When a gravitational wave passes through Earth, it causes space itself to stretch in one direction and compress in the other, so the two “arms” of the detector actually grow and shrink by tiny amounts. This means each beam of light travels a slightly different distance, which shows up in the recombined laser light pattern as a spike in frequency called a “cosmic chirp”—this is the gravitational wave signal.
To measure it, Virgo relies on state-of-the-art equipment. The mirrors at the end of each tunnel are made of a synthetic quartz so pure it absorbs only 1 in 3 million photons that hit it. It’s polished to an atomic level,
→ Continue reading at Wired - Science