3,524 research outputs found
Speed of light on rotating platforms
It is often taken for granted that on board a rotating disk it is possible to
operate a \QTR{it}{global}3+1 splitting of space-time, such that both lengths
and time intervals are \QTR{it}{uniquely} defined in terms of measurements
performed by real rods and real clocks at rest on the platform. This paper
shows that this assumption, although widespread and apparently trivial, leads
to an anisotropy of the velocity of two light beams travelling in opposite
directions along the rim of the disk; which in turn implies some recently
pointed out paradoxical consequences undermining the self-consistency of the
Special Theory of Relativity (SRT). A correct application of the SRT solves the
problem and recovers complete internal consistency for the theory. As an
immediate consequence, it is shown that the Sagnac effect only depends on the
non homogeneity of time on the platform and has nothing to do with any
anisotropy of the speed of light along the rim of the disk, contrary to an
incorrect but widely supported idea.Comment: Latex, 2 figure
From the elasticity theory to cosmology and vice versa
The paper shows how a generalization of the elasticity theory to four
dimensions and to space-time allows for a consistent description of the
homogeneous and isotropic universe, including the accelerated expansion. The
analogy is manifested by the inclusion in the traditional Lagrangian of general
relativity of an additional term accounting for the strain induced in the
manifold (i.e. in space-time) by the curvature, be it induced by the presence
of a texture defect or by a matter/energy distribution. The additional term is
sufficient to account for various observed features of the universe and to give
a simple interpretation for the so called dark energy. Then, we show how the
same approach can be adopted back in three dimensions to obtain the equilibrium
configuration of a given solid subject to strain induced by defects or applied
forces. Finally, it is shown how concepts coming from the familiar elasticity
theory can inspire new approaches to cosmology and in return how methods
appropriated to General Relativity can be applied back to classical problems of
elastic deformations in three dimensions.Comment: 11 pages, 3 figure
Gravitomagnetism, clocks and geometry
New techniques to evaluate the clock effect using light are described. These
are based on the flatness of the cylindrical surface containing the world lines
of the rays constrained to move on circular trajectories about a spinning mass.
The effect of the angular momentum of the source is manifested in the fact that
inertial observers must be replaced by local non rotating observers. Starting
from this an exact formula for circular trajectories is found. Numerical
estimates for the Earth environment show that light would be a better probe
than actual clocks to evidence the angular momentum influence. The advantages
of light in connection with some principle experiments are shortly reviewed.Comment: TCI Latex, 12 pages, 2 figures. To appear in European Journal of
Physic
Aging in short-ranged attractive colloids: A numerical study
We study the aging dynamics in a model for dense simple liquids, in which
particles interact through a hard-core repulsion complemented by a short-ranged
attractive potential, of the kind found in colloidal suspensions. In this
system, at large packing fractions, kinetically arrested disordered states can
be created both on cooling (attractive glass) and on heating (repulsive glass).
The possibility of having two distinct glasses, at the same packing fraction,
with two different dynamics offers the unique possibility of comparing --
within the same model -- the differences in aging dynamics. We find that, while
the aging dynamics of the repulsive glass is similar to the one observed in
atomic and molecular systems, the aging dynamics of the attractive glass shows
novel unexpected features.Comment: 8 pages, 11 figures, submited to Journal of Chemical Physic
Mode-Coupling Theory of Colloids with Short-range Attractions
Within the framework of the mode-coupling theory of super-cooled liquids, we
investigate new phenomena in colloidal systems on approach to their glass
transitions. When the inter-particle potential contains an attractive part,
besides the usual repulsive hard core, two intersecting liquid-glass transition
lines appear, one of which extends to low densities, while the other one, at
high densities, shows a re-entrant behaviour. In the glassy region a new type
of transition appears between two different types of glasses. The complex
phenomenology can be described in terms of higher order glass transition
singularities. The various glass phases are characterised by means of their
viscoelastic properties. The glass driven by attractions has been associated to
particle gels, and the other glass is the well known repulsive colloidal glass.
