LIGO quadruple suspension
A schematic of the lowest portion of an aLIGO quadruple suspension system. LIGO’s mirrors must be so well shielded from external vibration that the random motion of the atoms within the mirrors and their housings can be detected. To achieve this level of shielding, each of LIGO’s 40 kg test masses is suspended at the end of a 360 kg quadruple pendulum system (or ‘quad’). This system of multiple pendulums significantly reduces motion at the level of the test masses where it really counts. Suspensions employ both passive and active vibration isolation components. For more information on LIGO's suspension systems, visit: https://ligo.caltech.edu/page/vibration-isolation.