The aim of this thesis is to better understand the features of quantum matter that makes quantum gravity so hard to reach. In Chapter 3, we present two general models of decoherence, without specifying any Hamiltonian. The first one makes decoherence naturally appear as a geometrical phenomenon between reservoirs of dimensions. The second one specifically tackles the question of why, in a universe where decoherence is ubiquitous, things are not frozen by the quantum Zeno effect. In Chapter 4, we discuss the ontological problem of quantum physics, usuallyreferred to as the ‘measurement problem’. Our aim in this chapter is not to give a solution, but rather to formulate the problem independently of any interpretation of quantum mechanics, in the light of the theory of decoherence. In Chapter 5, a new ingredient is introduced in the discussion: spacetime. After investigating the apparent incompatibility between quantum mechanics and special relativity, we present a program of research and a toy model designed to build a backgroundless notion of position within the quantum formalism.
The aim of this thesis is to better understand the features of quantum matter that makes quantum gravity so hard to reach. In Chapter 3, we present two general models of decoherence, without specifying any Hamiltonian. The first one makes decoherence naturally appear as a geometrical phenomenon between reservoirs of dimensions. The second one specifically tackles the question of why, in a universe where decoherence is ubiquitous, things are not frozen by the quantum Zeno effect. In Chapter 4, we discuss the ontological problem of quantum physics, usuallyreferred to as the ‘measurement problem’. Our aim in this chapter is not to give a solution, but rather to formulate the problem independently of any interpretation of quantum mechanics, in the light of the theory of decoherence. In Chapter 5, a new ingredient is introduced in the discussion: spacetime. After investigating the apparent incompatibility between quantum mechanics and special relativity, we present a program of research and a toy model designed to build a backgroundless notion of position within the quantum formalism.
Le problème ontologique en physique quantique et l’émergence d’un espace-temps relationnel à la lumière de la théorie de la décohérence L’objectif de cette thèse est de mieux comprendre les caractéristiques de la matière quantique qui rendent la gravitation quantique si difficile à atteindre. Dans le Chapitre 3, nous présentons deux modèles généraux de décohérence, sans spécifier aucun Hamiltonien. Le premier modèle fait apparaître naturellement la décohérence comme un phénomène géométrique entre deux réservoirs de dimensions. Le second modèle tente d’expliquer pourquoi, dans un univers où la décohérence est omniprésente, la matière n’est pas figée par l’effet Zénon quantique. Le Chapitre 4 est consacré au problème ontologique de la physique quantique, communément appelé « problème de la mesure ». Notre objectif n’est pas d’en fournir une solution, mais plutôt de formuler le problème indépendamment de toute interprétation de la mécanique quantique, à la lumière de la théorie de la décohérence. Dans le Chapitre 5, un nouvel ingrédient est introduit dans la discussion : l’espace-temps. Après avoir éclairci l’incompatibilité apparente entre la mécanique quantique et la relativité restreinte, nous présentons un programme de recherche et un modèle jouet en vue de définir une notion backgroundless de position au sein du formalisme quantique.
