Phys. Rev. Lett. (in the press); preprint at

Special relativity assumes that laws of physics are the same in all reference frames, a principle known as Lorentz invariance. This principle has been subject to numerous experimental tests, but no sign of Lorentz violation has yet been spotted: either a reassuring or disappointing revelation, depending on your stance. These results are now reinforced by a new test using a fibre network of optical clocks, which pushes the existing bound on Lorentz violation in experiments measuring time dilation.

Pacôme Delva and colleagues used strontium optical lattice clocks located at the LNE-SYRTE, Observatoire de Paris in France, the National Metrology Institute in Germany and the National Physical Laboratory in the UK and connected via state-of-the-art optical fibre links. Looking at the frequency difference between the clocks, they were able to test whether time dilation varies between the reference frames of the three geographically remote locations. This approach improves on previous tests — including other atomic clock comparison experiments — by two orders of magnitude. Moreover, it is only limited by technical noise sources, so further improvements are certainly possible.