5,621 research outputs found
The Chrono-geometrical Structure of Special and General Relativity: a Re-Visitation of Canonical Geometrodynamics
A modern re-visitation of the consequences of the lack of an intrinsic notion
of instantaneous 3-space in relativistic theories leads to a reformulation of
their kinematical basis emphasizing the role of non-inertial frames centered on
an arbitrary accelerated observer. In special relativity the exigence of
predictability implies the adoption of the 3+1 point of view, which leads to a
well posed initial value problem for field equations in a framework where the
change of the convention of synchronization of distant clocks is realized by
means of a gauge transformation. This point of view is also at the heart of the
canonical approach to metric and tetrad gravity in globally hyperbolic
asymptotically flat space-times, where the use of Shanmugadhasan canonical
transformations allows the separation of the physical degrees of freedom of the
gravitational field (the tidal effects) from the arbitrary gauge variables.
Since a global vision of the equivalence principle implies that only global
non-inertial frames can exist in general relativity, the gauge variables are
naturally interpreted as generalized relativistic inertial effects, which have
to be fixed to get a deterministic evolution in a given non-inertial frame. As
a consequence, in each Einstein's space-time in this class the whole
chrono-geometrical structure, including also the clock synchronization
convention, is dynamically determined and a new approach to the Hole Argument
leads to the conclusion that "gravitational field" and "space-time" are two
faces of the same entity. This view allows to get a classical scenario for the
unification of the four interactions in a scheme suited to the description of
the solar system or our galaxy with a deperametrization to special relativity
and the subsequent possibility to take the non-relativistic limit.Comment: 33 pages, Lectures given at the 42nd Karpacz Winter School of
Theoretical Physics, "Current Mathematical Topics in Gravitation and
Cosmology", Ladek, Poland, 6-11 February 200
Discrete breathers for understanding reconstructive mineral processes at low temperatures
Reconstructive transformations in layered silicates need a high tem- perature
in order to be observed. However, very recently, some systems have been found
where transformation can be studied at temperatures 600 C below the lowest
experimental results previously reported, including sol-gel methods. We explore
the possible relation with the existence of intrinsic localized modes, known as
discrete breathers. We construct a model for nonlinear vibrations within the
cation layer, obtain their parameters and calculate them numerically, obtaining
their energies. Their statistics shows that although there are far less
breathers than phonons, there are much more above the activation energy, being
therefore a good candidate to explain the reconstructive transformations at low
temperature.Comment: 27 pages, 11 figure
Localizing gravity on exotic thick 3-branes
We consider localization of gravity on thick branes with a non trivial
structure. Double walls that generalize the thick Randall-Sundrum solution, and
asymmetric walls that arise from a Z_2-symmetric scalar potential, are
considered. We present a new asymmetric solution: a thick brane interpolating
between two AdS_5 spacetimes with different cosmological constants, which can
be derived from a ``fake supergravity'' superpotential, and show that it is
possible to confine gravity on such branes.Comment: Final version, minor changes, references adde
High nest failure but better nestling quality for early breeders in an alpine population of Northern Wheatear (Oenanthe oenanthe)
Social Preference of Building Materials: Decision-Making towards Low Carbon Housing Constructions
Material considerations are essential while trying to achieve low energy and carbon constructions.
According to our preliminary findings, decisions regarding materials in new buildings are mostly done by clients or by
self-builders often without the aid of an architect or planner. Therefore, community studies are important to understand
today’s building dynamics. In this exploratory study - carried out in USA and Mexico - we analysed the public’s
preferences for building materials for the exterior of dwellings and the factors that have more influence over their
decisions while choosing the materials. We also studied the update in the subjects’ preference after receiving certain
information concerning the materials. We wanted to know if the environmental factors play an important role in today's
public preference in the building sector and what type of information could greater affect their beliefs. Preliminary
results for Mexico and after a follow up analysis for USA show that acknowledging unfamiliar information (e.g. carbon
emissions or price) might have a greater influence on the public’s update of preference of materials. This could have
implications for the construction market dynamics in the usage promotion of low carbon building materials.
KEYWORDS: Building materials, decision-making, low carbon, social cognitio
Revision of the extinct Pleistocene tortoise Testudo lunellensis Almera and Bofill, 1903 from Cova de Gràcia (Barcelona, Spain).
A Method to Find Community Structures Based on Information Centrality
Community structures are an important feature of many social, biological and
technological networks. Here we study a variation on the method for detecting
such communities proposed by Girvan and Newman and based on the idea of using
centrality measures to define the community boundaries (M. Girvan and M. E. J.
Newman, Community structure in social and biological networks Proc. Natl. Acad.
