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 linguística

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