I present a thought experiment in quantum mechanics and tease out some of its implications for the doctrine of “peaceful coexistence”, which, following Shimony, I take to be the proposition that quantum mechanics does not force us to revise or abandon the relativistic picture of causality. I criticize the standard arguments in favour of peaceful coexistence on the grounds that they are question‐begging, and suggest that the breakdown of Lorentz‐invariant relativity as a principle theory would be a natural development, given the general trend of physics in this century.
I present a thought experiment in quantum mechanics and tease out some of its implications for the doctrine of “peaceful coexistence”, which, following Shimony, I take to be the proposition that quantum mechanics does not force us to revise or abandon the relativistic picture of causality. I criticize the standard arguments in favour of peaceful coexistence on the grounds that they are question‐begging, and suggest that the breakdown of Lorentz‐invariant relativity as a principle theory would be a natural development, given the general trend of physics in this century.
It may well be that a relativistic version of the theory, while Lorentz invariant and local at the observational level, may be necessarily non-local and with a preferred frame (or aether) at the fundamental level. – John Bell [1] Introduction Bell's theorem establishes a contradiction between locality (and a few other assumptions) and certain predictions of quantum mechanics [2]. There have been two main issues surrounding the deep implications of this celebrated theorem. The first issue is basic, and it concerns whether the theorem really establishes that nonlocality is a necessary feature of any empirically viable theory and hence a feature of nature itself. One aspect of the issue concerns exactly what the underlying assumptions of Bell's theorem are. For example, some argue that these assumptions include the assumption of counterfactual definiteness [3–5], while others disagree [6–8]. The other aspect of the issue concerns which assumption should be dropped due to the resulting contradiction. Some believe that standard quantum mechanics may avoid nonlocality by denying counterfactual definiteness [3–5]. The second issue is much deeper, and it concerns how to make sense of the strange nonlocality when assuming that Bell's theorem establishes that our world is nonlocal. In particular, it has yet to be determined whether such quantum nonlocality is compatible with the theory of relativity, e.g., whether the nonlocality requires the existence of a preferred Lorentz frame [7, 9, 10]. It seems that “there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light … Behind the apparent Lorentz invariance of the phenomena, there is a deeper level which is not Lorentz invariant” [11]. But it also seems possible that the existence of a preferred Lorentz frame is an illusion, since it cannot be detected according to standard quantum mechanics. A more subtle issue is whether quantum nonlocality permits superluminal signaling. It is widely thought that the answers to these questions will lead to a deeper understanding of Bell's theorem and quantum nonlocality. In this paper, we will present a new analysis of quantum nonlocality and its apparent incompatibility with relativity.
Traditionally causes come before effects, but according to modern physics things aren't that simple. Special relativity shows that `before' and `after' are relative, and quantum measurement is even more subtle. Since the nonlocality of Bell's theorem, it has been known that quantum measurement has an uneasy relation with special relativity, described by Shimony as `peaceful coexistence'. Hardy's theorem says that quantum measurement requires a preferred Lorentz frame. The original proofs of the theorem depended on there being no backward causality, even at the quantum level. In quant-ph/9803044 this condition was removed. It was only required that systems with classical inputs and outputs had no causal loops. Here the conditions are weakened further: there should be no forbidden causal loops as defined in the text. The theory depends on a transfer function analysis, which is introduced in detail before application to specific systems.
We provide a new formulation of the Local Friendliness no-go theorem of Bong et al. [Nat. Phys. 16, 1199 (2020)] from fundamental causal principles, providing another perspective on how it puts strictly stronger bounds on quantum reality than Bell's theorem. In particular, quantum causal models have been proposed as a way to maintain a peaceful coexistence between quantum mechanics and relativistic causality while respecting Leibniz's methodological principle. This works for Bell's theorem but does not work for the Local Friendliness no-go theorem, which considers an extended Wigner's Friend scenario. More radical conceptual renewal is required; we suggest that cleaving to Leibniz's principle requires extending relativity to events themselves.
