A test of special relativity with stored lithium ions

Laser spectroscopy at the heavy ion storage ring TSR in Heidelberg allows for precision experiments testing the limits of the special theory of relativity. With an optical Λ-type three-level system of⁷Li⁺ the Doppler shift has been measured by saturation spectroscopy as a test of the time dilatation factor γ = (1 - β ²)^-1/2 at an ion velocity of υ = 6.4% c. A precision of ∆ν/ν < 9 × 10^-9 has been obtained, which sets a second-order limit of 1.1 × 10^-6 for any deviation from the time dilatation factor. The fourth-order limit of this deviation is set below 2.7 × 10^-4 by the present experiment. These limits are given at a 1 σ confidence level.