Any quantum theory of gravity faces the measurement problem. Carlo Rovelli sees his relational interpretation as offering a solution to this problem when applied to his favored loop quantum gravity (LQG). I examine the prospects of Rovelli’s relationalism in LQG. In LQG it is not clear what physical systems there are at a fundamental level with no spacetime. But implementing Rovelli’s relational interpretation in the context of LQG requires an account of interaction and a model of observer systems whose relational states represent determinate outcomes. Even if no such account is forthcoming at a fundamental level in LQG, it might still be available in a limit at which spacetime has emerged. But to use this account effectively to address the measurement problem, it would be necessary to reconcile the observer-relativity of measurement outcomes with a basic norm of scientific objectivity.
Any quantum theory of gravity faces the measurement problem. Carlo Rovelli sees his relational interpretation as offering a solution to this problem when applied to his favored loop quantum gravity (LQG). I examine the prospects of Rovelli’s relationalism in LQG. In LQG it is not clear what physical systems there are at a fundamental level with no spacetime. But implementing Rovelli’s relational interpretation in the context of LQG requires an account of interaction and a model of observer systems whose relational states represent determinate outcomes. Even if no such account is forthcoming at a fundamental level in LQG, it might still be available in a limit at which spacetime has emerged. But to use this account effectively to address the measurement problem, it would be necessary to reconcile the observer-relativity of measurement outcomes with a basic norm of scientific objectivity.
Abstract Any quantum theory of gravity faces the measurement problem. Carlo Rovelli sees his relational interpretation as offering a solution to this problem when applied to his favored loop quantum gravity (LQG). This essay examines the prospects of Rovelli’s relationalism in LQG. In loop quantum gravity, it is not clear what physical systems there are at a fundamental level with no spacetime. But implementing Rovelli’s relational interpretation in the context of LQG requires an account of interaction and a model of observer systems whose relational states represent determinate outcomes. Even if no such account is forthcoming at a fundamental level in LQG, it might still be available in a limit at which spacetime has emerged. But to use this account effectively to address the measurement problem it would be necessary to reconcile the observer-relativity of measurement outcomes with a basic norm of scientific objectivity.
This article offers a critical analysis of Carlo Rovelli’s conceptual positions in works devoted to interpretations of quantum mechanics and the notion of time. Attention is paid to the relational interpretation developed by the author in “Helgoland: Making Sense of the Quantum Revolution” and to its philosophical foundations, which go back to the ideas of Ernst Mach and Nagarjuna. The article examines key elements of the quantum-mechanical formalism, including the superposition principle, interference effects, and commutation relations, with emphasis on their interpretive consequences. The article also analyzes Rovelli’s critical remarks on alternative approaches, such as Hugh Everett III’s many-worlds interpretation, hidden-variable theories associated with the works of Louis de Broglie and David Bohm, as well as models that propose a physical explanation of wave-function collapse. It is shown that some of the author’s arguments are limited in scope and do not consider contemporary experimental results, in particular studies that cast doubt on the epistemological interpretation of the wave function. Special attention is given to internal contradictions in Rovelli’s relational approach, including the problem of the ontological status of objects and the reconciliation of the principle of relationality with the existence of invariant physical quantities. The interpretation of time given by Rovelli in works such as The Order of Time and A Brief History of Time is also examined. The limitations of his ontological approach to time within loop quantum gravity are emphasized.
