5,334 research outputs found
Quantum Electrodynamics vacuum polarization solver
The self-consistent modeling of vacuum polarization due to virtual
electron-positron fluctuations is of relevance for many near term experiments
associated with high intensity radiation sources and represents a milestone in
describing scenarios of extreme energy density. We present a generalized
finite-difference time-domain solver that can incorporate the modifications to
Maxwell's equations due to vacuum polarization. Our multidimensional solver
reproduced in one dimensional configurations the results for which an analytic
treatment is possible, yielding vacuum harmonic generation and birefringence.
The solver has also been tested for two-dimensional scenarios where finite
laser beam spot sizes must be taken into account. We employ this solver to
explore different types of counter-propagating configurations that can be
relevant for future planned experiments aiming to detect quantum vacuum
dynamics at ultra-high electromagnetic field intensities
Progress in mass and clonal selection of grapevine varieties in Portugal
Since 1978 we have developed methods for mass and clonal selection of grapevine well adapted to the conditions of Portuguese viticulture. The most remarkable innovation of this methodology is the establishment of large experimental populations of clones in which good estimates of yield heritability and genetic gain can be obtained. The aim of the present paper is to clarify some methodological aspects such as (1) the ideal composition and structure of the experimental populations of clones and (2) the application of this methodology to a large number of Portuguese varieties, in order to maximize the rate of yield improvement. At present we can point out the following results: According to experimental data and computer simulation; we may conclude that the optimal number of clones to be included in each population is about 200-300, each one represented by 4 vines in 5 replications. By this way we can usually obtain heritability estimates between 30 and 80% and genetic gain values higher than 15%.Until now, we have succeeded in applying this methodology to 30 of the most important Portuguese varieties, which represent more than 80% of all varieties normally propagated in the country
Equality of opportunity and educational achievement in Portugal
Portugal has one of the highest levels of income inequality in Europe, and low wages and unemployment are concentrated among low skill individuals. Education is an important determinant of inequality. However, there are large differences in the educational attainment of different individuals in the population, and the sources of these differences emerge early in the life-cycle when families play a central role in individual development. We estimate that most of the variance of school achievement at age 15 is explained by family characteristics. Observed school inputs explain very little of adolescent performance. Children from highly educated parents benefit of rich cultural environments in the home and become highly educated adults. Education policy needs to be innovative: (1) it needs to explicitly recognize the fundamental long run role of families on child development; (2) it needs to acknowledge the failure of traditional input based policies
A Godel-Friedman cosmology?
Based on the mathematical similarity between the Friedman open metric and
Godel's metric in the case of nearby distances, we investigate a new scenario
for the Universe's evolution, where the present Friedman universe originates
from a primordial Godel universe by a phase transition during which the
cosmological constant vanishes. Using Hubble's constant and the present matter
density as input, we show that the radius and density of the primordial Godel
universe are close, in order of magnitude, to the present values, and that the
time of expansion coincides with the age of the Universe in the standard
Friedman model. In addition, the conservation of angular momentum provides, in
this context, a possible origin for the rotation of galaxies, leading to a
relation between the masses and spins corroborated by observational data.Comment: Extended version, accepted for publication in Physical Review
Non-singular inflation with vacuum decay
On the basis of a semi-classical analysis of vacuum energy in an expanding
spacetime, we describe a non-singular cosmological model in which the vacuum
density decays with time, with a concomitant production of matter. During an
infinitely long period we have an empty, inflationary universe, with H \approx
1. This primordial era ends in a fast phase transition, during which H and
\Lambda decrease to nearly zero in a few Planck times, with release of a huge
amount of radiation. The late-time scenario is similar to the standard model,
with the radiation phase followed by a long dust era, which tends
asymptotically to a de Sitter universe, with vacuum dominating again. An
analysis of the redshift-distance relation for supernovas Ia leads to
cosmological parameters in agreement with other current estimations.Comment: Work presented at IRGAC 2006, Barcelona, July 11-15 2006. To appear
in a special issue of Journal of Physics
Transformação genĂ©tica de sorgo com gene de tolerĂąncia ao alumĂnio ALMT1 isolado de trigo.
Avaliação do potencial alelopĂĄtico de trĂȘs espĂ©cies de Eucalyptus na germinação de espĂ©cies cultivadas e nativas do cerrado.
Realizaram-se ensaios de germinação com sementes de Lactuca sativa L. (alface) cv. AurĂ©lia, sorgo cv. BR 304, Anadenanthera peregrina (L.) Speg. (angico) e Enterolobium contortisiliquum (Vell.) (tamboril), com o objetivo de avaliar o potencial alelopĂĄtico de extratos aquosos foliares de trĂȘs espĂ©cies de Eucalyptus. Prepararam-se os extratos colocando as folhas senescentes, secas e moĂdas de E. camaldulensis Dehnh (EC), E. grandis W.Hill ex Maien (EG) e E. urophylla S.T.Blake (EU) em ĂĄgua destilada na concentração 5% (p/v). ApĂłs agitados e centrifugados, obtiveram-se extratos a 100% (v/v), com os quais diluiram-se para 75, 50, 25, 12,5, 0% (v/v) e medidos o pH e o potencial osmĂłtico em cada concentração. As sementes foram colocadas para germinar em placas gerbox, forradas com papel de filtro e umedecidas com os extratos, numa amostragem de 100 sementes por tratamento (4x25). As placas foram colocadas em cĂąmara germinadora a 25°C, na presença de luz. A germinação foi verificada a cada 48h, durante 7 dias, sendo o critĂ©rio germinativo a emergĂȘncia radicular. O pH dos extratos das trĂȘs espĂ©cies de Eucalyptus diminuiu com o aumento da concentração, porĂ©m, nĂŁo afetou a germinação das espĂ©cies receptoras testadas. Os extratos aquosos de folhas de EC, EG e EU nĂŁo interferiram na germinação das sementes de angico, tamboril e sorgo, porĂ©m inibiram a germinação das sementes de alface. Eucalyptus camaldulensis; E; grandis; E; urophyll
Avaliação do potencial alelopĂĄtico de trĂȘs espĂ©cies de Eucalyptus na germinação de espĂ©cies cultivadas e nativas do cerrado.
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