[en] The technique of using spinors in the proof of positive energy theorems in 4 dimensions is extended to the case of Kaluza-Klein theories in spaces of 4 + n dimensions. First a useful presentation of generalized Kaluza-Klein theories is introduced, in which just from the observation of conformal symmetries it is possible to detect a nice splitting of the Ricci tensor into a 4-dimensional Ricci part and a Yang-Mills part, among others. Consideration of linear dependence among the symmetries is not excluded in this treatment. Relevant to the introduction of spinors, a discussion of Clifford Algebras is presented. In particular a real representation of these algebras is introduced for spaces of higher dimensions and its structure is analyzed. The Lie derivative of spinors is presented probably more clearly than in former treatments. After the introduction of these preliminary themes, a brief review of the relevant aspects of positive energy theorems in 4 dimensions is presented, followed by the extension of these ideas to the case of 5 dimensions. Here an earlier result involving gravitational mass and electromagnetic charges is improved. Finally the results are generalized to spaces of 4 + n dimensions, and a more complicated condition to be satisfied by the usual matter tensor is discovered. This procedure leads to a natural definition of invariant Yang-Mills charges, which is compared with former studies

[en] The future null infinity for all non-contracting Robertson-Walker space time is studied systematically. A theorem is proved which establishes the expected relation between the nature of J⁺ and the appearance or absence of cosmic event horizons. (author). 7 refs, 1 tab

[en] This paper investigates the possibility of eliminating the supertranslation gauge freedom at scri by the introduction of a unique set of sections, which may be understood as giving a centre of mass system of reference. Our construction depends on the properties of the physical system at the fixed retarded time defined by these sections. (author)

[en] We give arguments to emphasise that methods for approximation around a priori stationary backgrounds representing radiation systems can only give a consistent description of the system at a fixed time. (author)

[en] After a review of fundamental concepts around the notion of isolated systems in curved spacetimes, we analyse the problem of the ambiguities of the super translations at future null infinity. We propose a tool that provides a clean way to fix the gauge problem; namely the notion of nice sections. We conclude with some resent results on nice sections on a particular class of radiant spacetimes. (author). 19 refs

[en] A new definition of angular momentum at future null infinity is proposed which is free of supertranslation ambiguities. A prescription is given to single out a unique Bondi system. (author)

[en] The properties of a simples quantum mechanical model for the decay of two equal mass particles are studied and related to some recent work on complex energy eigenvalues. It consists essentially in a generalization of the Lee-Friedrichs model for an unstable particle and gives a highly idealized version of the K⁰-anti K⁰ system, including CP violations. The model is completely solvable, thus allowing a comparison with the well known Weisskopf-Wigner formalism for the decay amplitudes. A different model, describing the same system is also briefly outlined. (Author)

[en] The gravitational waves, that are expected to be observed with the new generation of detectors, are supposed to be generated by systems involving very compact objects during processes like the coalescence of binary systems. First order estimation of the gravitational radiation emitted by those systems can be carried out from the quadrupole formula. A more precise description of the gravitational radiation requires improved formulae which would probably take into account further multipole structure of the sources. This conduces immediately to the issue of which is the appropriate frame of reference that should be used to define the multipole structure of the system. Even the notion of angular momentum is dependent on the frame that one is using to make the calculation. For example, if one uses an inappropriate frame, which is moving and is not round, to describe the asymptotics of Schwarzschild spacetime, one could find a nonzero angular momentum and a complicated multipole structure. Instead, when one wishes to describe the gravitational radiation, one is interested in determining the intrinsic structure of the sources, and therefore intends to avoid information associated to an inappropriate choice of frame. In a few words, one is interested in using a frame that captures the idea of being at rest with the sources, which is around the sources and that is centered in relation to the central distribution. All these properties are required to be satisfied at all times; which is not a trivial requirement for systems in which the back reaction due to gravitational radiation are important. The appropriate reference frame at future null infinity is constructed using the so-called nice sections that we introduced some time ago. In this occasion we would like to report on the latest results of our program; which allow us to provide a notion of center of mass frame, which is used to define intrinsic angular momentum, and can also be used to describe the multipolar structure of the sources of gravitational waves. (author)

[en] The positive-energy theorem is formulated in the context of Kaluza--Klein theories. Different cases are considered, including the situation in which no symmetry is assumed. This work offers a new technique for stability considerations in Kaluza--Klein theories

[en] A new theorem relating mass and charges is deduced, which can be applied to more general physical systems than those covered by the theorem of Gibbons and Hull. (orig.)