These correspondences, in associations with the new predictions of glassy
behaviour mean that such phenomena may be expected in colloidal systems with,
for example, strong depletion or other short-ranged attractive potentials.Comment: 17 pages, 8 figure
Geometric treatment of the gravitomagnetic clock effect
We have developed a general geometric treatment of the GCE valid for any
stationary axisymmetric metric. The method is based on the remark that the
world lines of objects rotating along spacely circular trajectories are in any
case, for those kind of metrics, helices drawn on the flat bidimensional
surface of a cylinder. Applying the obtained formulas to the special cases of
the Kerr and weak field metric for a spinning body, known results for time
delays and synchrony defects are recovered.Comment: 14 pages, LATEX, 2 figure
Prediction of Local Structural Stabilities of Proteins from Their Amino Acid Sequences
Hydrogen exchange experiments provide detailed information about the local stability and the solvent accessibility of different regions of the structures of folded proteins, protein complexes, and amyloid fibrils. We introduce an approach to predict protection factors from hydrogen exchange in proteins based on the knowledge of their amino acid sequences without the inclusion of any additional structural information. These results suggest that the propensity of different regions of the structures of globular proteins to undergo local unfolding events can be predicted from their amino acid sequences with an accuracy of 80% or better. © 2007 Elsevier Ltd. All rights reserved
Equilibrium cluster phases and low-density arrested disordered states: The role of short-range attraction and long-range repulsion
We study a model in which particles interact with short-ranged attractive and
long-ranged repulsive interactions, in an attempt to model the equilibrium
cluster phase recently discovered in sterically stabilized colloidal systems in
the presence of depletion interactions. At low packing fraction particles form
stable equilibrium clusters which act as building blocks of a cluster fluid. We
study the possibility that cluster fluids generate a low-density disordered
arrested phase, a gel, via a glass transition driven by the repulsive
interaction. In this model the gel formation is formally described with the
same physics of the glass formation.Comment: RevTeX4, 4 pages, 4 eps figure
A computational approach for the discovery of protein–RNA networks
Protein–RNA interactions play important roles in a wide variety of cellular processes, ranging from transcriptional and posttranscriptional regulation of genes to host defense against pathogens. In this chapter we present the computational approach catRAPID to predict protein–RNA interactions and discuss how it could be used to find trends in ribonucleoprotein networks. We envisage that the combination of computational and experimental approaches will be crucial to unravel the role of coding and noncoding RNAs in protein networks
Calibration of a granular matrix sensor for suction measurements in partially saturated pyroclastic soil
Field monitoring of soil moisture and matrix suction is a useful tool for the implementation of a reliable early warning system against rainfall-induced landslide occurrence. Several test fields have been set up in Campania region (southern Italy), frequently affected by flow-like landslides involving pyroclastic soil cover. In particular, at the Mount Faito test site (Lattari Mountains, southeast of Naples), field matric suctions were measured over two years by conventional jet-fill tensiometers and granular matrix sensors (Watermark, Irrometer®) at different depths. Granular matrix sensor is a resistive device that is more and more spread in agriculture applications and that may also be used for geotechnical purposes thanks to a suitable calibration. In order to gain the calibration curve of the Watermark sensor, two small tip tensiometers (STT) and one High Capacity Tensiometer (HCT) were installed at the same depth of the Watermark sensor in the partially saturated pyroclastic soil sampled at the topsoil of the Mount Faito test site. Tests were carried out in the laboratory by performing drying and wetting phases on undisturbed soil sample. By coupling resistance measurements by Watermark and matrix suction provided by the reference tensiometers, it was possible to derive the non-linear relationship between these two quantities. The soil retention curve was also determined thanks to the installation in the soil sample of a decagon probe previously calibrated in the same pyroclastic soil
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