On the Limits of “Undecidability”: Quantum Physics, Deconstruction, and Anti-realism Christopher Norris (bio) I Philosophers of science have responded in various ways to the conceptual and interpretative problems thrown up by quantum mechanics. These problems emerged a century ago from Planck’s researches into black-body radiation and have scarcely been resolved—rather, if anything, deepened and complicated—by the intensive debate that has surrounded them since that time.1 For some theorists, the resultant dilemmas (of observer “interference,” wave/particle dualism, quantum superposition, nonlocal causality, superluminal [faster-than-light] propagation, etc.) are such as to entail drastic revisions to our very idea of the physical world as existing and exerting its causal powers quite apart from our current best theories or investigative methods.2 For others again, quantum mechanics just is the best theory currently available, whatever the problems that it happens to create for our normal habits of logical thought or consistent reasoning on the evidence. Thus logic itself may have to be revised—right down to putative “laws of thought” such as noncontradiction and excluded middle—if these turn out to conflict with the results of quantum-mechanical theory or experiment.3 This is the view taken by Hilary Putnam in his writings on quantum logic and also by others, von Neumann among them, for whom the working rule in any such trade-off is to keep the physics as simple as possible even if the result is to complicate logic by introducing values other than those of determinate (bivalent) truth or falsehood.4 Very often it consorts with the argument that if quantum mechanics is right—at any rate on the standard Copenhagen interpretation—then we had best give up any hope of retaining a “classical” Einsteinian ontology, that is to say, a Lorentz-invariant account of space-time coordinates where the speed of light is the limiting value for assignments of position, velocity, and causal propagation.5 Hence Einstein’s famous series of debates with Niels Bohr when he devised a whole range of increasingly elaborate thought-experiments designed to prove that the quantum-mechnical description of reality had to be in some way “incomplete,” or not to have reckoned with some crucial factor that would bring it into line with logical reasoning and a causal-realist ontology. For on Einstein’s view there was simply no room in science for a theory—such as Bohr’s—that involved either the suspension of classical truth/falsehood [End Page 407] values or the idea of some “spooky action-at-a-distance” which contravened both commonsense realism and the requirements of Special Relativity.6 However it is Bohr who is generally thought to have carried the day in these debates. In which case—so it is argued—we are left with a choice between three possible “solutions.” These are (1) the hypothesis that quantum mechanics is “incomplete” or open to an alternative (“hidden-variables”) interpretation;7 (2) the adoption of an alternative (many-valued or non-bivalent) quantum logic that effectively conjures such problems away;8 or (3) the conclusion that realism may have to be abandoned in favour of some radically different ontology or worldview which counts this a price worth paying in order to conserve the predictive power of the standard quantum formalisms.9 On option (3) there is a further choice of interpretative views according to the theorist’s preferred choice between the various models (or possible-world scenarios) currently on offer. Thus the model may be one that embraces nonlocal simultaneous particle interaction, or which gives up space-time covariance (relative to the speed of light) as required by Einstein’s Special Theory. Or again it may be one that somehow envisions—strangest of all—the splitting-off of multiple coexistent but non-interacting “worlds” whenever the wave-packet collapses or whenever an act of measurement is performed upon any such quantum-system whose properties must be thought of as strictly indeterminate until that moment.10 For, according to its partisans, this theory is the most economical—i.e., involves least conflict with the standard quantum formalisms—despite its baroque proliferation of alternative worlds and its well-nigh unthinkable implications when applied...
The present conference takes place in the same year that celebrates the centenary of Albert Einstein. Hence it is a good occasion to reflect on those problems which have been at the core of Einstein’s intellectual activity. Undoubtedly the foundation of quantum mechanics (QM) is one of these problems. It is known that Einstein was never convinced by the interpretation of quantum mechanics accepted, in his times and still now, by the majority of physicists. The fact that he was sharing this skepticism with people like Schrödinger and, most of all, the fact that no convincing answer, to the doubts of these people, had emerged in a more than half a century old debate, helped in keeping alive the attention of a growing number of people on this problem.The crucial issue is that the standard interpretation of QM has some physical implications which are experimentally verifiable and which, for several years, have been thought to be incompatible with relativity theory (the so‐called “quantum nonlocality”). On the other hand alternative, more intuitive, interpretations (such as the ensemble interpretation) seemed to be ruled out from very well confirmed experimental data.The way out from this impasse has required a deep analysis of the connections between mathematics and physics as well as the emergence of new ideas both in mathematics (non‐Kolmogorovian probabilities) and in physics (the theory of adaptive systems).The Einstein centenary is a good occasion for a short survey of these developments with the goal of answering the intriguing question posed in the title of the present paper.
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.