that many apparent conflicts between quantum theory and relativity result from importing classical intuitions into domains where they no longer apply. Quantum entanglement does not permit controllable faster-than-light communication, yet it challenges classical separability. Relativity forbids superluminal signaling, yet it does not require that all physically meaningful correlations be reducible to local classical properties. Measurement introduces another difficulty because standard quantum theory often describes state update in a way that appears instantaneous, while relativistic space-time does not provide a universal present. The discussion develops a conceptual framework in which quantum measurement is treated as a physical interaction, relativistic causality as a constraint on operational influence, and space-time description as an organizing structure for events rather than a simple background stage. This analysis is relevant for quantum field theory, quantum information, relativistic quantum foundations, and future approaches to quantum gravity.
Perhaps the deepest mystery in all of physics is quantum non-locality – the ability of two distant systems to instantaneously know about each other. Non-locality means that two particles in a quantum system can be "entangled" such that the state of one particle affects the state of the other, regardless of how far apart they are. As well as challenging our common sense, non-locality suggests that the ultimate speed limit – the speed of light – is violated. Einstein spent many years worrying about this problem because such "superluminal" interactions seem to be incompatible with special relativity.
Introduction: The Theory of Local Beables John Bell's most celebrated contribution to the foundations of physics is his famous theorem. The theorem demonstrates that any physical theory capable of generating the predictions of the standard quantum-mechanical algorithm, in particular the prediction of violations of Bell's inequality for experiments done at spacelike separation, cannot be local. The sense of “locality” used here is the same sense that Einstein had in mind when he pointed out that the standard interpretation of the quantum algorithm was committed to “spooky action at a distance” To this day, the import of Bell's theorem is not universally appreciated. I have written about this elsewhere [1], and others in this volume will take up that task. It is properly the main focus during this 50th anniversary of that great achievement. But it is also important to recall and celebrate Bell's other achievements. In many of his later writings, including “The Theory of Local Beables,” “Quantum Mechanics for Cosmologists,” “On the Impossible Pilot Wave,” “Beables for Quantum Field Theory,” “Six Possible Worlds of Quantum Mechanics,” “Are There Quantum Jumps?” and “Against ‘Measurement,”’ 1 Bell turned his attention to the more general problem of physically construing the mathematical formalism used to derive these predictions. This activity is often denominated “interpreting quantum theory” as if there were some precise physical theory that might somehow be supplemented with an “interpretation” Once the issue is framed this way it is easy to ask: But if I already have a theory in hand, what can be gained by supplementing it with an “interpretation”? Many physicists, at this juncture, are happy to conclude that “interpretations” are not a matter of physics at all – maybe they are only of interest to philosophers – and that therefore the whole enterprise of “interpreting quantum theory” is not within the purview of physics per se. What then is in the purviewof physics proper?One answer to this question goes under the banner “instrumentalism”: all physics as such, is concerned about is predicting the outcomes of experiments .