694 research outputs found
Do static sources respond to massive scalar particles from the Hawking radiation as uniformly accelerated ones do in the inertial vacuum?
We revisit the recently found equivalence for the response of a static scalar
source interacting with a {\em massless} Klein-Gordon field when the source is
(i) static in Schwarzschild spacetime, in the Unruh vacuum associated with the
Hawking radiation and (ii) uniformly accelerated in Minkowski spacetime, in the
inertial vacuum, provided that the source's proper acceleration is the same in
both cases. It is shown that this equivalence is broken when the massless
Klein-Gordon field is replaced by a {\em massive} one.Comment: 4 pages, 2 figure
An international network to monitor the structure, composition and dynamics of Amazonian forests (RAINFOR)
The Amazon basin is likely to be increasingly affected by environmental changes: higher temperatures, changes in precipitation, CO2 fertilization and habitat fragmentation. To examine the important ecological and biogeochemical consequences of these changes, we are developing an international network, RAINFOR, which aims to monitor forest biomass and dynamics across Amazonia in a co-ordinated fashion in order to understand their relationship to soil and climate. The network will focus on sample plots established by independent researchers, some providing data extending back several decades. We will also conduct rapid transect studies of poorly monitored regions. Field expeditions analysed local soil and plant properties in the first phase (2001–2002). Initial results suggest that the network has the potential to reveal much information on the continental-scale relations between forest and environment. The network will also serve as a forum for discussion between researchers, with the aim of standardising sampling techniques and methodologies that will enable Amazonian forests to be monitored in a coherent manner in the coming decades
Interaction of Hawking radiation with static sources in deSitter and Schwarzschild-deSitter spacetimes
We study and look for similarities between the response rates and of a static scalar source
with constant proper acceleration interacting with a massless,
conformally coupled Klein-Gordon field in (i) deSitter spacetime, in the
Euclidean vacuum, which describes a thermal flux of radiation emanating from
the deSitter cosmological horizon, and in (ii) Schwarzschild-deSitter
spacetime, in the Gibbons-Hawking vacuum, which describes thermal fluxes of
radiation emanating from both the hole and the cosmological horizons,
respectively, where is the cosmological constant and is the black
hole mass. After performing the field quantization in each of the above
spacetimes, we obtain the response rates at the tree level in terms of an
infinite sum of zero-energy field modes possessing all possible angular
momentum quantum numbers. In the case of deSitter spacetime, this formula is
worked out and a closed, analytical form is obtained. In the case of
Schwarzschild-deSitter spacetime such a closed formula could not be obtained,
and a numerical analysis is performed. We conclude, in particular, that and do not coincide in
general, but tend to each other when or . Our
results are also contrasted and shown to agree (in the proper limits) with
related ones in the literature.Comment: ReVTeX4 file, 9 pages, 5 figure
Aharonov-Bohm interferometry with quantum dots: scattering approach versus tunneling picture
We address the question of how to model electron transport through closed
Aharonov-Bohm interferometers which contain quantum dots. By explicitly
studying interferometers with one and two quantum dots, we establish the
connection between a tunneling-Hamiltonian formulation on the one hand and a
scattering-matrix approach on the other hand. We prove that, under certain
circumstances, both approaches are equivalent, i.e., both types of models can
describe the same experimental setups. Furthermore, we analyze how the
interplay of the Aharonov-Bohm phase and the orbital phase associated with the
lengths of the interferometers' arms affect transport properties.Comment: 8 pages, 8 figures, published versio
Renormalization approach for quantum-dot structures under strong alternating fields
We develop a renormalization method for calculating the electronic structure
of single and double quantum dots under intense ac fields. The nanostructures
are emulated by lattice models with a clear continuum limit of the
effective-mass and single-particle approximations. The coupling to the ac field
