Improved Kennedy-Thorndike experiment to test special relativity

We have carried out a modern version of the Kennedy-Thorndike experiment by searching for sidereal variations between the frequency of a laser locked to an ${\mathit{l}}_2$ reference line and a laser locked to the resonance frequency of a highly stable cavity. No variations were found at the level of 2×${10}^{\mathrm{-}13}$. This represents a 300-fold improvement over the original Kennedy-Thorndike experiment and allows the Lorentz transformations to be deduced entirely from experiment at an accuracy level of 70 ppm.