Astronomical propagation
One of the most precise ways to test Lorentz symmetry is to study light from astronomical sources. Conventionally the two perpendicular components of the electric field of a light beam, E1 and E2, propagate with the same velocity, and the total electric-field vector (black arrow) rotates in a plane with its tip tracing an ellipse. The polarization of the light is determined by the shape and orientation (given by the angle ψ) of the ellipse, which does not change during propagation. If Lorentz symmetry is broken, however, the rotational invariance is lost and the two polarization components of light can travel at slightly different speeds. This effect is called birefringence. In this case, the orientation and shape of the polarization ellipse changes as the light travels through space from a distant galaxy (left image) to the Earth (right). This effect has a characteristic frequency dependence that can be searched for in experiments.