Can an observer really catch up with light

Given a null geodesic $\gamma_0(\lambda)$ with a point $r$ in $(p,q)$ conjugate to $p$ along $\gamma_0(\lambda)$, there will be a variation of $\gamma_0(\lambda)$ which will give a time-like curve from $p$ to $q$. This is a well-known theory proved in the famous book\cite{2}. In the paper we prove that the time-like curves coming from the above-mentioned variation have a proper acceleration which approaches infinity as the time-like curve approaches the null geodesic. This means no observer can be infinitesimally near the light and begin at the same point with the light and finally catch the light. Only separated from the light path finitely, does the observer can begin at the same point with the light and finally catch the light.Comment: 6 pages, no figures, submited to Physical Review

Contribution of increasing CO2 and climate change to the carbon cycle in China\u27s ecosystems

Atmospheric CO2 and China\u27s climate have changed greatly during 1961–2000. The influence of increased CO2 and changing climate on the carbon cycle of the terrestrial ecosystems in China is still unclear. In this article we used a process-based ecosystem model, Biome-BGC, to assess the effects of changing climate and elevated atmospheric CO2 on terrestrial China\u27s carbon cycle during two time periods: (1) the present (1961–2000) and (2) a future with projected climate change under doubled CO2 (2071–2110). The effects of climate change alone were estimated by driving Biome-BGC with a fixed CO2 concentration and changing climate, while the CO2 fertilization effects were calculated as the difference between the results driven by both increasing CO2 and changing climate and those of variable climate alone. Model simulations indicate that during 1961–2000 at the national scale, changes in climate reduced carbon storage in China\u27s ecosystems, but increasing CO2 compensated for these adverse effects of climate change, resulting in an overall increase in the carbon storage of China\u27s ecosystems despite decreases in soil carbon. The interannual variability of the carbon cycle was associated with climate variations. Regional differences in climate change produced differing regional carbon uptake responses. Spatially, reductions in carbon in vegetation and soils and increases in litter carbon were primarily caused by climate change in most parts of east China, while carbon in vegetation, soils, and litter increased for much of west China. Under the future scenario (2071–2110), with a doubling CO2, China will experience higher precipitation and temperature as predicted by the Hadley Centre HadCM3 for the Intergovernmental Panel on Climate Change Fourth Assessment. The concomitant doubling of CO2 will continue to counteract the negative effects of climate change on carbon uptake in the future, leading to an increase in carbon storage relative to current levels. This study highlights the role of CO2 fertilization in the carbon budget of China\u27s ecosystems, although future studies should include other important processes such as land use change, human management (e.g., fertilization and irrigation), environmental pollution, etc

Multi-frequency and multi-attribute GPR data fusion based on 2-D wavelet transform

High frequency GPR signals offer high resolution while low frequency GPR signals offer greater depth of penetration. Effective fusion of multiple frequencies can combine the advantages of both. In addition, GPR attribute analysis can improve subsurface imaging, but a single attribute can only partly highlight details of different physical and geometrical properties of subsurface potential targets. In order to overcome these challenges, we implement an advanced multi-frequency and multi-attribute GPR data fusion approach based on 2-D wavelet transform utilizing a dynamic fusion weight scheme derived from edge detection algorithm, which is tested on data from a small glacier in the north-eastern Alps by 250 & 500 MHz central frequency antennas. Besides, information entropy and spatial frequency are developed as quantitative evaluation parameters to analyze the fusion outcomes. The results demonstrate that the proposed approach can enhance the efficiency and scope of GPR data interpretation in an automatic and objective way

Evaluating water stress controls on primary production in biogeochemical and remote sensing based models

Water stress is one of the most important limiting factors controlling terrestrial primary production, and the performance of a primary production model is largely determined by its capacity to capture environmental water stress. The algorithm that generates the global near-real-time MODIS GPP/NPP products (MOD17) uses VPD (vapor pressure deficit) alone to estimate the environmental water stress. This paper compares the water stress calculation in the MOD17 algorithm with results simulated using a process-based biogeochemical model (Biome-BGC) to evaluate the performance of the water stress determined using the MOD17 algorithm. The investigation study areas include China and the conterminous United States because of the availability of daily meteorological observation data. Our study shows that VPD alone can capture interannual variability of the full water stress nearly over all the study areas. In wet regions, where annual precipitation is greater than 400 mm/yr, the VPD-based water stress estimate in MOD17 is adequate to explain the magnitude and variability of water stress determined from atmospheric VPD and soil water in Biome-BGC. In some dry regions, where soil water is severely limiting, MOD17 underestimates water stress, overestimates GPP, and fails to capture the intraannual variability of water stress. The MOD17 algorithm should add soil water stress to its calculations in these dry regions, thereby improving GPP estimates. Interannual variability in water stress is simpler to capture than the seasonality, but it is more difficult to capture this interannual variability in GPP. The MOD17 algorithm captures interannual and intraannual variability of both the Biome-BGC-calculated water stress and GPP better in the conterminous United States than in the strongly monsoon-controlled China

The relationship between lower-stratospheric ozone at southern high latitudes and sea surface temperature in the East Asian marginal seas in austral spring

Using satellite observations, reanalysis data, and model simulations, this study investigates the effect of sea surface temperature (SST) on interannual variations of lower-stratospheric ozone at southern high latitudes in austral spring. It is found that the SST variations across the East Asian marginal seas (5° S–35° N, 100–140° E) rather than the tropical eastern Pacific Ocean, where ENSO occurs, have the most significant correlation with the southern high-latitude lower-stratospheric ozone changes in austral spring. Further analysis reveals that planetary waves originating over the marginal seas in austral spring can propagate towards southern middle to high latitudes via teleconnection pathway. The anomalous propagation and dissipation of ultra-long Rossby waves in the stratosphere strengthen/cool (weaken/warm) the southern polar vortex, which produces more (less) active chlorine and enhances (suppresses) ozone depletion in the southern high-latitude stratosphere on one the hand and impedes (favors) the transport of ozone from the southern middle-latitude stratosphere to high latitudes on the other. The model simulations also reveal that approximately 17% of the decreasing trend in the southern high-latitude lower-stratospheric ozone observed over the past 5 decades may be associated with the increasing trend in SST over the East Asian marginal seas

Interaction of Individual Skyrmions in Nanostructured Cubic Chiral Magnet

We report the direct evidence of field-dependent character of the interaction between individual magnetic skyrmions as well as between skyrmions and edges in B20-type FeGe nanostripes observed by means of high resolution Lorentz transmission electron microscopy. It is shown that above certain critical values of external magnetic field the character of such long-range skyrmion interactions change from attraction to repulsion. Experimentally measured equilibrium inter-skyrmion and skrymion-edge distances as function of applied magnetic field shows quantitative agreement with the results of micromagnetic simulations. Important role of demagnetizing fields and internal symmetry of three-dimensional magnetic skyrmions are discussed in details.Comment: accepted in PR