Direct Terrestrial Test of Lorentz Symmetry in Electrodynamics to   10$^{-18}$
  Lorentz symmetry is a foundational property of modern physics, underlying the standard model of particles and general relativity. It is anticipated that these two theories are low energy approximations of a single theory that is unified and consistent at the Planck scale. Many unifying proposals allow Lorentz symmetry to be broken, with observable effects appearing at Planck-suppressed levels; thus precision tests of Lorentz invariance are needed to assess and guide theoretical efforts. Here, we use ultra-stable oscillator frequency sources to perform a modern Michelson-Morley experiment and make the most precise direct terrestrial test to date of Lorentz symmetry for the photon, constraining Lorentz violating orientation-dependent relative frequency changes $\Delta\nu$/$\nu$ to 9.2$\pm$10.7$\times10^{-19}$ (95$\%$ confidence interval). This order of magnitude improvement over previous Michelson-Morley experiments allows us to set comprehensive simultaneous bounds on nine boost and rotation anisotropies of the speed of light, finding no significant violations of Lorentz symmetry. 
M. Nagel, S. R. Parker, E. V. Kovalchuk, P. L. Stanwix, J. G.   Hartnett, E. N. Ivanov, A. Peters and M. E. Tobar
hep-ph gr-qc physics.ins-det physics.optics