1,720,961 research outputs found
Costruzione di stati di ground su spaziotempi statici dotati di bordo di tipo tempo: libertà nascoste e divergenze infrarosse.
No full textIn this work we review the construction of ground states focusing on a real scalar field whose dynamics is ruled by the Klein-Gordon equation on a large class of static spacetimes with a time-like boundary. As in the analysis of the classical equations of motion, when enough isometries are present, via a mode expansion the construction of two-point correlation functions boils down to solving a second order, ordinary differential equation on an interval of the real line. Using the language of Sturm-Liouville theory, most compelling is the scenario when one endpoint of such interval is classified as a limit circle, as it often happens when one is working on globally hyperbolic spacetimes with a timelike boundary. In this case, beyond initial data, one needs to specify a boundary condition both to have a well-defined classical dynamics and to select a corresponding ground state. Here, we take into account boundary conditions of Robin type by using well-known results from Sturm-Liouville theory, but we go beyond the existing literature by exploring an unnoticed freedom that emerges from the intrinsic arbitrariness of secondary solutions at a limit circle endpoint. Accordingly, we show that infinitely many one-parameter families of sensible dynamics are admissible. In other words, we emphasize that physical constraints guaranteeing the construction of full-fledged ground states do not, in general, fix one such state unambiguously. In addition, we provide, in full detail, an example on (1+1)-half Minkowski spacetime to spell out the rationale in a specific scenario where analytic formulae can be obtained.\\
Then, on , we construct the two-point correlation functions for the ground and thermal states admitting generalized -boundary conditions. We follow the prescription we have developed for two different choices of secondary solutions. For each of them, we obtain a family of admissible boundary conditions parametrized by . We study how they affect the response of a static Unruh-DeWitt detector. The latter not only perceives variations of , but also distinguishes between the two families of secondary solutions in a qualitatively different, and rather bizarre, fashion. Our results highlight once more the existence of a freedom in choosing boundary conditions at a timelike boundary which is greater than expected and with a notable associated physical significance.\\
To conclude, we observe that, depending on the assigned boundary condition of Robin type, this procedure does not always lead to the existence of a suitable bi-distribution due to the presence of infrared divergences. As a concrete example we consider a Bertotti-Robinson spacetime in two different coordinate patches. In one case we show that infrared divergences do not occur only for Dirichlet boundary conditions as one might expect a priori, while, in the other case, we prove that they occur only when Neumann boundary conditions are imposed at the time-like boundary
On the interplay between boundary conditions and the Lorentzian Wetterich equation
No full textIn the framework of the functional renormalization group and of the perturbative, algebraic approach to quantum field theory (pAQFT), in D'Angelo et al. [Ann. Henri Poinc. 25 (2024) 2295-2352] it has been derived a Lorentzian version of a flow equation à la Wetterich, which can be used to study nonlinear, quantum scalar field theories on a globally hyperbolic spacetime. In this work, we show that the realm of validity of this result can be extended to study interacting scalar field theories on globally hyperbolic manifolds with a timelike boundary. By considering the specific examples of half-Minkowski spacetime and of the Poincaré patch of Anti-de Sitter, we show that the form of the Lorentzian Wetterich equation is strongly dependent on the boundary conditions assigned to the underlying field theory. In addition, using a numerical approach, we are able to provide strong evidences that there is a qualitative and not only a quantitative difference in the associated flow and we highlight this feature by considering Dirichlet and Neumann boundary conditions on half-Minkowski spacetime
Hidden freedom in the mode expansion on static spacetimes
Get PDFWe review the construction of ground states focusing on a real scalar field
whose dynamics is ruled by the Klein-Gordon equation on a large class of static
spacetimes. As in the analysis of the classical equations of motion, when
enough isometries are present, via a mode expansion the construction of
two-point correlation functions boils down to solving a second order, ordinary
differential equation on an interval of the real line. Using the language of
Sturm-Liouville theory, most compelling is the scenario when one endpoint of
such interval is classified as a limit circle, as it often happens when one is
working on globally hyperbolic spacetimes with a timelike boundary. In this
case, beyond initial data, one needs to specify a boundary condition both to
have a well-defined classical dynamics and to select a corresponding ground
state. Here, we take into account boundary conditions of Robin type by using
well-known results from Sturm-Liouville theory, but we go beyond the existing
literature by exploring an unnoticed freedom that emerges from the intrinsic
arbitrariness of secondary solutions at a limit circle endpoint. Accordingly,
we show that infinitely many one-parameter families of sensible dynamics are
admissible. In other words, we emphasize that physical constraints guaranteeing
the construction of full-fledged ground states do not, in general, fix one such
state unambiguously. In addition, we provide, in full detail, an example on -half Minkowski spacetime to spell out the rationale in a specific
scenario where analytic formulae can be obtained.Comment: 24 pages, 3 fig
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Correction to: A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up (Microfluidics and Nanofluidics, (2018), 22, 8, (85), 10.1007/s10404-018-2101-1)
No full textThe article ‘A T-junction device allowing for two simultaneous orthogonal views: application to bubble formation and break-up’, written by Davide Caprini, Giorgia Sinibaldi, Luca Marino, Carlo Massimo Casciola was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 30 July 2018 without open access
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