12. Apr. 2018, 16:15 Uhr, Gebäude NW1, Raum H3 evtl. H2

## Physical Units Based on Fundamental Constants - Changing with Time?

Prof. Dr. Joachim Ullrich, PrĂ¤sident Physikalisch-Technische Bundesanstalt (PTB) Braunschweig

In 2018, on the occasion of the 25th meeting of the General Conference on Weights and Measures, CGPM, of the Metre Convention founded in 1875, it is envisaged to redefine the International System of Units (SI). In the future, as outlined by Max-Planck in his famous paper of 1900 postulating the 'Planck constant', it shall be based on fundamental constants of nature, the 'defining constants': the velocity of light, the charge of the electron, the Boltzmann, Avogadro and the Planck constants, the Cs hyperfine clock transition and the luminous efficacy.
In the talk I will provide an overview on the progress, challenges and future perspectives of the new 'Quantum SI', illustrated in Fig. 1, and discuss the question on whether or not the fundamental constants are indeed constant in time. New experiments are presently being devised, one of them based on next-generation optical clocks using transitions in highly charged ions that are read out via quantum-logic schemes. They bear the chance to trace potential changes in the fine structure constant Alpha on the level of Delta Alpha/Alpha≈10^20 per year.