LIGO Is a giant seismometer. Tom, don’t you know what seismic waves and seismic isolation are yet? Here’s a quick study sheet for you courtesy of LIGO isolation page.
For an instrument that needs to remain as still as possible, it is ironic that LIGO is so sensitive that it can feel the smallest vibrations from near and far. LIGO is essentially a giant seismometer capable of sensing vibrations from traffic on nearby roads, weather patterns on the other side of the continent, staff biking alongside detector arms, ocean waves crashing on shores hundreds of miles away, and of course nearly every significant earthquake on the planet. Since gravitational waves will make themselves known through vibrations in LIGO’s mirrors, the only way to make gravitational wave detection possible is to isolate LIGO’s components from environmental vibrations to unprecedented levels. The change in distance between LIGO’s mirrors (test masses) when a gravitational wave passes will be on the order of 10-19 m. To achieve this level of sensitivity, LIGO was constructed with multiple levels of active and passive vibration isolation systems. Many of LIGO’s larger infrastructure systems that provide some additional levels of isolation are discussed in previous sections. But LIGO’s most sensitive components (its optics) required even more complex and highly specialized mechanisms for isolating them from even the smallest imaginable vibrations.
Outside of its pre-stabilized laser, LIGO’s vibration isolation systems are comprised of two basic elements: Optics Suspensions and Seismic Isolation.