La flèche du temps : analyse philosophique d'une métaphore scientifique

Le problème de la direction du temps est un problème classique autant en physique qu’en philosophie. Quoiqu’il existe plusieurs façons de s’interroger sur ce problème, l’approche thermodynamique est la plus fréquemment utilisée. Cette approche consiste à considérer la flèche du temps thermodynamique comme la flèche fondamentale de laquelle les autres flèches ne sont que des manifestations. Ce mémoire vise à fournir une analyse philosophique de cette approche. Pour ce faire, nous esquisserons la problématique générale, nous exposerons les différentes approches et théories alternatives visant à résoudre ce problème et nous présenterons la thèse forte soutenant l’approche thermodynamique. Ensuite, nous évaluerons la pertinence du recours à la mécanique statistique et à la cosmologie visant à remédier aux déficiences de cette même approche. Enfin, nous analyserons en quoi cette approche, et plus particulièrement la notion d’entropie, est en mesure de fournir un cadre conceptuel pour la résolution du problème de la flèche du temps.

A historical approach to the new secular stagnation hypothesis

In this paper, we analyze the behavior of real interest rates over the long-run using historical data for nine developed economies, to assess the extent to which the recent decline observed in most advanced countries is at odds with the past data, as suggested by the Secular Stagnation hypothesis. By using data from 1703 and performing stationarity and structural breaks tests, we find that the recent decline in interest rates is not explained by a structural break in the time series. Our results also show that considering long-run data leads to different conclusions than using short-run data.

Omics meets hypothesis-driven research. Partnership for innovative discoveries in vascular biology and angiogenesis.

The emergence of omics technologies allowing the global analysis of a given biological or molecular system, rather than the study of its individual components, has revolutionized biomedical research, including cardiovascular medicine research in the past decade. These developments raised the prospect that classical, hypothesis-driven, single gene-based approaches may soon become obsolete. The experience accumulated so far, however, indicates that omic technologies only represent tools similar to those classically used by scientists in the past and nowadays, to make hypothesis and build models, with the main difference that they generate large amounts of unbiased information. Thus, omics and classical hypothesis-driven research are rather complementary approaches with the potential to effectively synergize to boost research in many fields, including cardiovascular medicine. In this article we discuss some general aspects of omics approaches, and review contributions in three areas of vascular biology, thrombosis and haemostasis, atherosclerosis and angiogenesis, in which omics approaches have already been applied (vasculomics).

Hepatitis C among former athletes: association with the use of injectable stimulants in the past

This study was performed with the purpose of testing the hypothesis that the high prevalence of hepatitis C among former athletes is associated with their past use of injectable stimulants. The study involved the participation of 208 former professional and amateur soccer and basketball players from the region of Ribeirão Preto, Brazil, who answered a questionnaire regarding their exposure to risk factors, including the use of injectable stimulants in the time they were engaged in sporting activities. ELISA tests were used to detect infection by the hepatitis C virus, and confirmed with PCR and genotyping for the positive cases. It was observed that the former use of injectable stimulants was a practice disseminated among the participants (24.5%), reaching 50.8% in the professionals. The overall prevalence for hepatitis C was 7.2%, with values of 11% among professionals and 5.5% among amateurs. In both categories, the presence of infection was markedly higher among those who admitted past use of injectable stimulants when compared to those who denied such practice (36% and 0.8% among amateurs; 21.9% and 0% among professionals, respectively). Multivariate analysis showed that the use of those substances was the only variable associated with the risk of hepatitis C. This confirms previous observations, performed with reduced sample sizes and without comparison groups, which indicated that the use of injectable vitamins was a risk factor of hepatitis C among former athletes.

