Marco Sanchioni

This paper examines the role of perspectivism in Relational Quantum Mechanics, situating it within the broader landscape of quantum interpretations and the scientific realism debate. We argue that, while interpretations such as QBism embrace strong forms of perspectivism, Relational Quantum Mechanics adopts a “soft” perspectivism, limiting the observer’s role to selecting experimental contexts without compromising its realist framework. We also explore the historical roots of Relational Quantum Mechanics, showing that relational ideas in the works of Bohr and other pioneers similarly avoided strong perspectivist commitments. By analyzing both contemporary and historical perspectives, we argue that Relational Quantum Mechanics offers a minimalist yet robust relational interpretation, distinct from more subjectivist approaches.

Since its inception in 1996, Rovelli’s Relational Quantum Mechanics (RQM) has left the status of non-interacting systems notably underexplored. When discussed at all, one prevailing view has maintained that attributing properties to isolated systems within RQM is simply meaningless. However, it has become increasingly evident that without a satisfying account of non-interacting systems, RQM risks sliding toward antirealist interpretations–at least insofar as such systems are concerned. In response to this crucial challenge, two main approaches have been advanced. One is a form of dispositionalism, which suggests that isolated physical systems possess only dispositional properties. The other appeals to a determinable-based account of metaphysical indeterminacy, suggesting that a non-interacting quantum system may bear a determinable property without possessing any unique determinate of that determinable at a given time. In this paper, we offer a new response to the question of what can be meaningfully said about quantum systems prior to measurement in the context of RQM. In particular, drawing on the formalism of Positive Operator-Valued Measures (POVMs), we emphasize that (1) preparation–like measurement–constitutes a physical interaction capable of assigning relational properties to a system, and that (2) POVMs are indispensable for the proper description of observables with continuous spectra. On this basis, we propose that RQM can accommodate an entity-realist interpretation of non-interacting systems by attributing to them what we term relational unsharp properties. As we will show, this theoretical move preserves the relational nucleus of RQM while enabling a physically and philosophically coherent account of non-interacting quantum systems.

This paper argues that von Neumann entropy plays two conceptually distinct roles in quantum theory. When applied to global mixed states, it expresses the quantum analogue of the informational entropy. But when applied to reduced states of entangled systems, it measures objective physical correlations. We propose that this second usage realizes a new informational kind, which we call relational information. Unlike information in communication theory, it reflects structural interdependence between systems, not probability about a certain outcome. We further suggest that this informational kind has ontological significance: relational information is a physically instantiated feature of entangled systems, and not merely an agent-relative concept.

This paper challenges the notion of emergent time in quantum cosmology by examining the reconciliation of the timeless Wheeler–DeWitt equation with the Universe’s dynamical evolution. We critically evaluate the analogy between the Wheeler–DeWitt and Klein–Gordon equations, highlighting challenges for the identification of an emergent time parameter. We conclude that refining this analogy may lead to a better understanding of emergent time in quantum cosmology, though it is still not free from complications.

Sebens (2025) has proposed a semiclassical “precursor” to quantum electrodynamics (QED) in which the electron’s anomalous magnetic moment arises from the self-interaction of an extended charge distribution governed by the Dirac equation. The calculation reproduces Schwinger’s leading-order value only for suitably tuned, spatially extended wave-packets, and thus yields a state-dependent magnetic moment. This paper offers a systematic critique of that result. After reviewing the standard QED derivation—where the anomaly is fixed by gauge symmetry, the Ward–Takahashi identity, and renormalization—we show that the semiclassical model lacks the structural resources that guarantee universality. Drawing on a general distinction between phenomenological dependence and theoretical fundamentality, we argue that Sebens’s construction attains intuitive, mechanical appeal at the cost of explanatory depth: its high phenomenologicality cannot compensate for its low fundamentality. What Sebens treats as a puzzle for QED—how the theory “nails down” a single value of $$g-2$$ —is instead a symptom of the precursor’s incompleteness. The episode illustrates a broader methodological point: in modern physics, structural principles, rather than classical pictures, underwrite genuine explanation.

