Quantum-relativity: How frames of reference are the key to gravity

The most challenging problem of modern physics is how to reconcile quantum theory and general relativity, namely, to find a consistent quantum theory in which gravity is quantized. This Progress Report focuses on such a tentative theory called the information-complete quantum theory (ICQT), in which (1) spacetime (gravity) as a physical quantum system plays a central role for formulating the theory, and (2) there are no any classical systems and concepts. Here universal space...

The Einstein Equivalence Principle (EEP), stating that all laws of physics take their special-relativistic form in any local inertial (classical) reference frame, lies at the core of general relativity. Because of its fundamental status, this principle could be a very powerful guide in formulating physical laws at regimes where both gravitational and quantum effects are relevant. The formulation of the EEP only holds when both matter systems and gravity are classical, and we ...

Observing spatial entanglement in the Bose-Marletto-Vedral (BMV) experiment would demonstrate the existence of non-classical properties of the gravitational field. We show that the special relativistic invariance of the linear regime of general relativity implies that all the components of the gravitational potential must be non-classical. This is simply necessary in order to describe the BMV entanglement consistently across different inertial frames of reference. On the othe...

The issue of whether some manifestations of gravitation in the quantum domain, are indicative or not of a non-geometrical aspect in gravitation is discussed. We examine some examples that have been considered in this context, providing a critical analysis of previous interpretations. The analysis of these examples is illustrative about certain details in the interpretation of quantum mechanics. We conclude that there are, at this time, no indications of such departure from th...

Recently, there has been increased interest in understanding entanglement and quantum communication in black hole spacetimes and in using quantum information techniques to address questions in gravity. Studies on relativistic entanglement show the emergence of conceptually important qualitative differences to a non-relativistic treatment. For instance, entanglement was found to be an observer-dependent property that changes from the perspective of accelerated observers moving...

Although the standard viewpoint in theoretical physics is that the unification of quantum theory and general relativity requires the quantization of gravity and spacetime, there is not consensus about whether spacetime must fundamentally have any quantum features. Here we show a theorem stating that spacetime degrees of freedom and a quantum system violate a Bell inequality in a background Minkowski spacetime if a few properties of general relativity and quantum theory have a...

We discuss the physical interpretation of the gravity mediated entanglement effect. We show how to read it in terms of quantum reference systems. We pinpoint the single gravitational degree of freedom mediating the entanglement. We clarify why the distinction between longitudinal and transverse degrees of freedom is irrelevant for the interpretation of the results. We discuss the relation between the LOCC theorem and the interpretation of the effect, its different relevance f...

A quantum frame is defined by a material object subject to the laws of quantum mechanics. The present paper studies the relations between quantum frames, which in the classical case are described by elements of the Poincare' group. The possibility of using a suitable quantum group is examined, but some arguments are given which show that a different mathematical structure is necessary. Some simple examples in lower dimensional spacetimes are treated. They indicate the necessi...

When two particles interact primarily through gravity and follow the laws of quantum mechanics, the generation of entanglement is considered a hallmark of the quantum nature of the gravitational interaction. However, we demonstrate that entanglement dynamics can also occur in the presence of a weak quantum interaction and non-linear corrections to local quantum mechanics, even if the gravitational interaction is classical or absent at short distances. This highlights the impo...

This paper elaborates on an intrinsically quantum approach to gravity, which begins with a general framework for quantum mechanics and then seeks to identify additional mathematical structure on Hilbert space that is responsible for gravity and other phenomena. A key principle in this approach is that of correspondence: this structure should reproduce spacetime, general relativity, and quantum field theory in a limit of weak gravitational fields. A central question is that of...