1,721,769 research outputs found
Pathology in the WKB wave function for tunneling assisted by gravity
No full textThere are several exotic tunneling processes that can be realized only by incorporating the effect of gravity. Here, we point out that we encounter difficulties in constructing the WKB wave function, once we try to describe quantum fluctuations around the semi-classical tunneling path. We present examples of pathology in the true vacuum decay/upward tunneling, the false vacuum decay catalyzed by a black hole and the tunneling with black hole/wormhole production
On the viability of higher-order theories
No full textIn physics, all dynamical equations that describe fundamental interactions are second-order differential equations in the time derivatives. In the literature, this property is traced back to a result obtained by Ostrogradski in the mid-nineteenth century, which is the technical basis of a no-go theorem for higher-order theories. In this work, we review the connection of symmetry properties with the order of dynamical equations, before reconsidering Ostrogradski's result. Then, we show how Ostrogradski's conclusion is reached by applying to higher-order theories concepts and methods that have been specifically developed for second-order theories. We discuss a potential lack of consistency in this approach, to support the claim that Ostrogradski's result applies to a class of higher-order theories that is nowhere representative of generic ones: we support this claim by giving an example of a higher-order Lagrangian that is asymptotically stable, but that would be unstable under Ostrogradski's criterion. We also conclude that, when considering higher-order theories as fundamental, we may need to reconsider and extend the conceptual framework on which our standard treatment of second-order theories is based. This article is part of the theme issue 'Newton, Principia, Newton Geneva Edition (17th-19th) and modern Newtonian mechanics: heritage, past & present'
Electromagnetic tests of horizonless rotating black hole mimickers
No full textThe interest in the implications that astrophysical observations have for the understanding of the structure of black holes has grown since the first detection of gravitational waves. Many arguments that are put forward in order to constrain alternative black hole models rely on substantial assumptions such as perfect spherical symmetry, which implies absence of rotation. However, given that astrophysical black holes will generally exhibit nonzero angular momentum, realistic constraints must take into account the effects of rotation. In this work we analyze the gravitational effect that rotation has on the emission from the surface of ultracompact objects, by studying how angular momentum affects the propagation of light rays. This allows us to evaluate the reliability of the constraints derived for supermassive black holes (more specifically, Sagittarius A∗ and M87∗) assuming lack of rotation [as presented in A. E. Broderick, A. Loeb, and R. Narayan, Astrophys. J. 701, 1357 (2009)ASJOAB0004-637X10.1088/0004-637X/701/2/1357; A. E. Broderick et al. Astrophys. J. 805, 179 (2015)ASJOAB0004-637X10.1088/0004-637X/805/2/179]. We find that for rapidly spinning objects rotation can significantly affect the escaping probability of a photon emitted from the surface of the object, with a significant increase at the equatorial regions and a decrease at the poles with respect to the nonrotating case. For not so rapidly spinning black hole candidates like Sagittarius A∗, such modifications do not affect significantly the present constraints, which are nevertheless weaker than originally supposed due to the relativistic lensing here considered and additional phenomenological parameters that describe basic processes such as absorption. However, taking into account the angular dependence of the superficial emission of rapidly spinning black hole mimickers will be necessary for future studies of objects like e.g., M87∗
Electromagnetic tests of horizonless rotating black hole mimickers
No full textThe interest in the implications that astrophysical observations have for the understanding of the structure of black holes has grown since the first detection of gravitational waves. Many arguments that are put forward in order to constrain alternative black hole models rely on substantial assumptions such as perfect spherical symmetry, which implies absence of rotation. However, given that astrophysical black holes will generally exhibit nonzero angular momentum, realistic constraints must take into account the effects of rotation. In this work we analyze the gravitational effect that rotation has on the emission from the surface of ultracompact objects, by studying how angular momentum affects the propagation of light rays. This allows us to evaluate the reliability of the constraints derived for supermassive black holes (more specifically, Sagittarius A∗ and M87∗) assuming lack of rotation [as presented in A. E. Broderick, A. Loeb, and R. Narayan, Astrophys. J. 701, 1357 (2009)ASJOAB0004-637X10.1088/0004-637X/701/2/1357; A. E. Broderick et al. Astrophys. J. 805, 179 (2015)ASJOAB0004-637X10.1088/0004-637X/805/2/179]. We find that for rapidly spinning objects rotation can significantly affect the escaping probability of a photon emitted from the surface of the object, with a significant increase at the equatorial regions and a decrease at the poles with respect to the nonrotating case. For not so rapidly spinning black hole candidates like Sagittarius A∗, such modifications do not affect significantly the present constraints, which are nevertheless weaker than originally supposed due to the relativistic lensing here considered and additional phenomenological parameters that describe basic processes such as absorption. However, taking into account the angular dependence of the superficial emission of rapidly spinning black hole mimickers will be necessary for future studies of objects like e.g., M87∗
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
Quantum fluctuations of the gravitational field and propagation of light: A heuristic approach
No full textQuantum gravity is quite elusive at the experimental level; thus a lot of interest has been raised by recent searches for quantum gravity effects in the propagation of light from distant sources, such as gamma ray bursters and active galactic nuclei, and also in Earth-based interferometers, such as those used for gravitational wave detection. Here we propose a simple heuristic picture of the quantum fluctuations of the gravitational field and use it to set up a mathematical framework to estimate quantum gravity effects in interferometers. We also discuss some other developments suggested by these heuristics
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
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