82 research outputs found
From quantum to classical instability in relativistic stars
It has been shown that gravitational fields produced by realistic
classical-matter distributions can force quantum vacuum fluctuations of some
nonminimally coupled free scalar fields to undergo a phase of exponential
growth. The consequences of this unstable phase to the background spacetime
have not been addressed so far due to known difficulties concerning
backreaction in semiclassical gravity. It seems reasonable to believe, however,
that the quantum fluctuations will "classicalize" when they become large
enough, after which backreaction can be treated in the general-relativistic
context. Here we investigate the emergence of a classical regime out of the
quantum field evolution during the unstable phase. By studying the appearance
of classical correlations and loss of quantum coherence, we show that by the
time backreaction becomes important the system already behaves classically.
Consequently, the gravity-induced instability leads naturally to initial
conditions for the eventual classical description of the backreaction. Our
results give support to previous analyses which treat classically the
instability of scalar fields in the spacetime of relativistic stars, regardless
whether the instability is triggered by classical or quantum perturbations.Comment: 16 pages. Minor changes to match the published versio
Etiquetagem de micromensagens no Twitter: uma abordagem linguÃstica
Hashtags are labels used by Twitter members in order to classify
messages posted in this social network. They are produced by the users
themselves without any interference from the platform, which generates
interest in studying them as linguistic elements since the appointment
of a hashtag is driven by linguistic and social factors that influence
the creation of new tags and the acceptance of labels proposed by other
members.
In this work, we present a sociolinguistic-based study about the
usage of hashtags on Twitter, assuming that its users' network has
common features with offline speech communities, i.e., groups of people
whose members linguistically influence each other. Initially, we analyze
the motivations that lead Twitter users to insert tags in their tweets.
We found that the main reasons for labeling on Twitter are to increase
the comprehensibility of the information and to raise the possibility of
effective content sharing. Then, we examine some linguistic factors
that contribute to success or failure of tags. Finally, we investigate
the role of a social factor - the user's gender - in the usage of
hashtags. Our results indicate that characteristics of some groups of
hashtags are able to contribute to genderize them. The outcomes show
similar features to those found in studies of offline speech, that leads
us to believe that free tagging in folksonomies can serve as a model
for characterizing the propagation of linguistic forms in other
contexts.
Our findings complement the knowledge about human behavior in free
tagging environments and may be useful to increase the effectiveness of
real-time streaming search algorithms and tag recommendation systems
based on users' collective preferences.FGW – Publications without University Leiden contrac
A comparison study of regional atmospheric simulations with an elastic backscattering Lidar and sunphotometry in an urban area
We describe a comparison study of Aerosol Optical Thickness (AOT) from numerical simulations using a regional atmospheric model with an elastic backscattering lidar operating at 532 nm and a sunphotometer belonging to the AERONET network at São Paulo (23° S 46° W) city, Brazil, a very populated urban area. The atmospheric model includes an aerosol emission, transport and deposition module coupled to a radiative transfer parameterization, which takes the interaction between aerosol particles and short and long wave radiation into account. A period of one week was taken as case study during the dry season (late August) when intense biomass burning activities occur at remote areas in South America, and meteorological conditions disfavor the pollution dispersion in the city of São Paulo. The situation presented here showed how smoke from biomass burning in remote areas is transported to the south-east part of Brazil and affects the optical atmospheric conditions in São Paulo. The numerical simulations are corroborated by in situ measurements of AOT obtained by lidar and sun photometry
Influence of detector motion in entanglement measurements with photons
We investigate how the polarization correlations of entangled photons
described by wave packets are modified when measured by moving detectors. For
this purpose, we analyze the Clauser-Horne-Shimony-Holt Bell inequality as a
function of the apparatus velocity. Our analysis is motivated by future
experiments with entangled photons designed to use satellites. This is a first
step towards the implementation of quantum information protocols in a global
scale
Sudden change in quantum and classical correlations and the Unruh effect
We use the Unruh effect to analyze the dynamics of classical and quantum
correlations for a two-qubit system when one of them is uniformly accelerated
for a finite amount of proper time. We show that the quantum correlation is
completely destroyed in the limit of infinite acceleration, while the classical
one remains nonzero. In particular, we show that such correlations exhibit the
so-called sudden-change behavior as a function of acceleration. Eventually, we
discuss how our results can be interpreted when the system lies in the vicinity
of the event horizon of a Schwarzschild black hole.Comment: Published versio
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