**4. The real detectors**

The interferometric gravitational wave detectors are very complex machines, besides being very big vacuum systems, need to have very powerful lasers, and so on. *Interferometric Gravitational Wave Detectors DOI: http://dx.doi.org/10.5772/intechopen.106417*

**Figure 3.**

*Schematics side view of the mirror suspension system of the LIGO detector showing the electrostatic actuator which is used to keep the detector locked in.*

The interferometer must be set in a dark fringe condition, but the mirrors are connected to the ground by the suspension, so they vibrate what could change the dark fringe condition. Then a very good suspension that attenuates the vibrations is used, **Figure 3** shows an example of such suspension (this example is about the LIGO detector). The Italian detector Virgo has a more sophisticated suspension which makes this detector more sensitive at lower frequencies.

The Virgo suspension is more complex, it is composed of an inverted pendulum and 6 masses suspended by its center, plus a collection of 18 LVDTs (Linear Variable Displacement Transducers), 5 accelerometers, 23 coils, three piezoelectric devices and 21 motor drivers. All these devices together are called the super attenuator. With a super attenuator for every suspended mirror.

These details show that it is very difficult to keep the interferometer locked in, it also depends on active systems.

But how sensitive the interferometer must be to make such measurements. The first measurement of gravitational waves was of displacement 10−18, as the arms are 4 km long, and the variation in length was 4 x 10−15. As the power inside the arms is 100 kW with an input power of 20 W, has a recycling factor of 5000 which makes the real sensitivity of the interferometer close to 10−12 m.
