Thanks to highly stabilised laser systems we can now make measurements with very great accuracy. For example, Hall has been able to verify what the theory of relativity predicts – that light has the same speed in all directions – with a degree of accuracy never achieved before. The frequency comb technique enables us to go even further; we can investigate whether the fundamental constants of nature really are constant. We can still sleep soundly at night because the measurements that have been made so far show that they are constant – with an accuracy of 15 digits. But the future may have surprises in store for us when the accuracy has become even greater.
Theodor W. Hänsch Max-Planck-Institut für Quanten optik, Garching and Ludwig-Maximilians- Universität, München, Germany
John L. Hall JILA, University of Colorado and National Institute of Standards and technology, Boulder, CO, USA.