2,645 research outputs found
Embedding Versus Immersion in General Relativity
We briefly discuss the concepts of immersion and embedding of space-times in
higher-dimensional spaces. We revisit the classical work by Kasner in which he
constructs a model of immersion of the Schwarzschild exterior solution into a
six-dimensional pseudo-Euclidean manifold. We show that, from a physical point
of view, this model is not entirely satisfactory since the causal structure of
the immersed space-time is not preserved by the immersion.Comment: 5 page
RF CMOS transceiver at 2.4 GHz in wearables for measuring the cardio-respiratory function
This paper presents a radio-frequency (RF) transceiver for operation in the 2.4 GHz ISM
band. The RF CMOS transceiver can be supplied with only 1.8 V, and it was designed to
establish wireless links for distances up to 10 m, for a maximum baud-rate of 250 Kbps
with a Bit Error Probability less than 10 6. The transmitter can deliver a output power of
0 dBm with a consumption of only 11.2 mW, while the receiver has sensitivity of 60
dBm and consumes only 6.3 mW. The goal of RF CMOS transceiver is for co-integration
with sensors in the same die using microsystems techniques. The target application of such
microsystems is in wearables (e.g., in wireless electronic shirts) for measuring biomedical
data of patients. The wireless electronic shirt (WES) measures the heart rate and the respiratory
frequency, and at the same time it allows patients to maintain their mobilit
Remarks on Legendrian Self-Linking
The Thurston-Bennequin invariant provides one notion of self-linking for any
homologically-trivial Legendrian curve in a contact three-manifold. Here we
discuss related analytic notions of self-linking for Legendrian knots in
Euclidean space. Our definition is based upon a reformulation of the elementary
Gauss linking integral and is motivated by ideas from supersymmetric gauge
theory. We recover the Thurston-Bennequin invariant as a special case.Comment: 42 pages, many figures; v2: minor revisions, published versio
On the embedding of spacetime in five-dimensional Weyl spaces
We revisit Weyl geometry in the context of recent higher-dimensional theories
of spacetime. After introducing the Weyl theory in a modern geometrical
language we present some results that represent extensions of Riemannian
theorems. We consider the theory of local embeddings and submanifolds in the
context of Weyl geometries and show how a Riemannian spacetime may be locally
and isometrically embedded in a Weyl bulk. We discuss the problem of classical
confinement and the stability of motion of particles and photons in the
neighbourhood of branes for the case when the Weyl bulk has the geometry of a
warped product space. We show how the confinement and stability properties of
geodesics near the brane may be affected by the Weyl field. We construct a
classical analogue of quantum confinement inspired in theoretical-field models
by considering a Weyl scalar field which depends only on the extra coordinate.Comment: 16 pages, new title and references adde
Fabrication methodology of microlenses for stereoscopic imagers using standard CMOS process
This paper presents the fabrication technology of microlenses
maintaining a high reproducibility of their characteristics with low
cost. The objective of microlenses is to be integrated into imagers
in CMOS technology to allow stereoscopic vision. The fabricated
microlenses form cylindrical arrays to be placed above the optical
filters and photodetectors, in order to potentiate stereoscopic vision
and at the same time maximizing the color fidelity. An array of
optical filters centered at the primary colors will enable a
multicolor usage. The AZ4562 material was the photoresist
selected for fabricating the microlenses. The cylindrical shape is
obtained by reflowing the photoresist using the hot-plate technique.This work and Rui Pedro Rocha were fully supported by the Portuguese Foundation for
Science and Technology under the project FCT/PTDC/EEA-ELC/109936/2009 and the
financial grant SFRH/BD/33733/2009, respectively
Geometric Mechanics of Curved Crease Origami
Folding a sheet of paper along a curve can lead to structures seen in
decorative art and utilitarian packing boxes. Here we present a theory for the
simplest such structure: an annular circular strip that is folded along a
central circular curve to form a three-dimensional buckled structure driven by
geometrical frustration. We quantify this shape in terms of the radius of the
circle, the dihedral angle of the fold and the mechanical properties of the
sheet of paper and the fold itself. When the sheet is isometrically deformed
everywhere except along the fold itself, stiff folds result in creases with
constant curvature and oscillatory torsion. However, relatively softer folds
inherit the broken symmetry of the buckled shape with oscillatory curvature and
torsion. Our asymptotic analysis of the isometrically deformed state is
corroborated by numerical simulations which allow us to generalize our analysis
to study multiply folded structures
The relative abundance of wheat Rubisco activase isoforms is post‑transcriptionally regulated
Diurnal rhythms and light availability affect transcription–translation feedback loops that regulate the synthesis of photosynthetic proteins. The CO2-fixing enzyme Rubisco is the most abundant protein in the leaves of major crop species and its activity depends on interaction with the molecular chaperone Rubisco activase (Rca). In Triticum aestivum L. (wheat), three Rca isoforms are present that differ in their regulatory properties. Here, we tested the hypothesis that the relative abundance of the redox-sensitive and redox-insensitive Rca isoforms could be differentially regulated throughout light–dark diel cycle in wheat. While TaRca1-β expression was consistently negligible throughout the day, transcript levels of both TaRca2-β and TaRca2-α were higher and increased at the start of the day, with peak levels occurring at the middle of the photoperiod. Abundance of TaRca-β protein was maximal 1.5 h after the peak in TaRca2-β expression, but the abundance of TaRca-α remained constant during the entire photoperiod. The redox-sensitive TaRca-α isoform was less abundant, representing 85% of the redox-insensitive TaRca-β at the transcript level and 12.5% at the protein level. Expression of Rubisco large and small subunit genes did not show a consistent pattern throughout the diel cycle, but the abundance of Rubisco decreased by up to 20% during the dark period in fully expanded wheat leaves. These results, combined with a lack of correlation between transcript and protein abundance for both Rca isoforms and Rubisco throughout the entire diel cycle, suggest that the abundance of these photosynthetic enzymes is post-transcriptionally regulated
Microlenses for stereoscopic image formation
This paper presents microlenses for integration on a stereoscopic image sensor in CMOS technology for use
in biomedical devices. It is intended to provide an image sensor with a stereoscopic vision. An array of
microlenses potentiates stereoscopic vision and maximizes the color fidelity. An array of optical filters
tuned at the primary colors will enable a multicolor usage. The material selected for fabricating the
microlens was the AZ4562 positive photoresist. The reflow method applied to the photoresist allowing the
fabrication of microlenses with high reproducibility.This work was fully supported by the Portuguese
Foundation for Science and Technology under the
project FCT/PTDC/EEA-ELC/109936/2009
Interface-mediated interactions: Entropic forces of curved membranes
Particles embedded in a fluctuating interface experience forces and torques
mediated by the deformations and by the thermal fluctuations of the medium.
Considering a system of two cylinders bound to a fluid membrane we show that
the entropic contribution enhances the curvature-mediated repulsion between the
two cylinders. This is contrary to the usual attractive Casimir force in the
absence of curvature-mediated interactions. For a large distance between the
cylinders, we retrieve the renormalization of the surface tension of a flat
membrane due to thermal fluctuations.Comment: 11 pages, 5 figures; final version, as appeared in Phys. Rev.
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