A laboratory test of the isotropy of light propagation at the 10 −17 level

Local Lorentz Invariance (LI) is a postulate of today's theories of the fundamental interactions. Theoretical work towards a unified theory suggests that LI may only be an approximate symmetry. This motivates experimental tests of LI with increased sensitivity. One aspect of LI is isotropy, the independence of the results of any local experiment from its orientation in space. We report on the results of a strongly improved test of the isotropy of light propagation (Michelson-Morley-type experiment).