Behavioural Flexibility: evolutionary past and its role in a changing world

A través de la historia de la vida, gran parte de los organismos han desarrollado estrategias para responder a un mundo en constante cambio. Hoy en día, las actividades humanas producen cambios ambientales a una velocidad sin precedentes, lo cual se traduce en grandes desafíos para la persistencia de biodiversidad. Esta investigación evalúa las respuesta de los animales a los cambios ambientales enfocándose en la flexibilidad del comportamiento como estrategia adaptativa. En una primera aproximación a una escala evolutiva, se otorgan evidencias del vínculo hasta ahora tenue entre la cognición e historias de vida, entregando un claro apoyo a la relación entre longevidad, vida reproductiva y el tamaño del cerebro en mamíferos. La longevidad es el centro de muchas hipótesis respecto a las ventajas de desarrollar un cerebro grande, como por ejemplo en la hipótesis del buffer cognitivo y las respuestas flexibles frente a nuevos ambientes. En un segundo nivel, se abordan factores extrínsecos e intrínsecos que podrían explicar las diferencias individuales en innovación, un componente clave en la flexibilidad del comportamiento. Por medio de una aproximación experimental, se evalúan potenciales escenarios que podrían conducir a consistentes diferencias individuales en uno de los principales factores subyacentes a la innovación (i.e. la motivación), y el potencial control endocrino sobre estos escenarios. Posteriormente, con el objetivo de evaluar la respuesta de los animales frente a los cambios ambientales actuales, se explora la respuesta de los animales frente a una de las actividades humanas mas disruptivas sobre los ecosistemas, la urbanización. Por medio de un analisis filogenetico comparativo a nivel global en aves se abordan los mecanismos implicados en la perdida de biodiversidad observada en ambientes urbanos. Los resultados entregan evidencias sobre la importancia de procesos de dispersión local junto con el papel clave de los rasgos de historia de vida, pero en un sentido diferente al clasicamente pensado. Finalmente por medio de una revisión bibliográfica se entregan evidencias teóricas y empíricas que respaldan el rol clave de la flexibilidad del comportamiento en confrontar los desafíos de una vida urbana. La integración de estos resultados muestra cómo el pasado evolutivo contribuye a hacer frente a los retos ambientales actuales, y pone de relieve posibles consecuencias ante un planeta más cambiante que nunca.

Behavioural Flexibility: evolutionary past and its role in a changing world

A través de la historia de la vida, gran parte de los organismos han desarrollado estrategias para responder a un mundo en constante cambio. Hoy en día, las actividades humanas producen cambios ambientales a una velocidad sin precedentes, lo cual se traduce en grandes desafíos para la persistencia de biodiversidad. Esta investigación evalúa las respuesta de los animales a los cambios ambientales enfocándose en la flexibilidad del comportamiento como estrategia adaptativa. En una primera aproximación a una escala evolutiva, se otorgan evidencias del vínculo hasta ahora tenue entre la cognición e historias de vida, entregando un claro apoyo a la relación entre longevidad, vida reproductiva y el tamaño del cerebro en mamíferos. La longevidad es el centro de muchas hipótesis respecto a las ventajas de desarrollar un cerebro grande, como por ejemplo en la hipótesis del buffer cognitivo y las respuestas flexibles frente a nuevos ambientes. En un segundo nivel, se abordan factores extrínsecos e intrínsecos que podrían explicar las diferencias individuales en innovación, un componente clave en la flexibilidad del comportamiento. Por medio de una aproximación experimental, se evalúan potenciales escenarios que podrían conducir a consistentes diferencias individuales en uno de los principales factores subyacentes a la innovación (i.e. la motivación), y el potencial control endocrino sobre estos escenarios. Posteriormente, con el objetivo de evaluar la respuesta de los animales frente a los cambios ambientales actuales, se explora la respuesta de los animales frente a una de las actividades humanas mas disruptivas sobre los ecosistemas, la urbanización. Por medio de un analisis filogenetico comparativo a nivel global en aves se abordan los mecanismos implicados en la perdida de biodiversidad observada en ambientes urbanos. Los resultados entregan evidencias sobre la importancia de procesos de dispersión local junto con el papel clave de los rasgos de historia de vida, pero en un sentido diferente al clasicamente pensado. Finalmente por medio de una revisión bibliográfica se entregan evidencias teóricas y empíricas que respaldan el rol clave de la flexibilidad del comportamiento en confrontar los desafíos de una vida urbana. La integración de estos resultados muestra cómo el pasado evolutivo contribuye a hacer frente a los retos ambientales actuales, y pone de relieve posibles consecuencias ante un planeta más cambiante que nunca.

