Abstract: |
Atomic clocks are used in various applications such as satellite navigation systems, high-speed communications and basic science. In the International System of Units (SI), one second (1 s) is determined based on a cesium atomic clock with an uncertainty of 5 x 10^-16. This corresponds to an error of 1 s in 60 million years. Recent atomic clocks have improved this uncertainty by 100 times. The optical lattice clock proposed in Japan (Katori, 2002) achieves an uncertainty of 1 x 10^-17 to 1 x 10^-18. Advances in the accuracy of atomic clocks have enabled practical applications to geodesy. One of the main goals of geodesy is to establish the geoid (an equipotential surface of the Earth’s gravity that agrees with the mean sea level) to serve as a height reference. According to the theory of general relativity, the length of 1 s (proper time) decreases as the gravitational potential increases at the measurement site. Near the surface of the Earth, the passage of time accelerates by 1 x 10^-18 for every 1-cm increment in altitude. Using this effect, a more precise determination of the geoid using atomic clocks in combination with traditional geodetic methods is under development in Europe. In Japan, a combination of optical lattice clocks and GNSS is proposed to improve the performance of crustal deformation monitoring. In this presentation, I will overview the roles of geodesy and introduce temporal gravity changes near the Earth’s surface due to representative geophysical phenomena as well as their measurement methods, from a point of view of classical mechanics. Next I will report the progress of chronometric geodesy using optical clocks based on the gravitational red shift in Europe and Japan. It will be understood that clocks serve as a complementary tool to observe gravity field variations in the realm of geodesy and geophysics, rather than as stand-alone instruments. Finally, I will mention a few geodetic topics related to relativity. Reference: Mehlstäubler et al 2018, Atomic clocks for geodesy, Rep. Prog. Phys. 81, 064401 |