Reichenbach’s importance to the development of modern philosophy can hardly be overstated. However, many themes and arguments originally developed by Reichenbach are either overlooked or not properly credited to him. In this article, we discuss an important but often forgotten argument of Reichenbach against the Kantian notion of synthetic a priori. We first give a detailed historical reconstruction of the argument and discuss the mild conventionalism that Reichenbach developed following this argument. Then, we recast this argument in the modern language of General Relativity. We show that this argument is still relevant for a modern attempt at articulating the notion of synthetic a priori: Friedman’s relativised synthetic a priori.

This Chapter explores the philosophical and ontological implications of F-theory, a non-perturbative extension of Type IIB string theory, mainly focusing on the apparent existence of two temporal dimensions. The paper proposes an interpretation that advocates for a single temporal dimension grounded in a brane-based ontology, challenging the conventional understanding of time and spacetime in F-theory. Moreover, it discusses the implications of a brane-based ontology for spacetime emergence, providing a novel understanding of the relations between spatiotemporal and non-spatiotemporal structures.

This paper aims to construct an ontology grounded in the interpretation of Relational Quantum Mechanics, which serves as a robust framework for a realist interpretation of quantum mechanics without the need for additional theoretical constructs. Using the notion of relational properties, we formulate Relational Quantum Mechanics’s ontology. Our analysis highlights the ontological significance of Rovelli (Int. J. Theor. Phys., 35, 1637–1678, 1996)’spostulates. Moreover, we apply this ontological perspective to quantum mechanics paradoxes and show that it provides new insights into those paradoxes, underscoring Relational Quantum Mechanics explanatory power.

The problem of recovering information from the interior of a black hole is crucial to any resolution of the information loss paradox. In this article, we critically evaluate the program of holographic interior reconstruction within the AdS/CFT correspondence, explaining the conceptual underpinnings and implicit assumptions behind the recovery of black hole interior information, in the face of the apparent impossibility of doing so due to the AMPSS paradox. We also show how the implicit assumptions behind holographic interior reconstruction are the same as those underpinning an apparently unrelated popular resolution of the firewall paradox. By doing so, we highlight how holographic interior reconstruction fits within a larger conceptual strategy for attacking the problem of describing black holes in Quantum Gravity.

The coexistence of Supersymmetry (SUSY) and the Spin-Statistics Theorem (SST) poses a challenge, given their seeming incompatibility. While SUSY connects particles with distinct spins, SST links particle’s spin to their statistics. We propose a solution to this puzzle: both spin and SST may emerge as low-energy phenomena. To do so, we look at SUSY breaking at lower energy scales, exploring if this mechanism aligns with the concept of emergence. The paper presents a comprehensive review of the SUSY-SST tension, a philosophical introduction to SUSY-breakig, and an argument for spin and SST emergence within SUSY. Overall, our aim is to show how SUSY and SST can coexist in the same theory and thus deepen our understanding of SUSY quantum field theory.

We consider the observables describing spatiotemporal properties in the context of two of the most popular approaches to quantum gravity (QG), namely String Theory and Loop QG. In both approaches these observables are described by non-commuting operators. In analogy with recent arguments put forward in the context of non-relativistic quantum mechanics [see Calosi and Mariani (Philos. Compass 16(4):e12731, 2021) for a review], we suggest that the physical quantities corresponding to those observables may be interpreted as ontologically indeterminate—i.e., indeterminate in a way that is non-epistemic and semantic-independent. This working hypothesis has not received enough attention in the current debate on QG, and yet it may prove explanatory useful in several respects. First, it provides a clear background for understanding how some features of QG are ontologically continuous to features of quantum mechanics. Second, it sets the stage for asking new interesting questions about QG, for instance concerning the status of the so-called Eigenstate-Eigenvalue link. Third, it indirectly shows how the debate on ontological indeterminacy may extend well beyond the non-relativistic case, contrary to what seems to be assumed. Fourth, and perhaps more importantly, it provides a promising alternative to the received view on QG [Wüthrich et al. (Philosophy Beyond Spacetime: Implications from Quantum Gravity, Oxford University Press, Oxford, 2021)] according to which spacetime is not fundamental. On the view we shall suggest, spacetime may be indeterminate and yet fundamental.

