Research Domains

Fundamental Gravity Theory − Applied General Relativity − Relativistic Geodesy − Relativistic Reference Frames − Global Gravity Field − Optical Clocks − Optical Fibers – Radio and Laser Links in Space− Time and Frequency Transfer

Soon after the advent of atomic clocks more than 50 years ago, their unprecedented uncertainties and widespread use required that the definition of time scales and clock comparison procedures be considered rigorously within the theoretical framework of general relativity. Today, the most advanced optical atomic clocks have reached levels of stability and accuracy that significantly surpass the performance of cesium primary standards, for which the best reported results are a stability of 1.4 x 10-14  τ-1/2 where τ is the averaging time, and an accuracy of  1.1 x 10-16 [Heavner et al., 2014]. Presently, optical clocks, based on ions or on neutral atoms, exhibit an accuracy of a few parts in 10-18 , and neutral lattice clocks achieve a stability down to a few parts in 10-18 in a few hours (Schioppo et al. 2017). The workshop team will discuss the reproducibility of optical clocks measurements coming from independent absolute frequency comparisons made in different laboratories, and how to achieve the remote comparison of clocks in consistency with their challenging accuracy level.

The team will also review the present procedures and analyze various aspects of the current relativistic modelling to ensure that it is adequate for the need of clocks and time transfer techniques. Accounting for the expected – in near future – improvements in clock and time transfer technology, the team will investigate possible implications in relation to the fundamental physics of relativistic effects influencing time and frequency comparisons between clocks at the10-18 level of accuracy, and beyond. The team will also explore how to establish a connection of time metrology to geodetic models of Earth’s gravitational field in order to describe the variation of the clock frequencies due to changes in the gravity potential. High accuracy optical atomic clocks are expected to have a huge impact on the field of Earth science such as geophysics, geodesy, navigation, etc. by using them to measure the gravity potential difference between two, well-defined locations with high temporal (daily) and geospatial resolution at the clock locations.  This new way to determine the gravity potential will help us to define the geoid [Philipp et al. 2017] and other geodetic parameter needed to describe the gravitational field and other properties of the Earth on a fully relativistic basis, to establish and connect regional and global height systems, to resolve their present discrepancies and to improve gravity field recovery on Earth’s surface and in space. On the long run Relativistic Geodesy will lead to novel services in geodetic and geophysical monitoring.

The workshop goal is to establish a tight connection between the theory and practice of precise space geodesy, spacetime metrology, atomic clocks and optical fibers network. This is a rapidly developing new branch of the fundamental physics of time and space with a wide range of applications from geophysics/geodesy – through realization of the next generation of terrestrial reference frames – to the most fundamental theory of gravity – general relativity. The workshop continues developments of the topics which were intensively discussed over a week 30 Nov – 4 Dec 2015 at a major ISSI/HISPAC workshop on “High Performance Clocks with Special Emphasis on Geodesy and Geophysics, and Applications to other bodies of the Solar System” [Müller et al. 2017].

Project Goals

  • Calculate all relativistic effects within the clock networks on Earth and in space
  • Investigate new and, likely, more powerful and effective concepts for height determination over long distances or in remote areas (islands) based on terrestrial clock networks (and with clocks in space)
  • Develop the geodetic modeling of time-dependent gravity signals, i.e. modeling of all relevant mass variations (including rotational and tidal effects) arising in the continuous measurement of time in clock networks
  • Develop relativistic models of reference ellipsoid, quasi-geoid, and normal gravity field as well as some important applications of general relativity for measuring the gravitomagnetic effects
  • Quantify the benefit of including clock measurements on ground and in space for gravity field recovery
  • Study and analyze how high precision clocks can be used for geophysics, planetary sciences as well as for new high-precision tests of special and general relativity