Atomic time, clocks, and clock comparisons in relativistic spacetime: a review

International audienc

Les enjeux de la narration dans l’immersion 3D archéologique

International audienceDigital documentation tools, helped by modelling, underscoring and animation techniques, allow us to recreate environments and objects that may be either partially degraded or inaccessible, with great geometric precision. This helps researchers with contextualisation, and for the public, this is an opportunity to access distant sites or sites with limited access, and to discover information that has already been interpreted.One of the first questions related to the 3D mediation of these environments is the matter of the display device. It may be a generic device, but more and more specific devices are being developed, raising many questions about the articulation between the content being displayed and the shape of the structure, way beyond the mere scenographic consideration.Of all these devices, the immersive devices offer the most interesting content. They let us experience the archaeological sites with their full context, whether they are outdoors or in enclosed spaces. Their conception immediately underlines narrative issues: chosen temporality, the processing of insufficient data, the insertion of live scenes and the treatment of atmospheres.Practice shows that, to solve those narrative issues in an immersive context, the classical visual or filmic grammar will not be sufficient. We must develop a 360° narrative grammar, a properly immersive grammar. By starting from a concrete realisation, currently in development, linked to the study of parietal arts and a corpus of references ranging from panoramas to digital tools, we will describe the constitutive parts of that 360° narrative grammar that pushes the archaeological representation beyond the descriptive visualisation and questions the relationship between the produced discourse and its visual transcription.Les outils de relevé numériques, prolongés par les techniques de modélisation, de mise en lumière et d’animation, permettent de restituer des environnements et des objets soit partiellement dégradés, soit inaccessibles, avec une grande précision géométrique. Pour le chercheur, c’est une aide à la contextualisation, pour le public, c’est la possibilité d’avoir accès à des sites parfois distants ou d’accès limité, et de découvrir de l’information déjà interprétée. Cet article cherche à identifier les enjeux de médiation de cette production scientifique. La médiation de ces environnements en 3D pose incidemment, dans un premier temps, la question du dispositif de présentation, puis dans un deuxième temps du contexte et des environnements de présentation. Notre propos interroge donc, en le référençant dans l’histoire des technologies immersives, l’articulation entre le fond présenté et la forme de la structure, et ce bien au-delà de la seule considération scénographique.Parmi les dispositifs de médiation, les dispositifs immersifs offrent les enjeux les plus stimulants. Considérés comme les derniers nés des technologies numériques, ils sont en fait les héritiers d’une longue généalogie passant par le cinéma multi-écran, les planétariums, les panoramas, les coupoles peintes et peut-être, quelques exemples de peintures pariétales. Aujourd’hui, les dispositifs immersifs donnent à expérimenter les sites archéologiques dans leur contexte, qu’ils soient à ciel ouvert ou dans des espaces clos. Leur conception fait d’emblée surgir des enjeux narratifs : la temporalité choisie, la gestion des données lacunaires, l’insertion de scènes du vivant, le traitement des ambiances. La pratique montre que pour répondre à ces enjeux narratifs on ne saurait se contenter, en contexte immersif, de recourir à la grammaire visuelle ou filmique classique. Il convient de développer une grammaire narrative à 360°, une grammaire proprement immersive. En partant d’une réalisation concrète en cours de développement, liée à l’étude pariétale, et d’un corpus de références allant des panoramas aux outils numériques, nous décrirons donc les constituants de cette grammaire narrative à 360° et essayerons de montrer comment dès lors la forme construite d’un dispositif peut être en cohérence avec le contenu narratif. Cette réflexion pousse la représentation en archéologie au-delà de la visualisation descriptive et questionne les relations entre les discours produits et leurs transcriptions visuelles

Geodetic methods to determine the relativistic redshift at the level of 10−18 in the context of international timescales: a review and practical results

International audienceThe frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in 1018 level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an accuracy of about 0.1m2s−2, equivalent to about 0.01 m in height. This contribution focuses on the static part of the gravity field, assuming that temporal variations are accounted for separately by appropriate reductions. Two geodetic approaches are investigated for the derivation of gravity potential values: geometric levelling and the Global Navigation Satellite Systems (GNSS)/geoid approach. Geometric levelling gives potential differences with millimetre uncertainty over shorter distances (several kilometres), but is susceptible to systematic errors at the decimetre level over large distances. The GNSS/geoid approach gives absolute gravity potential values, but with an uncertainty corresponding to about 2 cm in height. For large distances, the GNSS/geoid approach should therefore be better than geometric levelling. This is demonstrated by the results from practical investigations related to three clock sites in Germany and one in France. The estimated uncertainty for the relativistic redshift correction at each site is about 2×10−18

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed

Remote Optical and Fountain Clock Comparison Using BroadbandTWSTFT and GPS PPP

International audienceVarious Yb ion, Sr lattice and Cs fountain clocks at INRiM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a 3-week campaign using satellite-based techniques. With full 20 MChip/s modulation bandwidth TWSTFT links and PPP/iPPP analysis of GPS link data,instabilities of a few 1E-16 could be achieved for both techniques. For the calculation of the clock-to-clock ratios and their uncertainties, gap-tolerant weighted averaging with differentoptimized weighting functions was employed