Measurements play a crucial role in quantum mechanics because they serve two fundamental purposes. First, they enable the extraction of information about a quantum system; second, they give rise to new physical effects that do not appear in classical physics, one important example being the measurement-induced state update, where the act of measurement alters the system’s state. For these reasons, the mathematical description of measurements plays a primary role in the foundational postulates of the theory. However, the essence of what happens during a measurement remains poorly understood, even within the relatively easy realm of non-relativistic quantum theory. The situation becomes even less transparent in the relativistic setting, where the requirement to preserve causality and prevent faster-than-light signalling introduces both formal and interpretational challenges, complicating the very mathematical formulation of a measurement theory. As for quantum gravity, the idea of applying quantum measurements within or onto a quantum spacetime remains virtually unexplored, presenting a vast and uncharted frontier for theoretical inquiry. Building on recent advances across several areas at the intersection of quantum foundations, quantum information theory, relativistic quantum theory, and quantum gravity, the last few years saw a surge of interest in formulating a consistent framework for describing local measurements in a relativistic context. The main results of this endeavour are two distinct proposals for a relativistic measurement postulate for quantum field theory in generic spacetimes. While these two proposals are still being understood, and a growing research community is working to clarify their implications and potential interconnections, the field of relativistic quantum measurements has now reached a level of maturity that allows us to ask both formal and interpretive questions on a solid footing. All this only applies to the regime in which quantum field theory in curved spacetime remains valid: nothing can yet be said about measurements at the Planck energy, where quantum gravitational effects are expected to become important. This thesis presents an introduction to the theory of measurements in non-relativistic and relativistic quantum mechanics, along with four original research articles that explore aspects of measurements in both fields, as well as the potential extension of measurement theory to the domain of quantum gravity. The first article examines the limitations of realistic measurement apparatuses in non-relativistic quantum theory, establishing reasonable bounds for the precision that can be achieved with a given apparatus composed of many quantum components. The second article explores how to extract information from a newly introduced class of quantum systems, defined as those that cannot be replicated and thus fall outside the usual scope of quantum mechanics. Then, it applies the methodology to quantum field theory, demonstrating that classical results, such as the Unruh effect, can still be observed in this setting. The third article discusses detector-based measurements in quantum field theory. This is a recently proposed method that extends the non-relativistic measurement postulate to the relativistic regime by means of Unruh-de Witt detectors. The article highlights some conceptual and mathematical issues with the framework and proposes a solution using algebraic quantum field theory. The outcome is a self-consistent scheme for introducing local measurements in quantum field theory via measuring detectors. Finally, the fourth article uses the measurement scheme above to study how what we learned in the past years about local measurements can be extended to quantum gravity. This is achieved through the holographic principle realised in terms of the AdS-CFT correspondence. In conclusion, this thesis contributes to the development of both non-relativistic and relativistic measurement theories and introduces ideas and questions related to the role of measurement in quantum gravity. Though preliminary, this work opens a pathway toward studying measuring observers in quantum gravity.
The measurement problem in quantum mechanics and the emergence of spacetime in quantum gravity are traditionally treated as distinct structural crises in theoretical physics. This paper proposes a synthesis of Relational Quantum Mechanics (RQM) and the gauge-gravity duality (AdS/CFT) through a framework termed Perspectival Holographic Spacetime (PHS). By abandoning the assumption of an absolute, globally defined bulk geometry, PHS models spacetime as a sheaf of localized, observer-dependent holographic geometries driven by mutual information. To account for the generation of local Riemannian curvature from entanglement networks, we introduce Morphogenetic Quantum Resources (MQR)—the non-stabilizer complexity required to shape spacetime, directly analogous to biological morphogenesis. Under PHS, quantum measurement is formalized not as an objective collapse, but as a topological phase transition and structural localization within the observer's specific emergent reference frame, thereby providing a purely geometric resolution to the Extended Wigner's Friend paradox.
Conceptual Problems in Quantum Cosmology The application of quantum theory to cosmology raises a number of conceptual questions, such as the role of the quantum-mechanical notion of “observer” or the absence of a time variable in the Wheeler–DeWitt equation. I point out that a relational formulation of quantum mechanics, and more in general the observation that evolution is always relational, provides a coherent solution to this tangle of problems. A number of confusing issues appear when we try to apply quantum mechanics to cosmology. Quantum mechanics, for instance, is generally formulated in terms of an observer making measurements on a system. In a laboratory experiment, it is easy to identify the system and the observer: but what is the observer in quantum cosmology? Is it part of the same universe described by the cosmological theory, or should we think of it as external to the universe? Furthermore, the basic quantum dynamical equation describing a system including gravity is not the Schrödinger equation, but rather the Wheeler–DeWitt equation, which has no time parameter: is this related to the absence of an observer external to the universe? How do we describe the quantum dynamics of the universe without a time variable in the dynamical equation and without an observer external to the universe? I suggest that clarity on these issues can be obtained by simply recognizing the relational nature of quantum mechanics and more in general the relational nature of physical evolution. In the context of quantum theory, this nature is emphasized by the so-called relational interpretation of quantum mechanics (Rovelli, 1996). The relational nature of evolution, on the other hand, has been pointed out by the partial-observable formulation of general covariant dynamics (Rovelli, 2002). A central observation is that there is a common confusion between two different meanings of the expression “cosmology”. This is discussed below, in Section 15.1.1. A considerable amount of the difficulties mentioned above stems, I argue, from this confusion. The discussion clarifies on the role of the quantum mechanical observer in cosmology, which is considered in Section 15.1.2. In Section 15.2, I briefly describe the relational interpretation of quantum mechanics and some related aspects of quantum theory.
Quantum mechanics has been extraordinarily successful, yet fundamental problems remain, including the measurement problem, nonlocality, and the absence of a complete theory of quantum gravity. This paper proposes a measurement-centric framework in which spacetime is not fundamental. Instead, reality emerges from measurements occurring within a universal probability space composed of interconnected packets of information. Each measurement updates this structure while preserving its underlying symmetry. This framework offers a natural explanation for nonlocality, clarifies the role of observers, and connects general relativity to the conservation of symmetry.