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Foundations of Physics

Radical Perspectivalism, Locality and Relativity

Dennis Dieks

Published Apr 1, 2025• 1 citation
Publisher

Abstract

Abstract Perspectivalism is a natural ingredient of unitary one-world quantum mechanics. After briefly reviewing arguments for this thesis, we argue that a radical version of perspectivalism is able to provide local and relativistically covariant accounts of physical processes, and thus offers a way out of several no-go theorems. According to this radical perspectivalism, different perspectives are independent of each other and remain so even when they make causal contact. This leads to a worldview that is highly counter-intuitive, but does not lead to conflicts with experience. Moreover, locality and compatibility with relativity theory are positive points of radical perspectivalism.

DOI
10.1007/s10701-025-00844-3

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Abstract Perspectivalism is a natural ingredient of unitary one-world quantum mechanics. After briefly reviewing arguments for this thesis, we argue that a radical version of perspectivalism is able to provide local and relativistically covariant accounts of physical processes, and thus offers a way out of several no-go theorems. According to this radical perspectivalism, different perspectives are independent of each other and remain so even when they make causal contact. This leads to a worldview that is highly counter-intuitive, but does not lead to conflicts with experience. Moreover, locality and compatibility with relativity theory are positive points of radical perspectivalism.
Paul Busch has emphasized on various occasions the importance for physics of going beyond a merely instrumentalist view of quantum mechanics. Even if we cannot be sure that any particular realist interpretation describes the world as it actually is, the investigation of possible realist interpretations helps us to develop new physical ideas and better intuitions about the nature of physical objects at the micro level. In this spirit, Paul Busch himself pioneered the concept of “unsharp quantum reality”, according to which there is an objective non-classical indeterminacy—a lack of sharpness—in the properties of individual quantum systems. We concur with Busch’s motivation for investigating realist interpretations of quantum mechanics and with his willingness to move away from classical intuitions. In this article we try to take some further steps on this road. In particular, we pay attention to a number of prima facie implausible and counter-intuitive aspects of realist interpretations of unitary quantum mechanics. We shall argue that from a realist viewpoint, quantum contextuality naturally leads to “perspectivalism” with respect to properties of spatially extended quantum systems, and that this perspectivalism is important for making relativistic covariance possible.
Recent no-go theorems challenging the absoluteness of events have highlighted the possibility that dynamical facts in quantum theory are observer-relative. This thesis examines the metaphysical and conceptual foundations of quantum relativism, focusing on Relational Quantum Mechanics (RQM). It analyzes arguments for relativism through Wigner’s Friend and Extended Wigner’s Friend thought experiments, explores the link between relative physical facts and experiences, and offers the first metaphysical definition of "relative facts." The thesis argues: RQM supports a perspectival retro-determination of past events based on later acquisition of information; Quantum relativists must also relativize facts about observers’ experiences; the strongest arguments for relativism lead to unbounded higher-order relativity, precluding any absolute 'view from nowhere'; Only a metaphysical framework that relativized the composition of reality itself can accommodate such an unrestricted iteration. These conclusions deepen our grasp of a thoroughly perspectival quantum world.
Experimental evidene of the last decades has made the status of "collapses of the wave function" even more shaky than it already was on conceptual grounds: interference effects turn out to be detectable even when collapses are typically expected to occur. Non-collapse interpretations should consequently be taken seriously. In this paper we argue that such interpretations suggest a perspectivalism according to which quantum objects are not characterized by monadic properties, but by relations to other systems. Accordingly, physical systems may possess different properties with respect to different "reference systems". We discuss some of the relevant arguments, and argue that perspectivalism both evades recent arguments that single-world interpretations are inconsistent and eliminates the need for a privileged rest frame in the relativistic case.
Abstract The theories of pre-quantum physics are standardly seen as representing physical systems and their properties. Quantum mechanics in its standard form is a more problematic case: here, interpretational problems have led to doubts about the tenability of realist views. Thus, QBists and Quantum Pragmatists maintain that quantum mechanics should not be thought of as representing physical systems, but rather as an agent-centered tool for updating beliefs about such systems. It is part and parcel of such views that different agents may have different beliefs and may assign different quantum states. What results is a collection of agent-centered perspectives rather than a unique representation of the physical world. In this paper we argue that the problems identified by QBism and Quantum Pragmatism do not necessitate abandoning the ideal of representing the physical world. We can avail ourselves of the same puzzle-solving strategies as employed by QBists and pragmatists by adopting a perspectival quantum realism. According to this perspectivalism (close to the relational interpretation of quantum mechanics) objects may possess different, but equally objective properties with respect to different physically defined perspectives. We discuss two options for such a perspectivalism, a local and a nonlocal one, and apply them to Wigner’s friend and EPR scenarios. Finally, we connect quantum perspectivalism to the recently proposed philosophical position of fragmentalism.
According to various no-go results in the foundations of quantum mechanics, for any system associated to a Hilbert space of dimension higher than two, it is not possible to assign definite truth values to all propositions pertaining to the system without generating a Kochen-Specker contradiction. In this respect, the Bub-Clifton uniqueness theorem is utilized for arguing that truth-value definiteness is consistently restored with respect to a determinate sublattice of propositions defined by the state of the quantum system concerned and a particular observable to be measured. On this basis, a perspectivist/contextual account of truth valuation in the quantum domain is produced that satisfies Tarski's criterion of material adequacy for a theory of truth. In light of the latter, perspectivist truth conforms to perspective or context-bound correspondence of a de re nature, designating locally an objectively existing state of affairs. Such an account derives by virtue of the microphysical nature of physical reality in displaying a context-dependence of facts; thus, it essentially opposes a non-perspectival, metaphysically fixed point of reference, or a panoptical standpoint from which to state all facts of nature.
The present study investigates the tension between objectivism and perspectivism in light of the conceptual and philosophical implications of quantum theory. The guiding dictum of objectivism posits that science reveals the underlying structure of reality independently of human cognitive activity, that is, in terms of mind-independent, discourse-independent properties and relations occurring at any spatiotemporal location. It is shown that objectivism’s stance fits with the applicability of the bivalence principle as the semantic foundation of scientific discourse. It becomes then apparent that the non-Boolean logical structure of quantum theory systematically disrupts the objectivist framework. Along this vein, we provide succinct argumentation demonstrating quantum theory’s affinity to perspectivist reasoning. It is contended that the ubiquitous phenomenon of quantum contextuality, as established by Kochen-Specker’s celebrated theorem, strongly suggests that measurement values of quantum observables can, in general, be regarded as pertaining to an object under investigation only within a local probing frame or perspective from which the object-system is considered. The last remark is reinforced when considered within the endo-theoretic perspectivist framework of quantum mechanics. The essential philosophical meaning of the latter implies the view that the quantum world can be consistently comprehended through a multilevel structure of overlapping Boolean probing frames, realized as locally variable perspectives on a quantum system, being capable of forming jointly a coherent picture of the whole in a nontrivial way. Finally, in the appendix, the endo-theoretic perspectivist framework of quantum theory is critically compared to the relational interpretation of quantum mechanics, particularly in its recent development involving the postulate of “cross-perspective links”, revealing thereby the profoundly different picture of the quantum world’s architecture between the two proposals.