The Technical Challenges of Measuring Gravitational Waves - Rana Adhikari of LIGO

TL;DR

LIGO detects gravitational waves using laser interferometry with Fabry-Perot cavities to boost sensitivity, but the signals are extremely tiny (strain ~10^-21). The main challenges are radiation pressure causing mirror motion, scattered light from imperfect mirrors, and various noise sources that must be subtracted via linear filters and hardware feedback.

Key insights

• The Earth's entire diameter would stretch by only ~0.01 micron from a large gravitational wave, equivalent to 10^-21 strain.
• LIGO uses Fabry-Perot optical resonators (two mirrors facing each other) to make the laser travel hundreds of round trips, multiplying the phase shift from a gravitational wave by ~200.
• Higher laser power increases signal linearly but noise from photon shot noise grows only as sqrt(power), so there is a net benefit up to a point where radiation pressure starts shaking the mirrors.