Sci. USA 99, 7821-7826 (2002)). We develop an algorithm of hierarchical
clustering that consists in finding and removing iteratively the edge with the
highest information centrality. We test the algorithm on computer generated and
real-world networks whose community structure is already known or has been
studied by means of other methods. We show that our algorithm, although it runs
to completion in a time O(n^4), is very effective especially when the
communities are very mixed and hardly detectable by the other methods.Comment: 13 pages, 13 figures. Final version accepted for publication in
Physical Review
Locally Preferred Structure and Frustration in Glassforming Liquids: A Clue to Polyamorphism?
We propose that the concept of liquids characterized by a given locally
preferred structure (LPS) could help in understanding the observed phenomenon
of polyamorphism. ``True polyamorphism'' would involve the competition between
two (or more) distinct LPS, one favored at low pressure because of its low
energy and one favored at high pressure because of its small specific volume,
as in tetrahedrally coordinated systems. ``Apparent polyamorphism'' could be
associated with the existence of a poorly crystallized defect-ordered phase
with a large unit cell and small crystallites, which may be illustrated by the
metastable glacial phase of the fragile glassformer triphenylphosphite; the
apparent polyamorphism might result from structural frustration, i. e., a
competition between the tendency to extend the LPS and a global constraint that
prevents tiling of the whole space by the LPS.Comment: 11, 6 figures, Proceedings of the Conference "Horizons in Complex
Systems", Messina; in honor of the 60th birthday of H.E. Stanle
The York map as a Shanmugadhasan canonical transformation in tetrad gravity and the role of non-inertial frames in the geometrical view of the gravitational field
A new parametrization of the 3-metric allows to find explicitly a York map in
canonical ADM tetrad gravity, the two pairs of physical tidal degrees of
freedom and 14 gauge variables. These gauge quantities (generalized inertial
effects) are all configurational except the trace of
the extrinsic curvature of the instantaneous 3-spaces (clock
synchronization convention) of a non-inertial frame. The Dirac hamiltonian is
the sum of the weak ADM energy (whose density is coordinate-dependent due to the inertial
potentials) and of the first-class constraints. Then: i) The explicit form of
the Hamilton equations for the two tidal degrees of freedom in an arbitrary
gauge: a deterministic evolution can be defined only in a completely fixed
gauge, i.e. in a non-inertial frame with its pattern of inertial forces. ii) A
general solution of the super-momentum constraints, which shows the existence
of a generalized Gribov ambiguity associated to the 3-diffeomorphism gauge
group. It influences: a) the explicit form of the weak ADM energy and of the
super-momentum constraint; b) the determination of the shift functions and then
of the lapse one. iii) The dependence of the Hamilton equations for the two
pairs of dynamical gravitational degrees of freedom (the generalized tidal
effects) and for the matter, written in a completely fixed 3-orthogonal
Schwinger time gauge, upon the gauge variable ,
determining the convention of clock synchronization. Therefore it should be
possible (for instance in the weak field limit but with relativistic motion) to
try to check whether in Einstein's theory the {\it dark matter} is a gauge
relativistic inertial effect induced by .Comment: 90 page
Seawater quality control of microcontaminants in fish farm cage systems: Application of passive sampling devices
Increasingly, developed countries are imposing restrictions on chemicals used in aquaculture, and introducing residue monitoring programmes to ensure the highest possible seafood safety standards. Chemotherapeutants, additives or chemical residues in edible tissues of aquaculture products are now attracting attention, and a major issue is the accumulation of microcontaminants in seafood flesh. Environmental quality control is related to the provision of high-quality, safe products. The present paper evaluates the effectiveness of passive sampling devices as tools in environmental monitoring programmes for fish farm cage systems. Capability to detect trace levels of microcontaminants, sampling rates, and accumulation kinetic is assessed. Devices tested were Polar Organic Chemical Integrative Samplers (POCIS), for detecting pharmaceuticals, pesticides and hormone residues; Semi-Permeable Membrane Devices (SPMD), to detect bioaccumulable pollutants; and Diffusive Gradients in Thin films (DGT), for metals.Las restricciones que imponen los paÃses desarrollados al uso de sustancias quÃmicas en la acuicultura para asegurar la salubridad de sus productos son cada vez mayores. También es creciente la preocupación por el control de los aditivos, residuos quÃmicos o los preparados farmacéuticos que pudieran encontrarse en las partes comestibles de las especies acuÃcolas, asà como la acumulación de micro-contaminantes en las mismas. En este trabajo se presenta un estudio sobre el uso de los sistemas de muestreo pasivo para los programas de control ambiental de las piscifactorÃas de jaulas flotantes. Se valora su capacidad de detectar niveles traza, la tasa de muestreo y la cinética de acumulación de micro-contaminantes. Se han probado los POCIS (Polar Organic Chemical Integrative Samplers) para detectar productos farmacéuticos, pesticidas y residuos hormonales, los SPMD (Semi-Permeable Membrane Devices) para detectar contaminantes bioacumulables y las membranas DGT (Diffusive Gradients in Thin films) para metales.Instituto Español de OceanografÃ
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