Gravitational modulation of the optical length of long-baseline laser interferometers

The effect of quasistatic variations of the Earth’s gravitational field on the output signal of long-baseline gravitational-wave interferometers has been considered. The relativistic representation of the gravitational field in the form of a varying refractive index is used. An analytical calculation with numerical estimates of the tidal effect has been performed to explain the recent results observed on the LIGO interferometers.