5,241 research outputs found
Einstein-Cartan theory as a theory of defects in space-time
The Einstein-Cartan theory of gravitation and the classical theory of defects
in an elastic medium are presented and compared. The former is an extension of
general relativity and refers to four-dimensional space-time, while we
introduce the latter as a description of the equilibrium state of a
three-dimensional continuum. Despite these important differences, an analogy is
built on their common geometrical foundations, and it is shown that a
space-time with curvature and torsion can be considered as a state of a
four-dimensional continuum containing defects. This formal analogy is useful
for illustrating the geometrical concept of torsion by applying it to concrete
physical problems. Moreover, the presentation of these theories using a common
geometrical basis allows a deeper understanding of their foundations.Comment: 18 pages, 7 EPS figures, RevTeX4, to appear in the American Journal
of Physics, revised version with typos correcte
Light-cone fluctuations and the renormalized stress tensor of a massless scalar field
We investigate the effects of light-cone fluctuations over the renormalized
vacuum expectation value of the stress-energy tensor of a real massless
minimally coupled scalar field defined in a ()-dimensional flat space-time
with topology . For modeling the influence of
light-cone fluctuations over the quantum field, we consider a random
Klein-Gordon equation. We study the case of centered Gaussian processes. After
taking into account all the realizations of the random processes, we present
the correction caused by random fluctuations. The averaged renormalized vacuum
expectation value of the stress-energy associated with the scalar field is
presented
Gravitomagnetic Resonance Shift due to a Slowly Rotating Compact Star
The effect of a slowly rotating mass on a forced harmonic oscillator with two
degrees of freedom is studied in the weak field approximation. It is found that
according to the general theory of relativity there is a shift in the resonat
frequency of the oscillator which depends on the density and rotational
frequency of the gravitational source. The proposed shift is quite small under
normal physical situations however it is estimated that for compact x-ray
sources such as white dwarfs, pulsars, and neutron stars the shift is quite
appreciable.Comment: 8 pages, 2 figures, Accepted for Publication in Inter. Journal of
Modern Physics
On the Efficiency of Data Representation on the Modeling and Characterization of Complex Networks
Specific choices about how to represent complex networks can have a
substantial effect on the execution time required for the respective
construction and analysis of those structures. In this work we report a
comparison of the effects of representing complex networks statically as
matrices or dynamically as spase structures. Three theoretical models of
complex networks are considered: two types of Erdos-Renyi as well as the
Barabasi-Albert model. We investigated the effect of the different
representations with respect to the construction and measurement of several
topological properties (i.e. degree, clustering coefficient, shortest path
length, and betweenness centrality). We found that different forms of
representation generally have a substantial effect on the execution time, with
the sparse representation frequently resulting in remarkably superior
performance
Media devices in pre-school children: the recommendations of the Italian pediatric society
BACKGROUND: Young children are too often exposed to mobile devices (MD) and most of them had their own device. The adverse effects of a early and prolonged exposure to digital technology on pre-school children has been described by several studies. Aim of the study is to analyze the consequences of MD exposure in pre-school children. METHODS: We analyzed the documented effects of media exposure on children's mental and physical health. RESULTS: According to recent studies, MD may interfere with learning, children development, well being, sleep, sight, listening, caregiver-child relationship. DISCUSSION: Pediatricians should be aware of both the beneficial and side effects of MD and give advice to the families, according to children's age. CONCLUSION: In according to literature, the Italian Pediatric Society suggest that the media device exposure in childhood should be modulated by supervisors
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