  We observed the effect of radiation pressure on the angular sensing and
control system of the Laser Interferometer Gravitational-Wave Observatory
(LIGO) interferometer's core optics at LIGO Hanford Observatory. This is the
first measurement of this effect in a complete gravitational wave
interferometer. Only one of the two angular modes survives with feedback
control, since the other mode is suppressed when the control gain is
sufficiently large. We developed a mathematical model to understand the physics
of the system. This model matches well the dynamics that we observe.
