Drifting Fundamental Constants
By comparing data taken in 2003 [18] with earlier measurements in 1999 [15] we can set an upper limit on the variation of the 1S-2S transition frequency of (-29 ± 57) Hz within 44 months. To derive limits on the drift rates of fundamental constant such as the fine structure constant, we combine these measurements with other optical frequency measurements in Hg+ and in Yb+ performed at NIST, Boulder/USA [19] and at PTB, Braunschweig/Germany [20] respectively. This combined method gives precise and separate restrictions for the fractional time variation of the fine structure constant and the Cs nuclear magnetic moment measured in Bohr magnetons. The latter is a measure of the drift rate of the strong interaction.