The enterprise to construct a local causal model of quantum theory (QT), including quantum field theory (QFT), resulted in the identification of "quantum objects" as the elementary units of causality and locality.Quantum objects are collections of particles (including single particles) whose collective dynamics and measurement results can only be described by the laws of QT/ QFT.Quantum objects run autonomously with system state update frequency based on their local proper time and with no or minimal dependency on external parameters.The autonomy of quantum objects necessitates well-defined causal interrelationships between quantum objects and spacetime: 1) Quantum objects are embedded in space and move within space.2) In the proposed causal model, the dynamics of space is triggered by the dynamics of the quantum objects.The causal model of QT/QFT assumes discretized spacetime similar to the spacetime of causal dynamical triangulation.
The enterprise to construct a local causal model of quantum theory (QT), including quantum field theory (QFT), resulted in the identification of "quantum objects" as the elementary units of causality and locality.Quantum objects are collections of particles (including single particles) whose collective dynamics and measurement results can only be described by the laws of QT/ QFT.Quantum objects run autonomously with system state update frequency based on their local proper time and with no or minimal dependency on external parameters.The autonomy of quantum objects necessitates well-defined causal interrelationships between quantum objects and spacetime: 1) Quantum objects are embedded in space and move within space.2) In the proposed causal model, the dynamics of space is triggered by the dynamics of the quantum objects.The causal model of QT/QFT assumes discretized spacetime similar to the spacetime of causal dynamical triangulation.
The author's attempt to construct a local causal model of quantum theory (QT) that includes quantum field theory (QFT) resulted in the identification of "quantum objects" as the elementary units of causality and locality. Quantum objects are collections of particles (including single particles) whose collective dynamics and measurement results can only be described by the laws of QT and QFT. Local causal models of quantum objects' internal dynamics are not possible if a locality is understood as a space-point locality. Within quantum objects, state transitions may occur which instantly affect the whole quantum object. The identification of quantum objects as the elementary units of causality and locality has two primary implications for a causal model of quantum objects: (1) quantum objects run autonomously with system-state update frequencies based on their local proper times and with either no or minimal dependency on external parameters. (2) The laws of physics that describe global (but relativistic) interrelationships must be translated to a causal model of interactions between quantum objects and interactions between quantum objects and the space.
Ultimate goal of the work described in this paper has been the construction of a local causal model of quantum theory including quantum field theory.The original model has been refined in a number of refinement steps.As the so far latest refinement step, quantum loops are identified as the elementary processes of spacetime dynamics and quantum field dynamics.Quantum loops are processes that constitute quantum objects.In the causal model of quantum loops and quantum objects presented here, the collective behavioral processes involve the elementary units of spacetime and the associated elementary units of quantum of fields.As such, quantum loop processes are the sources of gravitational fields (i.e., spacetime curvature) and of the quantum objects (wave function).The model may be viewed as a derivative of loop quantum gravity, spin networks and causal dynamical triangulation, although significant deviations to these theories exist.The causal model of quantum loops is based on a causal model of spacetime dynamics where space (-time) consists of interconnected space points, each of which is connected to a small number of neighboring space points.The curvature of spacetime is expressed by the density of these space points and by the arrangement of the connections between them.The quantum loop emerges in a collective behavioral process from a collection of space points that carry energy and quantum field attributes.The paper is considered by the author to represent a fairly complete causal model, except for some processes whose detailed behaviour needs to be determined by use of further computer simulations.
This article presents an analysis of the extent to which local causal models or local realistic models of quantum theory (QT), including quantum field theory (QFT), are theoretically possible and practically feasible in light of the present state of these theories. Quantum physicists consider Bells famous inequality and its violation in experiments to be a strong indication that local realistic or local causal models of QT are not possible and that quantum theory as a whole is therefore not a local realistic or local causal theory. Based on a proposed definition of a "formal causal model" for a theory of physics (such as QT), this paper investigates the possibility of having a local causal model for QT. Areas of QT are identified in which the construction of a causal model is impeded because of deficiencies in the state of the respective theory. It is shown that the removal of the deficiencies can be achieved by the provision of a causal model. Whereas the construction of a causal model of QT, including QFT, appears to be feasible after the removal of certain deficiencies, the construction of a local (causal) model does not appear to be possible. As a consequence of the conclusion that local (causal) models of QT/QFT are not possible, if a strong interpretation of locality is assumed, a locality model is proposed in which the non-localities are confined to "quantum objects".
In quantum time (QT) schemes, time is promoted to a degree of freedom, allowing Lorentz covariance to be made explicit for single particles. We ask whether this can be lifted to QFT, so that Lorentz covariance becomes manifest at the Hilbert-space level, rather than being hidden as in the standard canonical formulation. We address this question by proposing a second-quantized approach in which the elementary particle is the QT particle itself, leading naturally to the notion of spacetime field algebras and of quantum action. We show, however, that a naive many-body construction runs into inconsistencies. To pinpoint their origin we introduce a classical counterpart of the second-quantized formalism, spacetime classical mechanics (SCM), and prove a no-go theorem: Dirac quantization of SCM collapses back to standard QFT and therefore hides covariance. We circumvent this problem by presenting a quantum-action-based quantization that yields a spacetime version of quantum mechanics (SQM), making covariance manifest for (interacting) QFTs. Finally, we show that this resolution is tied to a genuine spacetime generalization of the notion of quantum state, required by causality and closely connected to recent ``states over time'' proposals and, in dS/CFT-motivated settings, to microscopic notions of timelike entanglement and emergent time.
The non-localities in quantum theory (QT) (the most famous example is expressed in the violation of Bell's inequality in experiments) impede the construction of a local causal model of QT including quantum field theory (QFT).The laws of collective behavior may be considered to be types of non-local laws: laws that apply to the collection of system components as a whole.The article presents a proposal for the treatment of the non-localities that exist in QT/QFT by the concepts of collective behavior.The basic components of the collective behavior are the spatial elements of the causal model of QT/QFT proposed by the author.
In general relativity, the description of spacetime relies on idealised rods and clocks, which identify a reference frame. In any concrete scenario, reference frames are associated to physical systems, which are ultimately quantum in nature. A relativistic description of the laws of physics hence needs to take into account such quantum reference frames (QRFs), through which spacetime can be given an operational meaning. Here, we introduce the notion of a spacetime quantum reference frame, associated to a quantum particle in spacetime. Such formulation has the advantage of treating space and time on equal footing, and of allowing us to describe the dynamical evolution of a set of quantum systems from the perspective of another quantum system, where the parameter in which the rest of the physical systems evolves coincides with the proper time of the particle taken as the QRF. Crucially, the proper times in two different QRFs are not related by a standard transformation, but they might be in a quantum superposition one with respect to the other.Concretely, we consider a system of N relativistic quantum particles in a weak gravitational field, and introduce a timeless formulation in which the global state of the N particles appears "frozen", but the dynamical evolution is recovered in terms of relational quantities. The position and momentum Hilbert space of the particles is used to fix the QRF via a transformation to the local frame of the particle such that the metric is locally inertial at the origin of the QRF. The internal Hilbert space corresponds to the clock space, which keeps the proper time in the local frame of the particle. Thanks to this fully relational construction we show how the remaining particles evolve dynamically in the relational variables from the perspective of the QRF. The construction proposed here includes the Page-Wootters mechanism for non interacting clocks when the external degrees of freedom are neglected. Finally, we find that a quantum superposition of gravitational redshifts and a quantum superposition of special-relativistic time dilations can be observed in the QRF.