Abstract We aim to understand the extent to which something resembling the EPS approach can be applied when the inputs are quantum mechanical rather than classical. That is, we consider versions of EPS with all classical light ray signals replaced by quantum light signals, and all particle signals replaced by quantum particles. In making these substitutions and applying the EPS approach, one ultimately derives a superposition of metric structures as the relevant kinematical structure for quantum spacetime; moreover, as we will see, there is a way of interpreting these outputs in terms of branching spacetime structures.
Abstract We aim to understand the extent to which something resembling the EPS approach can be applied when the inputs are quantum mechanical rather than classical. That is, we consider versions of EPS with all classical light ray signals replaced by quantum light signals, and all particle signals replaced by quantum particles. In making these substitutions and applying the EPS approach, one ultimately derives a superposition of metric structures as the relevant kinematical structure for quantum spacetime; moreover, as we will see, there is a way of interpreting these outputs in terms of branching spacetime structures.
In this paper, I explore the feasibility of a realistic interpretation of the quantum mechanical path integral - that is, an interpretation according to which the particle actually follows the paths that contribute to the integral. I argue that an interpretation of this sort requires spacetime to have a branching structure similar to the structures of the branching spacetimes proposed by previous authors. I point out one possible way to construct branching spacetimes of the required sort, and I ask whether the resulting interpretation of quantum mechanics is empirically testable.
It is shown that, with some reasonable assumptions, the theory of general relativity can be made compatible with quantum mechanics by using the field equations of general relativity to construct a Robertson-Walker metric for a quantum particle so that the line element of the particle can be transformed entirely to that of the Minkowski spacetime, which is assumed by a quantum observer, and the spacetime dynamics of the particle described by a Minkowski observer takes the form of quantum mechanics. Spacetime structure of a quantum particle may have either positive or negative curvature. However, in order to be describable using the familiar framework of quantum mechanics, the spacetime structure of a quantum particle must be "quantised" by an introduction of the imaginary number i. If a particle has a positive curvature then the quantisation is equivalent to turning the pseudo-Riemannian spacetime of the particle into a Riemannian spacetime. This means that it is assumed the particle is capable of measuring its temporal distance like its spatial distances. On the other hand, when a particle has a negative curvature and a negative energy density then quantising the spacetime structure of the particle is equivalent to viewing the particle as if it had a positive curvature and a positive energy density.
All known physical systems are described by quantum fields, whose observable properties arise from measurable correlations among their states. In this interpretation, what we call information is not an abstract entity but the physical pattern formed by these correlations. Energy, momentum, and spacetime curvature are already known to be expressible through expectation values of quantum operators, and recent results in quantum information theory show that variations in entanglement correspond directly to variations in energy. From this perspective, macroscopic structures—including matter and spacetime geometry—can be understood as stable, evolving patterns within quantum fields. Apparent non-local correlations, such as quantum entanglement, do not transmit signals faster than light but reflect pre-existing informational structure consistent with relativistic causality. Gravitational effects traditionally attributed to unseen components may, in principle, also be described as consequences of information distribution within known fields, without invoking new particles or interactions. This interpretation does not modify established physical laws or introduce new constants. It remains fully compatible with quantum field theory and general relativity, and is intended as a conceptual framework for organizing existing results rather than a new predictive theory. Its empirical relevance lies in re-expressing known phenomena in terms of information patterns that are, in principle, measurable through correlations and entanglement.
Up to this point, our study of quantum mechanics has concerned itself with the behaviour of particles that inhabit a Galilean spacetime. For many purposes, in atomic, molecular and condensed matter physics, this theory is quite adequate. We saw in earlier chapters, however, that our actual spacetime has a structure which is much closer to that of the Minkowski spacetime of special relativity and that more general structures must be considered when gravitational phenomena are signicant. From a purely theoretical point of view, it is therefore important to formulate quantum theory in a way which is consistent with these more general spacetimes. The benets of constructing a relativistic quantum theory actually go far beyond the aesthetic satisfaction of making our geometrical and quantum-mechanical reasoning compatible. For one thing, we shall discover that the relativistic theory provides a deeper understanding of spin and the distinction between fermions and bosons, which in the non-relativistic theory appear simply as facts of life that we must strive to accommodate. Also, of course, there are many situations in which relativistic eects become observable, for which non-relativistic theory provides no explanation. The most obvious are high-energy scattering experiments, in which particles acquire kinetic energies comparable with or greater than their rest energies mc2, and the correct 4-momentum (3.33) must be used. There are, however, more subtle eects, such as the spin-orbit coupling that is essential for interpreting atomic spectra, which are also of relativistic origin.
We provide a possible fully geometric formulation of the core idea of quantum reference frames (QRFs) as it has been applied in the context of gravity, freeing its definition from unnecessary (though convenient) ingredients, such as coordinate systems. Our formulation is based on two main ideas. First, a QRF encodes uncertainty about what is the observer's (and, hence, the measuring apparatus's) perception of time and space at each spacetime point (i.e., event). For this, an observer at an event p is modeled, as usual, as a tetrad in the tangent space Tp. So a QRF at an event p is a complex function on the tetrads at p. Second, we use the result that one can specify a metric on a given manifold by stipulating that a basis one assigns at each tangent space is to be a tetrad in the metric one wants to specify. Hence a spacetime, i.e. manifold plus metric, together with a choice of "point of view" on it, is represented by a section of the bundle of bases, understood as taking the basis assigned to each point to be a tetrad. Thus a superposition of spacetimes gets represented as, roughly speaking, an assignment of complex amplitudes to sections of this bundle. A QRF, defined here as the collection of complex amplitudes assigned to bases at events--i.e., a complex function defined on the bundle of bases of the manifold--can describe, in a local way (i.e., attributing the amplitudes to bases at events instead of to whole sections), these superpositions. We believe that this formulation sheds some light on some conceptual aspects and possible extensions of current ideas about QRFs. For instance, thinking in geometric terms makes it clear that the idea of QRFs applied to the gravitational scenarios treated in the literature (beyond linear approximation) lacks predictive power due to arbitrariness which, we argue, can only be resolved by some further input from physics.
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.