Testing Einstein's time dilation under acceleration using M\"ossbauer spectroscopy

The Einstein time dilation formula was tested in several experiments. Many trials have been made to measure the transverse second order Doppler shift by M\"{o}ssbauer spectroscopy using a rotating absorber, to test the validity of this formula. Such experiments are also able to test if the time dilation depends only on the velocity of the absorber, as assumed by Einstein's clock hypothesis, or the present centripetal acceleration contributes to the time dilation. We show here that the fact that the experiment requires $\gamma$-ray emission and detection slits of finite size, the absorption line is broadened; by geometric longitudinal first order Doppler shifts immensely. Moreover, the absorption line is non-Lorenzian. We obtain an explicit expression for the absorption line for any angular velocity of the absorber. The analysis of the experimental results, in all previous experiments which did not observe the full absorption line itself, were wrong and the conclusions doubtful. The only proper experiment was done by K\"{u}ndig (Phys. Rev. 129 (1963) 2371), who observed the broadening, but associated it to random vibrations of the absorber. We establish necessary conditions for the successful measurement of a transverse second order Doppler shift by M\"{o}ssbauer spectroscopy. We indicate how the results of such an experiment can be used to verify the existence of a Doppler shift due to acceleration and to test the validity of Einstein's clock hypothesis.Comment: 11 pages, 4 figure

Identification of potentially dangerous glacial lakes in the northern Tien Shan

Like in many other parts of the world, the glaciers in northern Tien Shan are receding, and the permafrost is thawing. Concomitantly, glacial lakes are developing. Historically, outbursts of these glacial lakes have resulted in severe hazards for infrastructures and livelihood. Multi-temporal space imageries are an ideal means to study and monitor glaciers and glacial lakes over large areas. Geomorphometric analysis and modelling allows to estimate the potential danger for glacial lake outburst floods (GLOFs). This paper presents a comprehensive approach by coupling of remote sensing, geomorphometric analyses aided with GIS modelling for the identification of potentially dangerous glacial lakes. We suggest a classification scheme based on an additive ratio scale in order to prioritise sites for detailed investigations. The identification and monitoring of glacial lakes was carried out semi-automatically using band ratioing and the normalised difference water index (NDWI) based on multi-temporal space imagery from the years 1971 to 2008 using Corona, ASTER and Landsat data. The results were manually edited when required. The probability of the growth of a glacial lake was estimated by analysing glacier changes, glacier motion and slope analysis. A permafrost model was developed based on geomorphometric parameters, solar radiation and regionalised temperature conditions which permitted to assess the influence of potential permafrost thawing. Finally, a GIS-based model was applied to simulate the possibly affected area of lake outbursts. The findings of this study indicate an increasing number and area of glacial lakes in the northern Tien Shan region. We identified several lakes with a medium to high potential for an outburst after a classification according to their outburst probability and their downstream impact. These lakes should be investigated more in detail

Multi-image 3D reconstruction: A photogrammetric and structure from motion comparative analysis

Virtual Web Reconstruction of cultural heritage is one of the most interesting and innovative tool to preserve historical, architectural and artistic memory of many sites, particularly the ones prone to disappear, as well as to promote territories and tourism development. Recently, high-resolution 3D models are realized through improved technology and integration of survey techniques such as laser scanning and photogrammetry. However, the large and complex volume of 3D data makes difficult to access and handle them for either experts and citizens. In particular, the rendering of large 3D models may influence the performance of web publication and browsing. Considering this background, the goal of this paper is the comparison between the level of accuracy and realism of 3D models optimized using two different mesh simplification. The metric used is based on the Hausdorff distance which is a generic technique to define a distance between two nonempty sets, considering 3D scanner mesh as a reference in the measure. The “Casale di Pacciano” near Bisceglie (Apulia region, Italy), has been investigated as study case

Structure-From-Motion Photogrammetry to Support the Assessment of Collapse Risk in Alpine Glaciers

The application of Structure-from-Motion (SfM) Photogrammetry with ground-based and UAV camera stations may be exploited for modelling the topographic surface of Alpine glaciers. Multi-temporal repeated surveys lead to geometric models that may be applied to analyze the glacier retreat under global warming conditions. Thanks to the integration of point clouds obtained from ground-based and UAV imaging platforms, a complete 3D reconstruction also including vertical and sub-vertical surfaces may be achieved. These 3D models may be also exploited to understand the precursory signals of local collapse that might represent a risk for tourists and hikers visiting glaciers. In this paper a review on the application of SfM Photogrammetry in the field of glaciological studies is reported. The case of Forni Glacier in the Italian Alps is presented as emblematic study. Photogrammetric data sets obtained from measurement campaigns carried out in 2014, 2016, 2017 and 2018 have been processed using a common workflow. Attention is paid to a few crucial aspects, such as image orientation and calibration, dense surface matching, georeferencing and data fusion. In the end, the use of output point clouds to evaluate the risk of collapse in the Forni Glacier is addressed