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

## TL;DR

This paper reports on long-term observations of changes in the effective refractive index of light in a gravitational wave detector, revealing effects caused by external gravity gradients and an unexpected biannual modulation.

## Contribution

It presents the first detection of long-term refractive index variations in a gravitational wave detector, highlighting external gravity gradients as a significant factor.

## Key findings

- Detected long-term refractive index changes over 14 months
- Identified external gravity gradients as a cause
- Observed an unexpected twice-annual modulation

## Abstract

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.