If a gravitational wave passes through a detector it will move the mirrors by a tiny amount; for example, just by 10-20m. Even something as ordinary as ocean waves, or passing cars can move the mirrors around 10-9m, which is huge compared to what we want to measure.

To have any hope of detecting a gravitational wave the mirrors must be set up so that unwanted motion can be reduced. The surface of the Earth is in constant motion with different parts of the environment moving it at different frequencies. Human activities, like cars and trains, tend to cause vibrations that propagate through the Earth's surface. The ground motion is generally called `seismic noise'. When the ground moves, so does the detector and this creates noise in the detector output. To attempt to reduce this effect real detectors place the mirrors on mechanical suspension systems. In a way this is similar to suspension systems in cars that isolate the passengers from shocks and vibrations of the tyres.

A very effective isolation from seismic noise can be achieved through a clever combinations of pendulums. The amount a pendulum moves reaches a maximum at a frequncy known as the resonance frequency. When a pendulum is oscillated faster than its resonance then it is displaced less and less. Therefore, suspending a mirror as a pendulum makes the mirror quieter than the ground (at high frequencies). By adding more pendulums, the motion can be reduced even further. The pendulums must be made so that their resonance frequency is well below that of the oscillations from the environment, and especially that of a gravitational wave.

GEO 600 mirror suspension system

GEO 600 mirror suspension system. Image credits: Albert Einstein Institute Hannover

Slower vibrations however, like those from ocean waves, may still do their damage. Making the pendulum longer helps because this lowers the resonance frequency.

Overly complex systems with lots of pendulum stages or incredibly long pendulums can be very difficult to control and make it a very expensive business. Consequently the pendulums in real suspension systems for gravitational wave detectors are designed as a trade off between length, number of stages and complexity.

Our Processing sketch Pendulum allows you to play with a pendulum online and see how it can be used to suppress seismic noise: