Time: Astronomical time - atomic time - redefining second based on constant

Abstract

The evolution history of modern time from the astronomical time to the atomic second is briefly reviewed herein. The progress and status of realizing the current definition of the second using cesium cold atomic fountain primary frequency standards, the process of generating the international atomic time (TAI) and coordinated universal time (UTC), and trend of releasing the tolerance on inserting leap-seconds into the UTC are sketched. The present situation on steering the TAI regularly from January 2016 by cesium fountain frequency standards from five countries is reported. Research progress of local and global optical frequency standards is summarized, and steering TAI by six optical frequency standards starting from 2016 is reported. Requirements and responses on steering TAI regularly by optical frequency standards are introduced according to the roadmap, which is under discussion internationally, or the future redefinition of the second. Furthermore, two types of optical frequency standards, namely the basic-research and the metrological type are suggested, and the performance and technical qualifications for the metrological type of optical frequency standards are recommended. For the optical frequency standards of the metrological type, with uncertainties better than 3×10−17, not only static relative gravity but also the alternative tide frequency shifts should be considered in their frequency evaluations. Finally, a scheme employing frequency comparison, across a long longitude-distance, between two highly stable optical frequency standards to demonstrate the relativistic tidal frequency shift directly and experimentally is proposed.