Test of relativistic time dilation with fast optical atomic clocks at different velocities

Nature Physics. Published online: 11 November 2007 | doi:10.1038/nphys778

Moving clocks are observed to tick more slowly. This effect known as time dilation is one of the most fascinating aspects of Special Relativity as it abolishes the notion of absolute time. In 1907, Einstein proposed to verify the existence of time dilation using the Doppler effect — the frequency shift perceived by an observer moving relative to a source of radiation, realized in 1938 by Ives and Stilwell. Here we report on a new experiment with fast optical atomic clocks testing relativistic time dilation with unprecedented precision, combining cutting-edge ion storage and cooling techniques with frequency counting in the optical domain using a frequency comb. 7Li+ ions with a suitable clock transition are prepared at two different velocities in a storage ring. Their time is read with an accuracy of 2 x 10-10 using laser saturation spectroscopy and referenced to a clock in the lab frame. The comparison of relativistic Doppler shifts from ions at 6.4% and 3.0% of the speed of light yields a self-contained measurement of time dilation represented by a Mansouri-Sexl parameter |alpha| < 8.4 x 10-8 and is consistent with Special Relativity. This result puts constraints on the existence of a preferred cosmological reference frame such as the cosmic microwave background frame and yields unique limits on CPT & Lorentz violating ‘new’ physics beyond the Standard Model of particle physics, emerging for example from string theory. Proper inclusion of time dilation is critical to the accuracy of the global positioning system GPS, and we provide the only independent test that is more sensitive than those derived from GPS itself, making our work not only relevant to relativity, cosmology and particle physics, but also to ubiquitous technology.