1,720,961 research outputs found
A time machine for free fall into the past
No full textInspired by some recent works of Tippett-Tsang and Mallary-Khanna-Price, we present a new spacetime model containing closed timelike curves (CTCs). This model is obtained postulating an ad hoc Lorentzian metric on R^4, which differs from the Minkowski metric only inside a spacetime region bounded by two concentric tori. The resulting spacetime is topologically trivial, free of curvature singularities and is both time and space orientable; besides, the inner region enclosed by the smaller torus is flat and displays geodesic CTCs. Our model shares some similarities with the time machine of Ori and Soen but it has the advantage of a higher symmetry in the metric, allowing for the explicit computation of a class of geodesics. The most remarkable feature emerging from this computation is the presence of future-oriented timelike geodesics starting from a point in the outer Minkowskian region, moving to the inner spacetime region with CTCs, and then returning to the initial spatial position at an earlier time; this means that time travel to the past can be performed by free fall across our time machine. The amount of time travelled into the past is determined quantitatively; this amount can be made arbitrarily large keeping non-large the proper duration of the travel. An important drawback of the model is the violation of the classical energy conditions, a common feature of many time machines. Other problems emerge from our computations of the required (negative) energy densities and of the tidal accelerations; these are small only if the time machine is gigantic
On the Casimir Effect with delta-Like Potentials, and a Recent Paper by K. Ziemian (Ann. Henri Poincare, 2021)
No full textThe local and global aspects of the Casimir effect for a scalar field in the presence of a point-like impurity were treated in our papers (Fermi and Pizzocchero in Symmetry 10(2):38, 2018; Fermi in Mod. Phys. Lett. A 35(03):2040008, 2020), using the zeta regularization method. A paper by Ziemian, recently published in (Ann. Henri Poincare 22:1751-1781, 2021), discusses the Casimir effect for a scalar field in presence of one or two, extended or point-like impurities, using the Herdegen-Stopa approach. Ziemian claims that his result for the energy density with a single point-like impurity differs from that derived in Fermi and Pizzocchero (2018), ascribing the mismatch to a basic conceptual discrepancy. In the present work, we show that the formula in Ziemian (2021) for the energy density in the presence of a point-like impurity coincides (upon amending minor computational errors) with the formula of Fermi and Pizzocchero (2018) for the same quantity. In order to make our discussion self-contained, in the present paper we also survey some basic facts related to Fermi and Pizzocchero (2018), Fermi (2020), Ziemian (2021). This survey includes Schrodinger's operators with point-like interactions, as described in the celebrated book (Solvable models in quantum mechanics, Springer, New York, 1988) by Albeverio et al., the zeta regularization method for a canonically quantized scalar field and the implementation of point-like interactions in this setting
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
Local Casimir Effect for a Scalar Field in Presence of a Point Impurity
Full text linkThe Casimir effect for a scalar field in presence of delta-type potentials has been
investigated for a long time in the case of surface delta functions, modelling semi-transparent boundaries. More recently Albeverio, Cacciapuoti, Cognola, Spreafico and Zerbini have considered some configurations involving delta-type potentials concentrated at points of R^3; in particular, the case with an isolated point singularity at the origin can be formulated as a field theory on R^3 \ {0 } with self-adjoint boundary conditions at the origin for the Laplacian. However, the above authors have discussed only global aspects of the Casimir effect, focusing their attention on the vacuum expectation value (VEV) of the total energy. In the present paper we analyze the local Casimir effect with a point delta-type potential, computing the renormalized VEV of the stress-energy tensor at any point of R^3 \ {0 }; for this purpose we follow the zeta regularization approach, in the formulation already employed for different configurations in previous works of ours
Local Zeta Regularization and the Scalar Casimir Effect : A General Approach based on Integral Kernels
No full textLocal zeta regularization and the scalar Casimir effect IV : the case of a rectangular box
No full textApplying the general framework for local zeta regularization proposed in Part I of this series of papers, we compute the renormalized vacuum expectation value of several observables (in particular, of the stress-energy tensor) for a massless scalar field confined within a rectangular box of arbitrary dimension
Local zeta regularization and the Casimir effect
Full text linkThe local zeta regularization allows to treat local divergences appearing in quantum
field theory; these are renormalized by pure analytic continuation (in the parameter of the regulator), with no need to remove or subtract divergent terms. This approach can be applied to the stress-energy tensor of the Casimir effect, and works as well on curved space-times.
It is not useless to illustrate the power and elegance of this method in a simple case. In the present paper, our attention is devoted to the case of a neutral, massless scalar field in flat space-time, on a space domain with suitable (e.g., Dirichlet) boundary conditions. After a general outline of the local zeta method for the Casimir effect, we exemplify it in the typical
case of a (Dirichlet) field between two parallel plates, or outside them. The results agree with the ones obtained by more popular methods, such as point splitting regularization. Connections with the existing literature on this subject are indicated
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
Local zeta regularization and the scalar Casimir effect III : the case with a background harmonic potential
No full textApplying the general framework for local zeta regularization proposed in Part I of this series of papers, we renormalize the vacuum expectation value of the stress-energy tensor (and of the total energy) for a scalar field in presence of an external harmonic potential
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