1,720,959 research outputs found
The emergence of time with interactions in quantum and classical mechanics
No full textPhysicists are nowadays able to produce and measure light pulses with durations on the inconceivably small scale of attoseconds. While this allows them to probe and explore the dynamics of the most fundamental processes of the physical world on ever shorter time scales, the explicit nature of time itself still remains a mystery even today. This unresolved issue divides the scientific community into two distinct camps. Those in the first camp view time as a fundamental entity of nature, while those in the second camp view it as an effective or emergent quantity derived from more elementary principles. Proponents of the latter view advocate for a static global state as the fundamental entity, in which the whole encompasses a principal “system” and a “clock” as separate subsystems. In this context, the central notion is that of a state of the principal system being conditioned on the physical state of the clock. As a result, time or, more concretely, dynamics emerges from the inherent correlations between both subsystems contained in the global state. Two prominent quantum mechanical approaches for this “timeless” theory have been developed during the last decades. The first relies on a semiclassical treatment in the form of Born-Oppenheimer and Wentzel-Kramers-Brillouin approximations, but allows one to include interactions between system and clock. A second treatment, devised by Page and Wootters, works in abstract Hilbert space instead. Unfortunately, it is as of yet unable to generally deal with interactions and often relies on Hamiltonians of a special form.
The main objective of this thesis is to close the gap between both approaches through construction of an overarching framework in full generality, without relying on specific choices for the underlying subsystems or the Hamiltonians involved. To this end, we single out three central principles on which the previous approaches build, namely a global energy constraint, the existence of subsystems and a definition of the conditional state of the principal system. Based on them, we derive unitary system dynamics and the time-dependent Schrödinger equation with an exact memoryless effective potential from a timeless quantum mechanical formulation, namely the time-independent Schrödinger equation. We find that the traditionally presupposed time is replaced by a scalar path variable parametrizing a global state invariance generated by the energy constraint. Yet, this derivation deals exclusively with pure states and, to widen its scope, we extend the timeless approach to quantum mechanical mixed states and classical probability densities as well. A key concept is the minimization of a unitarity-violating term in the evolution equations, appearing in the presence of an interaction term between system and clock. Useful approximations and analytical and numerical examples complement our results and corroborate our derivations. As a further demonstration of the versatility of our approach, we explain how a long-suspected connection between imaginary-time formulations and inverse temperature in canonical ensembles could be traced to a common physical origin. Finally, we discuss the consequences of our newly developed framework, especially similarities to conventional open systems theory, and conclude with an outlook for future investigations
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
The emergence of time with interactions in quantum and classical mechanics
No full textPhysicists are nowadays able to produce and measure light pulses with durations on the inconceivably small scale of attoseconds. While this allows them to probe and explore the dynamics of the most fundamental processes of the physical world on ever shorter time scales, the explicit nature of time itself still remains a mystery even today. This unresolved issue divides the scientific community into two distinct camps. Those in the first camp view time as a fundamental entity of nature, while those in the second camp view it as an effective or emergent quantity derived from more elementary principles. Proponents of the latter view advocate for a static global state as the fundamental entity, in which the whole encompasses a principal “system” and a “clock” as separate subsystems. In this context, the central notion is that of a state of the principal system being conditioned on the physical state of the clock. As a result, time or, more concretely, dynamics emerges from the inherent correlations between both subsystems contained in the global state. Two prominent quantum mechanical approaches for this “timeless” theory have been developed during the last decades. The first relies on a semiclassical treatment in the form of Born-Oppenheimer and Wentzel-Kramers-Brillouin approximations, but allows one to include interactions between system and clock. A second treatment, devised by Page and Wootters, works in abstract Hilbert space instead. Unfortunately, it is as of yet unable to generally deal with interactions and often relies on Hamiltonians of a special form.
The main objective of this thesis is to close the gap between both approaches through construction of an overarching framework in full generality, without relying on specific choices for the underlying subsystems or the Hamiltonians involved. To this end, we single out three central principles on which the previous approaches build, namely a global energy constraint, the existence of subsystems and a definition of the conditional state of the principal system. Based on them, we derive unitary system dynamics and the time-dependent Schrödinger equation with an exact memoryless effective potential from a timeless quantum mechanical formulation, namely the time-independent Schrödinger equation. We find that the traditionally presupposed time is replaced by a scalar path variable parametrizing a global state invariance generated by the energy constraint. Yet, this derivation deals exclusively with pure states and, to widen its scope, we extend the timeless approach to quantum mechanical mixed states and classical probability densities as well. A key concept is the minimization of a unitarity-violating term in the evolution equations, appearing in the presence of an interaction term between system and clock. Useful approximations and analytical and numerical examples complement our results and corroborate our derivations. As a further demonstration of the versatility of our approach, we explain how a long-suspected connection between imaginary-time formulations and inverse temperature in canonical ensembles could be traced to a common physical origin. Finally, we discuss the consequences of our newly developed framework, especially similarities to conventional open systems theory, and conclude with an outlook for future investigations
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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