In the last decade decoherence has become a very popular topic mainly due to the progress in experimental techniques which allow monitoring of the process of decoherence for single microscopic or mesoscopic systems. The other motivation is the rapid development of quantum information and quantum computation theory where decoherence is the main obstacle in the implementation of bold theoretical ideas. All that makes the second improved and extended edition of this book very timely. Despite the enormous efforts of many authors decoherence with its consequences still remains a rather controversial subject. It touches on, namely, the notoriously confusing issues of quantum measurement theory and interpretation of quantum mechanics. The existence of different points of view is reflected by the structure and content of the book. The first three authors (Joos, Zeh and Kiefer) accept the standard formalism of quantum mechanics but seem to reject orthodox Copenhagen interpretation, Giulini and Kupsch stick to both while Stamatescu discusses models which go beyond the standard quantum theory. Fortunately, most of the presented results are independent of the interpretation and the mathematical formalism is common for the (meta)physically different approaches. After a short introduction by Joos followed by a more detailed review of the basic concepts by Zeh, chapter 3 (the longest chapter) by Joos is devoted to the environmental decoherence. Here the author considers mostly rather `down to earth' and well-motivated mechanisms of decoherence through collisions with atoms or molecules and the processes of emission, absorption and scattering of photons. The issues of decoherence induced superselection rules and localization of objects including the possible explanation of the molecular structure are discussed in details. Many other topics are also reviewed in this chapter, e.g., the so-called Zeno effect, relationships between quantum chaos and decoherence, the role of decoherence in quantum information processing and even decoherence in the brain. The next chapter, written by Kiefer, is devoted to decoherence in quantum field theory and quantum gravity which is a much more speculative and less explored topic. Two complementary aspects are studied in this approach: decoherence of particle states by the quantum fields and decoherence of field states by the particles. Cosmological issues related to decoherence are discussed, not only within the standard Friedmann cosmology, but also using the elements of the theory of black holes, wormholes and strings. The relations between the formalism of consistent histories defined in terms of decoherence functionals and the environmental decoherence are discussed in chapter 5, also written by Kiefer. The Feynman--Vernon influence functional for the quantum open system is presented in detail as the first example of decoherence functional. Then the general theory is outlined together with possible interpretations including cosmological aspects. The next chapter by Giulini presents an overview of the superselection rules arising from physical symmetries and gauge transformations both for nonrelativistic quantum mechanics and quantum field theory. Critical discussion of kinematical superselection rules versus dynamical ones is illustrated by numerous examples like Galilei invariant quantum mechanics, quantum electrodynamics and quantum gravity. The introduction to the theory of quantum open systems and its applications to decoherence models is given in chapter 7 by Kupsch. Generalized master equations, Markovian approximation and a few Hamiltonian models relevant for decoherence are discussed. Some mathematical tools, e.g., complete positivity and entropy inequalities are also presented. The last chapter by Stamatescu is devoted to stochastic collapse models which can be interpreted either as certain representations of the dynamics of open quantum systems or as fundamental modifications of the Schr\"odinger equation. The final part of the book consists of remarks by Zeh on related concepts and methods and seven appendices. The broad spectrum, mathematically-friendly presentation, inclusion of the very recent developments and the extensive bibliography (about 550 references) make this book a valuable reference for all researchers, graduate and PhD students interested in the foundations of quantum mechanics, quantum open systems and quantum information. The relative independence of the chapters and numerous redundancies allow for selective reading, which is very helpful for newcomers to this field.
“For me then this is the real problem with quantum theory: the apparently essential conflict between any sharp formulation and fundamental relativity. That is to say, we have an apparent incompatibility, at the deepest level, between the two fundamental pillars of contemporary theory...” J. S. Bell (2004, p. 171) The inability to delineate a unified physical ontology that accounts simultaneously for the laws of special relativity and the results of quantum experiments has been a defining problem in physics for more than 100 years. Although various primitive ontologies (including Bohmian pilot wave, spontaneous collapse, and objective collapse theories) along with wave function realism ontologies, many-worlds interpretations, and multi-field theories address the tension between special relativity and quantum mechanics, none has successfully resolved it. This analysis addresses the problem by positing an ontic, mixed ontology composed of a “discrete” 4D spacetime and a physical, ultra-high dimensional (3 x N) “Planck Space.” Together, Planck Space and the three spatial dimensions of 4D spacetime form a tightly integrated ((3 x N) + 3) hyperspace (the “Dual Ontology”). 4D spacetime’s three spatial dimensions and the N dimensions of Planck Space are composed of two substructures: discrete, three-dimensional, quantized units of space ("Planck Spheres") and an ontic State of Absolute Nothingness (the "SOAN") whose sole physical characteristic is onticness. Critically, the Dual Ontology’s structure replaces 1) the continuous, differentiable manifold of 4D spacetime with a discrete 4D spacetime and 2) mathematical 3N configuration spaces with a physical (3 x N) Planck Space. Moreover, structurally and dynamically, the Dual Ontology is predicated on the one-to-one mapping and identity between the Planck Spheres that simultaneously form 4D spacetime and Planck Space. The one-to-one mapping and identity physically and theoretically support an integrated quantum dynamics based upon the dynamic evolution of single and N-body quantum states in 4D spacetime in full compliance with the laws of special relativity and the instantaneous collapse of all quantum states in an ontic Planck Space, where special and general relativity and more generally, 4D spacetime’s laws of physics, do not apply.
