Observation of long-term changes in the effective refractive index of light

During the LIGO S5 run (April 2006 to June 2007) preliminary data from the H1 interferometer recorded long-term changes in the effective refractive index of light over a 14 month period. These are due to the presence of external gravity gradients along the arms of the interferometer. After accounting for the effect of the tides, an unexpected twice-annual modulation was observed.