6,798 research outputs found
Particle Creation by a Moving Boundary with Robin Boundary Condition
We consider a massless scalar field in 1+1 dimensions satisfying a Robin
boundary condition (BC) at a non-relativistic moving boundary. We derive a
Bogoliubov transformation between input and output bosonic field operators,
which allows us to calculate the spectral distribution of created particles.
The cases of Dirichlet and Neumann BC may be obtained from our result as
limiting cases. These two limits yield the same spectrum, which turns out to be
an upper bound for the spectra derived for Robin BC. We show that the particle
emission effect can be considerably reduced (with respect to the
Dirichlet/Neumann case) by selecting a particular value for the oscillation
frequency of the boundary position
Duality and interval analysis over idempotent semirings
In this paper semirings with an idempotent addition are considered. These
algebraic structures are endowed with a partial order. This allows to consider
residuated maps to solve systems of inequalities . The
purpose of this paper is to consider a dual product, denoted , and the
dual residuation of matrices, in order to solve the following inequality . Sufficient conditions ensuring the
existence of a non-linear projector in the solution set are proposed. The
results are extended to semirings of intervals
Quantum radiation in a plane cavity with moving mirrors
We consider the electromagnetic vacuum field inside a perfect plane cavity
with moving mirrors, in the nonrelativistic approximation. We show that low
frequency photons are generated in pairs that satisfy simple properties
associated to the plane geometry. We calculate the photon generation rates for
each polarization as functions of the mechanical frequency by two independent
methods: on one hand from the analysis of the boundary conditions for moving
mirrors and with the aid of Green functions; and on the other hand by an
effective Hamiltonian approach. The angular and frequency spectra are discrete,
and emission rates for each allowed angular direction are obtained. We discuss
the dependence of the generation rates on the cavity length and show that the
effect is enhanced for short cavity lengths. We also compute the dissipative
force on the moving mirrors and show that it is related to the total radiated
energy as predicted by energy conservation.Comment: 17 pages, 1 figure, published in Physical Review
Mass distribution and structural parameters of Small Magellanic Cloud star clusters
In this work we estimate, for the first time, the total masses and mass
function slopes of a sample of 29 young and intermediate-age SMC clusters from
CCD Washington photometry. We also derive age, interstellar reddening and
structural parameters for most of the studied clusters by employing a
statistical method to remove the unavoidable field star contamination. Only
these 29 clusters out of 68 originally analysed cluster candidates present
stellar overdensities and coherent distribution in their colour-magnitude
diagrams compatible with the existence of a genuine star cluster. We employed
simple stellar population models to derive general equations for estimating the
cluster mass based only on its age and integrated light in the B, V, I, C and
T1 filter. These equations were tested against mass values computed from
luminosity functions, showing an excellent agreement. The sample contains
clusters with ages between 60 Myr and 3 Gyr and masses between 300 and 3000 Mo
distributed between ~0.5 deg. and ~2 deg. from the SMC optical centre. We
determined mass function slopes for 24 clusters, of which 19 have slopes
compatible with that of Kroupa IMF (2.3 +/- 0.7), considering the
uncertainties. The remaining clusters - H86-188, H86-190, K47, K63 and NGC242 -
showed flatter MFs. Additionally, only clusters with masses lower than ~1000 Mo
and flatter MF were found within ~0.6 deg. from the SMC rotational centre.Comment: 12 pages, 19 figures. Includes another 29 full-page figures of
supplementary material. Accepted for publication in the MNRA
Future dynamics in f(R) theories
The gravity theories provide an alternative way to explain the current
cosmic acceleration without invoking dark energy matter component. However, the
freedom in the choice of the functional forms of gives rise to the
problem of how to constrain and break the degeneracy among these gravity
theories on theoretical and/or observational grounds. In this paper to proceed
further with the investigation on the potentialities, difficulties and
limitations of gravity, we examine the question as to whether the future
dynamics can be used to break the degeneracy between gravity theories by
investigating the future dynamics of spatially homogeneous and isotropic dust
flat models in two gravity theories, namely the well known gravity and another by A. Aviles et al., whose motivation comes
from the cosmographic approach to gravity. To this end we perform a
detailed numerical study of the future dynamic of these flat model in these
theories taking into account the recent constraints on the cosmological
parameters made by the Planck team. We show that besides being powerful for
discriminating between gravity theories, the future dynamics technique
can also be used to determine the fate of the Universe in the framework of
these gravity theories. Moreover, there emerges from our numerical
analysis that if we do not invoke a dark energy component with
equation-of-state parameter one still has dust flat FLRW solution
with a big rip, if gravity deviates from general relativity via . We also show that FLRW dust solutions with do not
necessarily lead to singularity.Comment: 12 pages, 8 figures. V2: Generality and implications of the results
are emphasized, connection with the recent literature improved, typos
corrected, references adde
Google Glass as a learning tool: sharing evaluation results for the role of optical head mounted displays in education
This paper provides an overview of the findings from an evaluation of the role of Google Glass in education over the past three years. The authors have experimented with Optical Head Mounted Displays as a support tool for various learning activities over the past few years. The study described in the paper commenced back in 2014 and continued despite the fact that the development of the Google Glass technology was paused and then shifted towards enterprise clientele. This was a result of our confidence that the future of learning interfaces is aligned to the proliferation of augmented reality and the fact that the Google Glass interface offers an ideal tool for learners due to its light structure and seamless wearing experience. The paper discusses how Google Glass has been used for a range of learning activities and describes the learners’ experiences from using the device. The main contribution of the paper is in the form of measuring the success of the specific interface by sharing the results of three years of evaluations. The evaluation results are further analysed taking under consideration a number of profiling techniques of the learners involved including their personality type and learning style
Compressive Phase Contrast Tomography
When x-rays penetrate soft matter, their phase changes more rapidly than
their amplitude. In- terference effects visible with high brightness sources
creates higher contrast, edge enhanced images. When the object is piecewise
smooth (made of big blocks of a few components), such higher con- trast
datasets have a sparse solution. We apply basis pursuit solvers to improve SNR,
remove ring artifacts, reduce the number of views and radiation dose from phase
contrast datasets collected at the Hard X-Ray Micro Tomography Beamline at the
Advanced Light Source. We report a GPU code for the most computationally
intensive task, the gridding and inverse gridding algorithm (non uniform
sampled Fourier transform).Comment: 5 pages, "Image Reconstruction from Incomplete Data VI" conference
7800, SPIE Optical Engineering + Applications 1-5 August 2010 San Diego, CA
United State
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