Purinergic signalling: past, present and future

The discovery of non-adrenergic, non-cholinergic neurotransmission in the gut and bladder in the early 1960's is described as well as the identification of adenosine 5'-triphosphate (ATP) as a transmitter in these nerves in the early 1970's. The concept of purinergic cotransmission was formulated in 1976 and it is now recognized that ATP is a cotransmitter in all nerves in the peripheral and central nervous systems. Two families of receptors to purines were recognized in 1978, P1 (adenosine) receptors and P2 receptors sensitive to ATP and adenosine diphosphate (ADP). Cloning of these receptors in the early 1990's was a turning point in the acceptance of the purinergic signalling hypothesis and there are currently 4 subtypes of P1 receptors, 7 subtypes of P2X ion channel receptors and 8 subtypes of G protein-coupled receptors. Both short-term purinergic signalling in neurotransmission, neuromodulation and neurosecretion and long-term (trophic) purinergic signalling of cell proliferation, differentiation, motility, death in development and regeneration are recognized. There is now much known about the mechanisms underlying ATP release and extracellular breakdown by ecto-nucleotidases. The recent emphasis on purinergic neuropathology is discussed, including changes in purinergic cotransmission in development and ageing and in bladder diseases and hypertension. The involvement of neuron-glial cell interactions in various diseases of the central nervous system, including neuropathic pain, trauma and ischemia, neurodegenerative diseases, neuropsychiatric disorders and epilepsy are also considered.

Fire history and the Global Charcoal Database: a new tool for hypothesis testing and data exploration

Version 1 of the Global Charcoal Database is now available for regional fire history reconstructions, data exploration, hypothesis testing, and evaluation of coupled climate–vegetation–fire model simulations. The charcoal database contains over 400 radiocarbon-dated records that document changes in charcoal abundance during the Late Quaternary. The aim of this public database is to stimulate cross-disciplinary research in fire sciences targeted at an increased understanding of the controls and impacts of natural and anthropogenic fire regimes on centennial-to-orbital timescales. We describe here the data standardization techniques for comparing multiple types of sedimentary charcoal records. Version 1 of the Global Charcoal Database has been used to characterize global and regional patterns in fire activity since the last glacial maximum. Recent studies using the charcoal database have explored the relation between climate and fire during periods of rapid climate change, including evidence of fire activity during the Younger Dryas Chronozone, and during the past two millennia.

On the evolutionary origins of obesity: a new hypothesis

Obesity is an escalating threat of pandemic proportions, currently affecting billions of people worldwide and exerting a devastating socioeconomic influence in industrialized countries. Despite intensive efforts to curtail obesity, results have proved disappointing. Although it is well recognized that obesity is a result of gene-environment interactions and that predisposition to obesity lies predominantly in our evolutionary past, there is much debate as to the precise nature of how our evolutionary past contributed to obesity. The “thrifty genotype” hypothesis suggests that obesity in industrialized countries is a throwback to our ancestors having undergone positive selection for genes that favored energy storage as a consequence of the cyclical episodes of famine and surplus after the advent of farming 10 000 years ago. Conversely, the “drifty genotype” hypothesis contends that the prevalence of thrifty genes is not a result of positive selection for energy-storage genes but attributable to genetic drift resulting from the removal of predative selection pressures. Both theories, however, assume that selection pressures the ancestors of modern humans living in western societies faced were the same. Moreover, neither theory adequately explains the impact of globalization and changing population demographics on the genetic basis for obesity in developed countries, despite clear evidence for ethnic variation in obesity susceptibility and related metabolic disorders. In this article, we propose that the modern obesity pandemic in industrialized countries is a result of the differential exposure of the ancestors of modern humans to environmental factors that began when modern humans left Africa around 70 000 years ago and migrated through the globe, reaching the Americas around 20 000 years ago. This article serves to elucidate how an understanding of ethnic differences in genetic susceptibility to obesity and the metabolic syndrome, in the context of historic human population redistribution, could be used in the treatment of obesity in industrialized countries

