The Measurement of Time in the 21st Century

Christophe Salomon

Laboratoire Kastler Brossel
Ecole Normale Superieure - Department de Physique

We will first review the recent progress in precision time measurements. Atomic clocks using laser cooled atoms realize the definition of the SI second with a relative accuracy of 3 x 10^-16 thanks to a number of atomic fountains worldwide. Optical clocks have recently reached a frequency stability and accuracy in the 10^-18 range opening new perspectives for time keeping and fundamental tests.

In a second part we will describe tests of fundamental physical laws using ultra-stable clocks in space and on the ground. An improved measurement of Einstein's gravitational red-shift will be made with a precision of two parts per million using the space mission ACES that will carry in orbit a cold atom cesium clock. By comparing clocks of different nature at the 10^-17/year level, new limits are obtained for the time variation of fundamental constants of physics such as the fine structure constant alpha and the ratio of electron to proton mass. The ability to compare microwave and optical clocks using the recently developed frequency comb technique and optical fibers opens a wide range of possibilities in clock comparisons. Finally a new kind of relativistic geodesy based on the Einstein effect will provide information on the Earth geoid, complementing the recent determinations obtained by space geodesy methods.