1,720,977 research outputs found
The gravitational energy-momentum pseudo-tensor in higher-order theories of gravity
No full textThe problem of non-localizability and the non-uniqueness of gravitational energy in general relativity has been considered by many authors. Several gravitational pseudo-tensor prescriptions have been proposed by physicists, such as Einstein, Tolman, Landau, Lifshitz, Papapetrou, Moller, and Weinberg. We examine here the energy-momentum complex in higher-order theories of gravity applying the Noether theorem for the invariance of gravitational action under rigid translations. This, in general, is not a tensor quantity because it is not a covariant object but only an affine tensor, that is, a pseudo-tensor. Therefore we propose a possible prescription of gravitational energy and momentum density for square(k) gravity governed by the gravitational Lagrangian L-g = ((R) over bar + a(0)R(2) + Sigma(p)(k=1) a(k)R square R-k) root-g and generally for n-order gravity described by the gravitational Lagrangian L = L (g(u,v),g(uv,)i(1,)g(uv,)i(1,)i(2,)i(3,) ..., g(uv),i(1)i(2)i(3) ... i(n) ) . The extended pseudo-tensor reduces to the one introduced by Einstein in the limit of general relativity where corrections vanish. Then, we explicitly show a useful calculation, i.e., the power per unit solid angle Omega emitted by a massive system and carried by a gravitational wave in the direction (x) over cap for a fixed wave number k. We fix a suitable gauge, by means of the average value of the pseudo-tensor over a spacetime domain and we verify that the local pseudo-tensor conservation holds. The gravitational energy-momentum pseudo-tensor may be a useful tool to search for possible further gravitational modes beyond the two standard ones of general relativity. Their finding could be a possible observable signatures for alternative theories of gravity
Some remarks about underused Loedel diagrams
No full textWe emphasize that it can be didactically very useful for students to realize how a space-time diagram of an observer, moving with a constant velocity with respect to another observer, can be obtained easily by means of a standard matrix of rotation, without recourse to imaginary axes and angles. These diagrams were introduced for the first time by Loedel and their main advantage over Minkowski diagrams is that a scale factor is not necessary to convert the units of an observer to those of another observer. We show this well-known property of Loedel diagrams using a new geometric approach. © 2013 IOP Publishing Ltd
A little help for a better understanding and application of Faraday's law
No full textIn this letter, we examine Faraday's law of induction, analysing the electromotive force generated by a Lorentz force and the one generated by an electric field due to a changing magnetic field. We obtain the result in a didactically simple and appealing way. The final formula is derived considering explicitly the dependence of the magnetic field on the space coordinates, which is often neglected in standard textbooks
Ex situ germination and growth of Posidonia oceanica L. Delile (Monocotyledones, Potamogentonales) seeds.
No full textIs Dark Energy due to an excited quantum state of the Universe?
No full textOne of the mysteries of modern cosmology is the origin of dark energy. The current cosmological model, that is in agreement with the experimental data, predicts that the expansion of the galaxies is accelerated. In order to take this acceleration into account, an additional term is introduced in the Einstein equations invoking the existence of a cosmological constant. The origin of this additional term is still unknown. The aim of this paper is to explain dark energy as the energy of the quantum state of the universe that, instead of being in a fundamental state with zero energy, is in an excited state with a non-vanishing value of energy. The experimental value found for the cosmological constant is shown to be compatible with the model, taking, in our equations, the only free parameter of the order of Planck length
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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