All those theories that have the possible Lorentz violation will have to deal with a new lower and more stringent limit on light speed isotropy. A new study base on the Compton edge measurement at the ESRF has imposed a new limit on such isotropy.
Abstract: When the electrons stored in the ring of the European Synchrotron Radiation Facility (ESRF, Grenoble) scatter on a laser beam (Compton scattering in flight) the lower energy of the scattered electron spectra, the Compton Edge (CE), is given by the two body photon-electron relativistic kinematics and depends on the velocity of light. A precision measurement of the position of this CE as a function of the daily variations of the direction of the electron beam in an absolute reference frame provides a one-way test of Relativistic Kinematics and the isotropy of the velocity of light. The results of GRAAL-ESRF measurements improve the previously existing one-way limits, thus showing the efficiency of this method and the interest of further studies in this direction.
The new limit is Delta(c)/c of less than 1 e-14.