Abstract In this article, I argue that modern spacetime physics causes problems for a number of traditional accounts of modality, but also offers important new ideas about the connection between modal and non-modal features of reality. I suggest that recent work on relational observables in general relativity and quantum gravity can help us understand how non-modal features of reality could arise from modal features of reality within some form of modal ontic structural realism. In particular, I argue that the notion of an ‘internal view,’ as employed in the partial/complete observables formalism and the quantum reference frame formalism, is an important conceptual insight which can help address outstanding problems in the philosophy of lawhood and modality.
Abstract In this article, I argue that modern spacetime physics causes problems for a number of traditional accounts of modality, but also offers important new ideas about the connection between modal and non-modal features of reality. I suggest that recent work on relational observables in general relativity and quantum gravity can help us understand how non-modal features of reality could arise from modal features of reality within some form of modal ontic structural realism. In particular, I argue that the notion of an ‘internal view,’ as employed in the partial/complete observables formalism and the quantum reference frame formalism, is an important conceptual insight which can help address outstanding problems in the philosophy of lawhood and modality.
Abstract Ontic structuralism or ontic structural realism in the philosophy of physics can be broadly considered as an interpretative strategy providing a set of conceptual and metaphysical tools—or, more ambitiously, an ontological framework—in order to account for central features of current fundamental physics. This article aims to review the main structuralist interpretative moves in the context of our two best fundamental physical theories of matter and spacetime, namely, quantum theory (quantum mechanics and quantum field theory) and general relativity. We highlight in particular the structuralist understanding of permutation invariance, entanglement and nonlocality in quantum theory, and of the dynamical features of spacetime, (gauge‐theoretic) diffeomorphism invariance and background independence in general relativity.
In this paper we investigate the history of relationalism and its present use in some interpretations of quantum mechanics. In the first part of this article we will provide a conceptual analysis of the relation between substantivalism, relationalism and relativism in the history of both physics and philosophy. In the second part, we will address some relational interpretations of quantum mechanics, namely, Bohr's relational approach, the modal interpretation by Kochen, the perspectival modal version by Bene and Dieks and the relational interpretation by Rovelli. We will argue that all these interpretations ground their understanding of relations in epistemological terms. By taking into account the analysis on the first part of our work, we intend to highlight the fact that there is a different possibility for understanding quantum mechanics in relational terms which has not been yet considered within the foundational literature. This possibility is to consider relations in (non-relativist) ontological terms. We will argue that such an understanding might be capable of providing a novel approach to the problem of representing what quantum mechanics is really talking about.
Standard quantum mechanics poses serious challenges to some principles deeply entrenched in traditional metaphysics. In particular, the theory defies the ontological category of individual object through three of its features: contextuality, non-separability, and indistinguishability. The difficulties have prompted the development of non-traditional ontological frameworks intended to account for the quantum domain. Some approaches advocate replacing the classical conception of a realm of objects endowed with properties by an ontology that either dispenses with the category of object altogether or treats it as a derivative notion. The Modal-Hamiltonian Interpretation (MHI) of quantum mechanics and Ontic Structural Realism (OSR) exemplify this position. MHI proposes an ontology in which quantum systems are non-objectual bundles of properties linked by definite physical relations. The OSR defends the ontological primacy of structures, within which objects are construed as nodes of relations. The aim of this article is to compare these two ontological approaches, highlighting both their points of convergence and their divergences, particularly in light of the ontological challenges posed by quantum mechanics. With this aim, the article is structured as follows. Section 2 outlines the ontology of MHI, introducing the distinction between type-properties and case-properties, characterizing quantum systems as non-objectual bundles, and analyzing the role of symmetry groups in providing physical content to the theory and governing actualization. Section 3 turns to OSR, examining its motivations, its distinction between physical and mathematical structures, and the modal character of structures. Section 4 offers a systematic comparison of the two approaches, considering their respective motivations and scope, their shared commitment to an object-free ontology, the role played by symmetry groups within each framework, and their contrasting interpretations of modality. Finally, Section 5 presents some concluding remarks.
In General Relativity, relational constructions yield gauge-invariant Dirac observables by expressing physical fields relative to reference frames. This leaves open how the physical situations represented in different frames should be understood. I articulate two ontological interpretations within a common fibre-bundle framework, distinguishing \emph{frame-independence} from \emph{frame-freedom} to clarify appeals to perspective-neutrality. The \emph{View from Nowhere} treats the situations characterised by relational observables as partial aspects of a shared physical situation represented by a frame-free gauge-equivalence class. The \emph{View from Everywhere} takes each relational observable to characterise a comprehensive physical situation in its own right, without commitment to a shared frame-free reality. They articulate a qualified realisation of moderate and strong physical perspectivalism within GR, respectively. Using two GPS reference frames as a working example, I show that a physically constrained, frame-independent inter-frame map supplies a constructive counterexample to the claim that connecting perspectives requires frame-free structures in the ontology. The choice between the two interpretations remains open, with their ontological commitments made explicit. I conclude by outlining possible implications for quantum reference frames and relational quantum mechanics.
It is mandatory to know how to operationally define and translate a reference frame into mathematics, in order that a physical interpretation of theory calculations in terms of observational data is possible. The situation is particularly challenging for gauge systems such as General Relativity where spacetime coordinates are subject to spacetime diffeomorphisms considered as gauge transformations turning coordinates into non-observables. This motivates the idea of operationally defined (material) reference frames which specify coordinates in terms of matter or geometry reference fields leading to the concept of relational observables, relational reference frames and gauge reduction. Upon quantisation, all fields become operator valued distributions. Now new conceptual and technical questions arise such as: Should one reduce before or after quantisation and how are the reference fields quantised respectively in either route? Is a reference frame itself subject to quantisation and how are different quantum reference frames related? How does the gauge reduction fit into this, i.e. how can it be that a certain reference field is considered a non-observable in one reference frame and an observable in another which upon quantisation even displays fluctuations? How precisely are gauge dependent fields interpreted in terms of the relational observables in a given reference frame? What is the relative dynamics, e.g. how exactly are physical Hamiltonians of two relational reference frames related? The present conceptual work addresses these and related questions in a non-perturbative field theory context of sufficient generality to cover General Relativity coupled to standard matter. A central role is played by the concept of the relational reference frame transformation (RRFT) for which a general formula is derived and its properties are explored.
Traditional physical ontology relies on the assumption of observer-independence, regarding a physical state as an intrinsic property that exists prior to measurement, with observation acting merely as a passive retrieval of pre-existing facts. This paper challenges this view and articulates a relational paradigm of physics: at fundamental levels, physical states are not absolute inherent attributes, but structural outputs generated through mutual interactions among the system, the environment, and the observational framework. By re-evaluating quantum collapse, quantum decoherence, and relational quantum mechanics, this study demonstrates how the choice of measurement basis and environmental interactions selectively determine pointer states and establish classicuality. Furthermore, the relational character of physical states is corroborated in macroscopic domains, including the dependency of thermodynamic entropy on coarse-graining and the reference-frame relativity of spatiotemporal states in general relativity and the Unruh effect. By reframing physics around interaction boundaries and information exchange, this paper establishes a relational realism that transcends both rigid objective realism and subjective idealism, offering insight into foundational problems in quantum gravity and quantum cosmology without appealing to external observers.