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In a didactic way, we obtain the Lorentz's transformations without using the speed of light constancy postulate which is the most difficult to understand. The postulate of the constancy of the speed of light is obtained as a consequence of Eisntein's principle of relativity and the properties of space-time. We show that the Lorentz transformations, obtained from the properties of space-time and Eisten's principle of relativity, form a continuous group of symmetry of the system and as a direct consequence the postulate of the constancy of the velocity of the light. [ABSTRACT FROM AUTHOR]

In a view to explore blood vessels neo formation as a possible adaptive response to hypobaric hypoxia in the chicken lung, mRNA expression of the following genes was examined: hypoxia inducible factor-1α (HIF-1α) and -2α (HIF-2α), vascular endothelial growth factor (VEGF), receptors for VEGF (Flk- 1 and Flt-1), hepatocyte growth factor (HGF) and its receptor (HGFR), epidermal growth factor (EGF) and its receptor (EGFR). The molecular regulation of these genes remains completely unknown in these conditions. One group of birds was exposed to relative normoxia and the other group, to hypobaric hypoxia. Comparisons were made among non-pulmonary hypertensive chickens under relative normoxic conditions (NHCN) and non-pulmonary hypertensive chickens (NHCH) and pulmonary hypertensive chickens (PHCH) under hypobaric hypoxic conditions, respectively. For HIF-2α, VEGF, Flk-1, HGF and HGFR genes, no statistical differences were found in the relative mRINA expression between PHCH and NHCH, but those were lower when compared to NHCN (P<0,05). For Flt-1 mRNA expression, a significant difference between NHCH and NHCN was noted (P<0.05), but relative mRNA expression in the lungs of PHCH did not show differences with findings in the lungs of NHCN or NHCH. HIF-1α and EGFR mRNA expression analysis indicated significant differences between NHCN and PHCH, but not between NHCH and PHCH. There was a general trend towards a lower mRNA expression of genes involved in the response to hypoxia and vascular proliferation in the lungs of chickens exposed to chronic hypoxia in PHCH and NHCH when compared with NHCN. These results show consistency with those registered in a parallel study which indicated a lower number of blood vessels with a diameter range ≥50-100μm in the lungs of PHCH and NHCH when compared with observations obtained in NHCN. [ABSTRACT FROM AUTHOR]

This paper evaluates the consistency of the Normalized Difference Vegetation Index (NDVI) records derived from Advanced Very High Resolution Radiometer (AVHRR), SPOT-Vegetation, SeaWiFS, Moderate Resolution Imaging Spectroradiometer, and Landsat ETM+. We used independently derived NDVI from atmospherically corrected ETM+ data at 13 Earth Observation System Land Validation core sites, eight locations of drought, and globally aggregated one-degree data from the four coarse resolution sensors to assess the NDVI records agreement. The objectives of this paper are to: 1) compare the absolute and relative differences of the vegetation signal across these sensors from a user perspective, and, to a lesser degree, 2) evaluate the possibility of merging the AVHRR historical data record with that of the more modern sensors in order to provide historical perspective on current vegetation activities. The statistical and correlation analyses demonstrate that due to the similarity in their overall variance, it is not necessary to choose between the longer time series of AVHRR and the higher quality of the more modern sensors. The long-term AVHRR-NDVI record provides a critical historical perspective on vegetation activities necessary for global change research and, thus, should be the basis of an intercalibrated, sensor-independent NDVI data record. This paper suggests that continuity is achievable given the similarity between these datasets. [ABSTRACT FROM AUTHOR]

Vegetation seasonality assessment through remote sensing data is crucial to understand ecosystem responses to climatic variations and human activities at large-scales. Whereas the study of the timing of phenological events showed significant advances, their recurrence patterns at different periodicities has not been widely study, especially at global scale. In this work, we describe vegetation oscillations by a novel quantitative approach based on the spectral analysis of Normalized Difference Vegetation Index (NDVI) time series. A new set of global periodicity indicators permitted to identify different seasonal patterns regarding the intra-annual cycles (the number, amplitude, and stability) and to evaluate the existence of pluri-annual cycles, even in those regions with noisy or low NDVI. Most of vegetated land surface (93.18%) showed one intra-annual cycle whereas double and triple cycles were found in 5.58% of the land surface, mainly in tropical and arid regions along with agricultural areas. In only 1.24% of the pixels, the seasonality was not statistically significant. The highest values of amplitude and stability were found at high latitudes in the northern hemisphere whereas lowest values corresponded to tropical and arid regions, with the latter showing more pluri-annual cycles. The indicator maps compiled in this work provide highly relevant and practical information to advance in assessing global vegetation dynamics in the context of global change. [ABSTRACT FROM AUTHOR]

Abstract: We compared tree-ring width (TRW) and maximum latewood density (MXD) chronologies to remotely sensed indices of productivity (NDVI) and snowmelt since 1981 and to the instrumental temperature record at four arctic treeline sites in North America. Our results show that at these sites, TRW chronologies reflect temperatures less consistently than the MXD chronologies do and that the NDVI does not correlate significantly with TRW at high-frequency, i.e. when comparing yearly values. In contrast, the MXD chronologies correlate positively and significantly with NDVI and temperature during the growing season at all sites. Neither TRW or MXD chronologies appeared consistently influenced by the annual timing of snowmelt. A comparison of tree-ring chronologies and temperatures since 1900 confirms that MXD has tracked growing season temperature at these treeline sites throughout the past century. A spatial evaluation of the correlations further reveals that each of the MXD chronologies investigated here reflects interannual variation in NDVI and growing season temperatures across a large geographic region. As a result, they collectively provide a spatially comprehensive record of historic early-season canopy status as well as growing season temperatures for the high latitudes of North America. [Copyright &y& Elsevier]