is treated non-perturbatively by means of the Floquet Hamiltonian. The
renormalization approach allows the study of dressed states of the nanoscopic
system with realistic geometries as well arbitrary strong ac fields. We give
examples of a single quantum dot, emphasizing the analysis of the
effective-mass limit for lattice models, and double-dot structures, where we
discuss the limit of the well used two-level approximation.Comment: 6 pages, 7 figure
Horizons, Constraints, and Black Hole Entropy
Black hole entropy appears to be ``universal''--many independent
calculations, involving models with very different microscopic degrees of
freedom, all yield the same density of states. I discuss the proposal that this
universality comes from the behavior of the underlying symmetries of the
classical theory. To impose the condition that a black hole be present, we must
partially break the classical symmetries of general relativity, and the
resulting Goldstone boson-like degrees of freedom may account for the
Bekenstein-Hawking entropy. In particular, I demonstrate that the imposition of
a ``stretched horizon'' constraint modifies the algebra of symmetries at the
horizon, allowing the use of standard conformal field theory techniques to
determine the asymptotic density of states. The results reproduce the
Bekenstein-Hawking entropy without any need for detailed assumptions about the
microscopic theory.Comment: 16 pages, talk given at the "Peyresq Physics 10 Meeting on Micro and
Macro structures of spacetime
The Hahn Quantum Variational Calculus
We introduce the Hahn quantum variational calculus. Necessary and sufficient
optimality conditions for the basic, isoperimetric, and Hahn quantum Lagrange
problems, are studied. We also show the validity of Leitmann's direct method
for the Hahn quantum variational calculus, and give explicit solutions to some
concrete problems. To illustrate the results, we provide several examples and
discuss a quantum version of the well known Ramsey model of economics.Comment: Submitted: 3/March/2010; 4th revision: 9/June/2010; accepted:
18/June/2010; for publication in Journal of Optimization Theory and
Application
Análise de variáveis preditivas do deslize aquatic através de um modelo de path-analysis
A velocidade de nado depende da propulsão
e arrasto hidrodinâmico. Para um nadador
se deslocar no meio aquático tem de
produzir a mesma quantidade de propulsão
e arrasto hidrodinâmico (Barbosa et al.,
2010)info:eu-repo/semantics/publishedVersio
Soil biochemistry and microbial activity in vineyards under conventional and organic management at Northeast Brazil.
The São Francisco Submedium Valley is located at the Brazilian semiarid region and is an important center for irrigated fruit growing. This region is responsible for 97% of the national exportation of table grapes, including seedless grapes. Based on the fact that orgThe São Francisco Submedium Valley is located at the Brazilian semiarid region and is an important center for irrigated fruit growing. This region is responsible for 97% of the national exportation of table grapes, including seedless grapes. Based on the fact that organic fertilization can improve soil quality, we compared the effects of conventional and organic soil management on microbial activity and mycorrhization of seedless grape crops. We measured glomerospores number, most probable number (MPN) of propagules, richness of arbuscular mycorrhizal fungi (AMF) species, AMF root colonization, EE-BRSP production, carbon microbial biomass (C-MB), microbial respiration, fluorescein diacetate hydrolytic activity (FDA) and metabolic coefficient (qCO2). The organic management led to an increase in all variables with the exception of EE-BRSP and qCO2. Mycorrhizal colonization increased from 4.7% in conventional crops to 15.9% in organic crops. Spore number ranged from 4.1 to 12.4 per 50 g-1 soil in both management systems. The most probable number of AMF propagules increased from 79 cm-3 soil in the conventional system to 110 cm-3 soil in the organic system. Microbial carbon, CO2 emission, and FDA activity were increased by 100 to 200% in the organic crop. Thirteen species of AMF were identified, the majority in the organic cultivation system. Acaulospora excavata, Entrophospora infrequens, Glomus sp.3 and Scutellospora sp. were found only in the organically managed crop. S. gregaria was found only in the conventional crop. Organically managed vineyards increased mycorrhization and general soil microbial activity
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