# Angular instability due to radiation pressure in the LIGO gravitational wave detector

## TL;DR

This paper reports the first measurement of radiation pressure-induced angular instability in a complete LIGO interferometer, highlighting the impact on the angular control system and validating a mathematical model of the system dynamics.

## Contribution

It provides the first experimental observation of radiation pressure effects on LIGO's angular control, supported by a validated mathematical model.

## Key findings

- Radiation pressure affects the angular modes of LIGO's optics.
- One angular mode remains active under feedback control.
- The developed model accurately predicts observed dynamics.

## Abstract

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.