Optical Pathlength Noise in Sensitive Interferometers Due to Residual Gas

An analysis of the noise in the apparent optical pathlength in an interferometer due to the presence of statistical fluctuations in the passage of gas molecules through a laser beam is presented An experimental test using a 40 m long prototype interferometer confirms the main predictions of the model regarding the dependences on frequency, pressure, molecular polarizability and molecular speed The results are used to derive the vacuum requirements for the planned Laser Interferometer Gravitational-wave Observatory (LIGO).