This paper conducts a Bayesian analysis of the Hubble tension, which addresses the discrepancy between local measurements of the Hubble Constant $$H_0$$ and the value predicted by the $$\Lambda $$ CDM model based on Cosmic Microwave Background data. By incorporating new, independent data from the James Webb Space Telescope released in August 2024, the analysis shows that, unlike before, there is no longer strong evidence to suggest that the $$\Lambda $$ CDM model is incorrect. As a result, the Hubble tension appears to ease somewhat, indicating that the previously perceived crisis in cosmology may have been overstated, and the standard model remains robust.

Canonically, ‘classic’ tests of general relativity (GR) include perihelion precession, the bending of light around stars, and gravitational redshift; ‘modern’ tests have to do with, _inter alia_, relativistic time delay, equivalence principle tests, gravitational lensing, strong field gravity, and gravitational waves. The orthodoxy is that both classic and modern tests of GR afford experimental confirmation of that theory _in particular_. In this article, we question this orthodoxy, by showing there are classes of both relativistic theories (with spatiotemporal geometrical properties different from those of GR) and non-relativistic theories (in which the lightcones of a relativistic spacetime are ‘widened’) which would also pass such tests. Thus, (a) issues of underdetermination in the context of GR loom much larger than one might have thought, and (b) given this, one has to think more carefully about what exactly such tests in fact _are_ testing.

This paper examines the tension between wave function realism and interpretations of gauge symmetries within quantum mechanics. We explore how traditional views of gauge symmetries as descriptive redundancies challenge the principles of wave function realism, which regards the wave function as a real entity. By noting that, through the case study of a quantum particle in an electromagnetic field, gauge transformations impact the wave function’s phase, we present a dilemma for wave function realism. We discuss potential resolutions, including redefining ontological commitments to accommodate gauge-invariance.

This paper presents a philosophical analysis of the structure of black holes, focusing on the event horizon and its fundamental status. While black holes have been at the centre of countless paradoxes arising from the attempt to merge quantum mechanics and general relativity, recent experimental discoveries have emphasised their importance as objects for the development of Quantum Gravity. In particular, the statistical mechanical underpinning of black hole thermodynamics has been a central research topic. The Quantum Membrane Paradigm, proposed by Wallace (Stud Hist Philos Sci Part B 66:103-117, 2019), posits a real membrane made of black hole microstates at the black hole horizon to provide a statistical mechanical understanding of black hole thermodynamics from an exterior observer’s point of view. However, we argue that the Quantum Membrane Paradigm is limited to low-energy Quantum Gravity and needs to be modified to avoid reference to geometric notions, such as the event horizon, which presumably do not make sense in the non-spatiotemporal context of full Quantum Gravity. Our proposal relies on the central dogma of black hole physics. It considers recent developments, such as replica wormholes and entanglement wedge reconstruction, to provide a new framework for understanding the nature of black hole horizons in full Quantum Gravity.

This paper aims to characterise properly entanglement as an external relation obtaining between multiple quantum degrees of freedom. In particular, we argue that the entanglement relation is a unique relation fully characterised by mutual information, i.e. a quantity standardly used as a measure of entanglement. This analysis leads us to propose a new metaphysical account of entanglement, which we call Relational Entanglement Tesseract. Such an account characterises entanglement for both bipartite and multipartite cases, and, at the same time, it satisfies what we argue are three important desiderata of any metaphysical account of entanglement.

A common assumption in non-relativistic quantum mechanics is that self-adjoint operators mathematically represent properties of quantum systems. Focusing on spin, we argue that a natural view considers observables as determinable properties and their eigenvalues as their corresponding determinates. We provide a taxonomy of the different views that one can hold, once it is accepted that spin can be modelled with the determinable-determinate relation. In particular, we present the two main families of views, dubbed Spin Monism and Pluralism, and we show that the current literature does not take a stance between the two. Then we put forward two arguments in favour of the former. Finally, we present a new account of Spin Monism, that is absent in current literature; such a view is worth discussing, or so we contend, because several compelling considerations support it, and it opens new ways of thinking about the ontology of quantum mechanics.