The Usefulness of Caries Frequency, Depth, and Location in Determining Cariogenicity and Past Subsistence: A Test on Early and Later Agriculturalists From the Peruvian Coast

Dental markers have been used to unravel particularities of paleodiet, subsistence, social structure, and health. This article aims to compare oral pathology among four pre-Columbian groups with different degrees of agricultural and socio-cultural development but comparable ecological conditions who lived on the coastal desert of Peru. Three of these groups are assigned to distinct phases of the Formative Period (2500-1 BC), a time critical for our understanding of the development of agriculture and social complexity. The fourth group corresponds to the Late Intermediate Period (1000-1470 AD), when agriculture had its apogee and society was highly stratified. In this study we test whether there is an increase (1) in the frequency of carious lesions and (2) in caries depth, and (3) if there is a shift from occlusal to extra-occlusal caries locations with the development of agriculture. Therefore, we analyze the frequencies of carious lesions and antemortem tooth loss (AMTL), the caries distributions by age, sex, and type of tooth, as well as the tissues affected by, and the location of the carious lesions. Since there are no significant differences in the frequencies of carious lesions and AMTL between the groups, we reject hypothesis 1. In contrast, caries depth does increase, and caries location changes from occlusal to extra-occlusal sites with agricultural development. However, we can only corroborate hypothesis 2 and 3 when taking into consideration dental wear. Thus, we recommend that caries depth and locations should be used with evaluations of dental wear to reconstruct subsistence in ancient populations. Am J Phys Anthropol 143:75-91, 2010. (C) 2010 Wiley-Liss, Inc.

A new perspective on the PPP hypothesis

This paper the stastistical properties of the real exchange rates of G-5 countries for the Bretton-Woods peiod, and draw implications on the purchasing power parity (PPP) hypothesis. In contrast to most previous studies that consider only unit root and stationary process to describe the real exchange tae, this paper also considers two in-between processes, the locally persistent process ans the fractionally integrated process, to complement past studies. Seeking to be consistent with tha ample evidence of near unit in the real exchange rate movements very well. This finding implies that: 1) the real exchange movement is more persistent than the stationary case but less persistent than the unit root case; 2) the real exchange rate is non-stationary but the PPP reversion occurs and the PPP holds in the long run; 3) the real exchange rate does not exhibit the secular dependence of the fractional integration; 4) the real exchange rate evolves over time in a way that there is persistence over a range of time, but the effect of shocks will eventually disappear over time horizon longer than order O (nd), that is, at finite time horizon; 5) shocks dissipation is fasters than predicted by the fractional integracion, and the total sum of the effects of a unit innovation is finite, implying that a full PPP reversion occurs at finite horizons. These results may explain why pasrt empirical estudies could not provide a clear- conclusion on the real exchange rate processes and the PPP hypothesis.

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive nuclide 40K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5–1 billion years within a factor of ∼1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.