“For me then this is the real problem with quantum theory: the apparently essential conflict between any sharp formulation and fundamental relativity. That is to say, we have an apparent incompatibility, at the deepest level, between the two fundamental pillars of contemporary theory...” J. S. Bell (2004, p. 171) The inability to delineate a unified physical ontology that accounts simultaneously for the laws of special relativity and the results of quantum experiments has been a defining problem in physics for more than 100 years. Although various primitive ontologies (including Bohmian pilot wave, spontaneous collapse, and objective collapse theories) along with wave function realism ontologies, many-worlds interpretations, and multi-field theories address the tension between special relativity and quantum mechanics, none has successfully resolved it. This analysis addresses the problem by positing an ontic, mixed ontology composed of a “discrete” 4D spacetime and a physical, ultra-high dimensional (3 x N) “Planck Space.” Together, Planck Space and the three spatial dimensions of 4D spacetime form a tightly integrated ((3 x N) + 3) hyperspace (the “Dual Ontology”). 4D spacetime’s three spatial dimensions and the N dimensions of Planck Space are composed of two substructures: discrete, three-dimensional, quantized units of space ("Planck Spheres") and an ontic State of Absolute Nothingness (the "SOAN") whose sole physical characteristic is onticness. Critically, the Dual Ontology’s structure replaces 1) the continuous, differentiable manifold of 4D spacetime with a discrete 4D spacetime and 2) mathematical 3N configuration spaces with a physical (3 x N) Planck Space. Moreover, structurally and dynamically, the Dual Ontology is predicated on the one-to-one mapping and identity between the Planck Spheres that simultaneously form 4D spacetime and Planck Space. The one-to-one mapping and identity physically and theoretically support an integrated quantum dynamics based upon the dynamic evolution of single and N-body quantum states in 4D spacetime in full compliance with the laws of special relativity and the instantaneous collapse of all quantum states in an ontic Planck Space, where special and general relativity and more generally, 4D spacetime’s laws of physics, do not apply. Keywords: Quantum Cosmology; Special Relativity; Quantum Mechanics, Wave Function Collapse; Mixed Ontology; Dual Ontology; (3 x N) Planck Space; Planck Identity; Bell Identity
The dissertation investigates some relevant metaphysical issues arising in the context of spacetime theories. In particular, the inquiry focuses on general relativity and canonical quantum gravity. A formal definition of spacetime theory is proposed and, against this framework, an analysis of the notions of general covariance, symmetry and background independence is performed. It is argued that many conceptual issues in general relativity and canonical quantum gravity derive from putting excessive emphasis on general covariance as an ontological prin-ciple. An original metaphysical position grounded in scientific essential- ism and causal realism (weak essentialism) is developed and defended. It is argued that, in the context of general relativity, weak essentialism supports spacetime substantivalism. It is also shown that weak essentialism escapes arguments from metaphysical underdetermination by positing a particular kind of causation, dubbed geometric. The proposed interpretive framework is then applied to Bohmian mechanics, pointing out that weak essentialism nicely fits into this theory. In the end, a possible Bohmian implementation of loop quantum gravity is considered, and such a Bohmian approach is interpreted in a geometric causal fashion. Under this interpretation, Bohmian loop quantum gravity straightforwardly commits us to an ontology of elementary extensions of space whose evolution is described by a non-local law. The causal mechanism underlying this evolution clarifies many conceptual issues related to the emergence of classical spacetime from the quantum regime. Although there is as yet no fully worked out physical theory of quantum gravity, it is argued that the proposed approach sets up a standard that proposals for a serious ontology in this field should meet.
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.