2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/104024A test of Lorentz invariance for electromagnetic waves was performed by comparing the resonance frequencies of two stable optical resonators as a function of orientation in space. The crystalline resonators were operated at 3.4 K in a cryostat employing a pulse-tube refrigerator. A new analysis yields the Robertson-Mansouri-Sexl theory parameter combination $β-δ-1/2=(-0.6\pm 2.1\pm1.2)\cdot10^{-10}$ and one parameter of the Standard Model Extension theory, $(\tildeκ_{e-})^{ZZ} = (-2.9\pm2.2)\cdot 10^{-14}$.15 pages, 9 figures. Improved analysis of the paper Antonini et al, Phys. Rev. A 71, 050101 (2005). To appear in: "Special Relativity: will it hold another 100 years", Springer Lect. Notes in Physics, J. Ehlers and C. Lämmerzahl, eds. (2006)Classical PhysicsOpticstext