Ocean dynamics, not dust, have controlled equatorial Pacific productivity over the past 500,000 years

Biological productivity in the equatorial Pacific is relatively high compared with other low-latitude regimes, especially east of the dateline, where divergence driven by the trade winds brings nutrient rich waters of the Equatorial Undercurrent to the surface. The equatorial Pacific is one of the three principal high-nutrient low chlorophyll ocean regimes where biological utilization of nitrate and phosphate is limited, in part, by the availability of iron. Throughout most of the equatorial Pacific, upwelling of water from the Equatorial Undercurrent supplies far more dissolved iron than is delivered by dust, by as much as two orders of magnitude. Nevertheless, recent studies have inferred that the greater supply of dust during ice ages stimulated greater utilization of nutrients within the region of upwelling on the equator, thereby contributing to the sequestration of carbon in the ocean interior. Here we present proxy records for dust and for biological productivity over the past 500 ky at three sites spanning the breadth of the equatorial Pacific Ocean to test the dust fertilization hypothesis. Dust supply peaked under glacial conditions, consistent with previous studies, whereas proxies of export production exhibit maxima during ice age terminations. Temporal decoupling between dust supply and biological productivity indicates that other factors, likely involving ocean dynamics, played a greater role than dust in regulating equatorial Pacific productivity.

Analysis of air flow past and through the 2415-3S airfoil for an unmanned aerial vehicle with internal propulsion system

This paper deals with the prediction of velocity fields on the 2415-3S airfoil which will be used for an unmanned aerial vehicle with internal propulsion system and in this way analyze the air flow through an internal duct of the airfoil using computational fluid dynamics. The main objective is to evaluate the effect of the internal air flow past the airfoil and how this affects the aerodynamic performance by means of lift and drag forces. For this purpose, three different designs of the internal duct were studied; starting from the base 2415-3S airfoil developed in previous investigation, basing on the hypothesis of decreasing the flow separation produced when the propulsive airflow merges the external flow, and in this way obtaining the best configuration. For that purpose, an exhaustive study of the mesh sensitivity was performed. It was used a non-structured mesh since the computational domain is three-dimensional and complex. The selected mesh contains approximately 12.5 million elements. Both the computational domain and the numerical solution were made with commercial CAD and CFD software, respectively. Air, incompressible and steady was analyzed. The boundary conditions are in concordance with experimental setup in the AF 6109 wind tunnel. The k-e model is utilized to describe the turbulent flow process as followed in references. Results allowed obtaining velocity contours as well as lift and drag coefficients and also the location of separation and reattachment regions in some cases for zero degrees of angle of attack on the internal and external surfaces of the airfoil. Finally, the selection of the configuration with the best aerodynamic performance was made, selecting the option without curved baffles.

Numerical analysis of air flow past the 2415-3S airfoil for an unmanned aerial vehicle with internal propulsion system

This paper deals with the prediction of pressure and velocity fields on the 2415-3S airfoil which will be used for and unmanned aerial vehicle with internal propulsion system and in this way analyze the air flow through an internal duct of the airfoil using computational fluid dynamics. The main objective is to evaluate the effect of the internal air flow past the airfoil and how this affects the aerodynamic performance by means of lift and drag forces. For this purpose, three different designs of the internal duct were studied; starting from the base 2415-3S airfoil developed in previous investigation, basing on the hypothesis of decreasing the flow separation produced when the propulsive airflow merges the external flow, and in this way obtaining the best configuration. For that purpose, an exhaustive study of the mesh sensitivity was performed. It was used a non-structured mesh since the computational domain is tridimensional and complex. The selected mesh contains approximately 12.5 million elements. Both the computational domain and the numerical solution were made with commercial CAD and CFD software respectively. Air, incompressible and steady was analyzed. The boundary conditions are in concordance with experimental setup in the AF 6109 wind tunnel. The k-ε model is utilized to describe the turbulent flow process as followed in references. Results allowed obtaining pressure and velocity contours as well as lift and drag coefficients and also the location of separation and reattachment regions in some cases for zero degrees of angle of attack on the internal and external surfaces of the airfoil. Finally, the selection of the configuration with the best aerodynamic performance was made, selecting the option without curved baffles.