Lorentz violation is a candidate quantum-gravity signal, and the Standard-Model Extension (SME) is a widely used parametrization of such violation. In the gravitational SME sector, there is an elusive coefficient for which no effects have been found. This is is known as the t puzzle and, to date, it has no compelling explanation. This paper analyzes if there is a fundamental explanation for the t puzzle. To tackle this question, several approaches are followed. Mainly, redefinitions of the dynamical fields are studied, showing that other SME coefficients can be moved to nongravitational sectors. It is also found that the gravity SME sector can be consistently treatedà la Palatini, and that, in the presence of spacetime boundaries, it is possible to correct its action to get the desired equations of motion. Moreover, through a reformulation as a Lanczos-type tensor, some problematic features of the t term, that should arise at the phenomenological level, are revealed. The most important conclusion of the paper is that there is no evidence of a fundamental explanation for the t puzzle, suggesting that it may be linked to the approximations taken at the phenomenological level.

Lorentz invariance is one of the fundamental principles of physics, and, as such, it must be experimentally tested. The purpose of this work is to obtain, within the Standard-Model Extension, the dynamics of a Lorentzviolating spinor in a uniform Newtonian gravitational field. This is achieved by treating the spinor as a test particle and introducing the gravitational field through a uniformly accelerated observer. The nonrelativistic Hamiltonian is obtained, and some experimental consequences are discussed. One unexpected outcome of this work is that the gravitational field helps disentangle bounds on coefficients for Lorentz violation.

We describe a scheme for the exploration of quantum gravity phenomenology focussing on effects that could be thought as arising from a fundamental granularity of space-time. In contrast with the simplest assumptions, such granularity is assumed to respect Lorentz Invariance but is otherwise left unspecified. The proposal is fully observer covariant, it involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. We present the effective Hamiltonian which could be used to analyze concrete experimental situations, some of which are briefly described, and we shortly discuss the degree to which the present proposal is in line with the fundamental ideas behind the equivalence principle.

The interface between gravitation and quantum theory is a fascinating subject. However, it is also riddled with subtleties and slight confusion can easily lead to questionable conclusions. A dramatic example in this regard is provided by [1] where it is claimed that gravitational effects generically produce a novel form of decoherence for systems with internal degrees of freedom, which would account for the emergence of classicality. The effect is supposed to arise from the different gravitational redshifts suffered by such systems when placed in superpositions of positions along the direction of the gravitational field. There are, however, serious issues with the arguments of the paper.To begin with, the results of [1] cannot be right in light of the equivalence principle, which is valid, by construction, in the frameworks employed. This is because the only external force acting on all studied systems is that of a gravitational field with no relevant space-time curvature. As a result, the situations of interest can be analyzed in a free falling frame, in which the systems under study are gravity-free and isolated. Clearly, such scenarios cannot lead to decoherence, as, without gravity, there is nothing to cause it. One might claim that all this shows is that decoherence is frame-dependent. Note however that in order to claim that decoherence has occurred, it is not enough to show that, in the description of the situation according to certain observers, the reduced density matrix for the center of mass is almost diagonal. What one would need to show is that interference is not present in a concrete experiment, a fact that is clearly frame-independent, and not at all shown in [1]. Moreover, since the systems described in [1] are subject to gravity, they will not remain static when placed in a superposition of fixed positions. Of course, one could achieve this by including a compensating force generated by an external device, and this additional interaction may lead to decoherence, but this effect cannot be ascribed to gravity.Next, notice that the results presented in [1] crucially depend on considering systems where the various internal energy levels contribute to their effective mass, which thus can have more than one value. However, as shown in [2], the nonrelativistic treatment of such situations can lead to spurious relative phases, calling for extreme care in dealing with nonrelativistic approximations. Such a matter is completely ignored by the free use of nonrelativistic quantum mechanics in [1].Finally, the widespread believe that decoherence can explain the quantum-to-classical transition, which is key in the analysis of the paper, is unjustified [3]. The confusion arises from the fact that the density matrix of an improper mixture (which represents the partial description of a subsystem that is part of a larger system in a pure state) has, after decoherence takes place, the same form as that of a proper mixture (which represents an actual ensemble of systems) [4]. However, from such formal similarity i...

We describe a refined version of a previous proposal for the exploration of quantum gravity phenomenology. Unlike the original scheme, the one presented here is free from sign ambiguities while it shares with the previous one the essential features. It focuses on effects that could be thought as arising from a fundamental granularity of quantum space-time. The sort of schemes we consider are in sharp contrast with the simplest scenarios in that such granularity is assumed to respect Lorentz Invariance but it remains otherwise unspecified. The proposal is fully observer covariant, it involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. We present the effective Hamiltonian which could be used to analyze concrete experimental situations, and we shortly review the degree to which this proposal is in line with the fundamental ideas behind the equivalence principle.