Comparisons between Atomic Clocks (IMAGE) Physikalisch-Technische Bundesanstalt (PTB) Caption Comparisons between atomic clocks with cesium and ytterbium, respectively, confirm the constancy of the mass ratio of protons to electrons. Credit (Fig.: PTB) Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.