L Essen-J Parry 1955 cesium-atomic-clock; modern modern foundational text + 1967 SI-second + 2024 Sr-optical-clock 10^-19 surpasses Cs.
L Essen-J Parry 1955 cesium-atomic-clock; modern modern foundational text + 1967 SI-second + 2024 Sr-optical-clock 10^-19 surpasses Cs.