900 research outputs found
Superfluid Motion of Light
Superfluidity, the ability of a fluid to move without dissipation, is one of
the most spectacular manifestations of the quantum nature of matter. We explore
here the possibility of superfluid motion of light. Controlling the speed of a
light packet with respect to a defect, we demonstrate the presence of
superfluidity and, above a critical velocity, its breakdown through the onset
of a dissipative phase. We describe a possible experimental realization based
on the transverse motion through an array of waveguides. These results open new
perspectives in transport optimization.Comment: 4 pages, 3 figure
Sistema de controle de horas extras.
Editores técnicos: Joseani Mesquita Antunes, Ana Lídia Variani Bonato, Márcia Barrocas Moreira Pimentel
Evolving Lorentzian wormholes supported by phantom matter with constant state parameters
In this paper we study the possibility of sustaining an evolving wormhole via
exotic matter made out of phantom energy. We show that this exotic source can
support the existence of evolving wormhole spacetimes. Explicitly, a family of
evolving Lorentzian wormholes conformally related to another family of
zero-tidal force static wormhole geometries is found in Einstein gravity.
Contrary to the standard wormhole approach, where first a convenient geometry
is fixed and then the matter distribution is derived, we follow the
conventional approach for finding solutions in theoretical cosmology. We derive
an analytical evolving wormhole geometry by supposing that the radial tension
(which is negative to the radial pressure) and the pressure measured in the
tangential directions have barotropic equations of state with constant state
parameters. At spatial infinity this evolving wormhole, supported by this
anisotropic matter, is asymptotically flat, and its slices constant are
spaces of constant curvature. During its evolution the shape of the wormhole
expands with constant velocity, i.e without acceleration or deceleration, since
the scale factor has strictly a linear evolution.Comment: 9 pages, 2 figures, Accepted for publication in Phys. Rev.
Using a Penalized Likelihood to Detect Mortality Deceleration
In this paper, we suggest a novel method for detecting mortality
deceleration. We focus on the gamma-Gompertz frailty model and suggest the
subtraction of a penalty in the log-likelihood function as an alternative to
traditional likelihood inference and hypothesis testing. Over existing methods,
our method offers advantages, such as avoiding the use of a p-value, hypothesis
testing, and asymptotic distributions. We evaluate the performance of our
approach by comparing it with traditional likelihood inference on both
simulated and real mortality data. Results have shown that our approach is more
accurate in detecting mortality deceleration and provides more reliable
estimates of the underlying parameters. The proposed method is a significant
contribution to the literature as it offers a powerful tool for analyzing
mortality patterns
Makeham Mortality Models as Mixtures
Mortality modeling is crucial to understanding the complex nature of
population aging and projecting future trends. The Makeham term is a commonly
used constant additive hazard in mortality modeling to capture background
mortality unrelated to aging. In this manuscript, we propose representing
Makeham mortality models as mixtures that describe lifetimes in a
competing-risk framework: an individual dies either according to a baseline
mortality mechanism or an exponential distribution, whatever strikes first. The
baseline can describe mortality at all ages or just mortality due to aging. By
using this approach, we can estimate the share of non-senescent mortality at
each adult age, which is an essential contribution to the study of premature
and senescent mortality. Our results allow for a better understanding of the
underlying mechanisms of mortality and provide a more accurate picture of
mortality dynamics in populations
Coulomb's law modification in nonlinear and in noncommutative electrodynamics
We study the lowest-order modifications of the static potential for
Born-Infeld electrodynamics and for the -expanded version of the
noncommutative U(1) gauge theory, within the framework of the gauge-invariant
but path-dependent variables formalism. The calculation shows a long-range
correction (-type) to the Coulomb potential in Born-Infeld
electrodynamics. However, the Coulomb nature of the potential (to order )
is preserved in noncommutative electrodynamics.Comment: 14 pages, 1 figur
Living bacteria rheology: population growth, aggregation patterns and cooperative behaviour under different shear flows
The activity of growing living bacteria was investigated using real-time and
in situ rheology -- in stationary and oscillatory shear. Two different strains
of the human pathogen Staphylococcus aureus -- strain COL and its isogenic cell
wall autolysis mutant -- were considered in this work. For low bacteria
density, strain COL forms small clusters, while the mutant, presenting
deficient cell separation, forms irregular larger aggregates. In the early
stages of growth, when subjected to a stationary shear, the viscosity of both
strains increases with the population of cells. As the bacteria reach the
exponential phase of growth, the viscosity of the two strains follow different
and rich behaviours, with no counterpart in the optical density or in the
population's colony forming units measurements. While the viscosity of strain
COL keeps increasing during the exponential phase and returns close to its
initial value for the late phase of growth, where the population stabilizes,
the viscosity of the mutant strain decreases steeply, still in the exponential
phase, remains constant for some time and increases again, reaching a constant
plateau at a maximum value for the late phase of growth. These complex
viscoelastic behaviours, which were observed to be shear stress dependent, are
a consequence of two coupled effects: the cell density continuous increase and
its changing interacting properties. The viscous and elastic moduli of strain
COL, obtained with oscillatory shear, exhibit power-law behaviours whose
exponent are dependent on the bacteria growth stage. The viscous and elastic
moduli of the mutant have complex behaviours, emerging from the different
relaxation times that are associated with the large molecules of the medium and
the self-organized structures of bacteria. These behaviours reflect
nevertheless the bacteria growth stage.Comment: 9 pages, 10 figure
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