TITLE: What is the point of the reaction mass in the LIGO mirror suspension system?
QUESTION [8 upvotes]: I learned from the LIGO official website that the LIGO mirror suspension system consists of a "main chain" and a "reaction chain", and there are small electric motors gently pushing the masses on the main chain (i.e., the mirror and other suspension masses) against their counterparts on the reaction chain to "keep them in place". Here is a link to the material (see the second paragraph under the schematics of the suspension system).
I am wondering why this is a good strategy to achieve better isolation. It appears to me that the whole point of isolation is to keep the masses  free  (at least in some frequency band and along some direction), and the idea of installing motors seems to be ruining it. Also, I do not understand how these motors are controlled to distinguish between unwanted disturbances which should be corrected for, and true gravitational wave signals which should not be corrected for.

REPLY [2 votes]: The vibration control systems are actuating in all the degrees of freedom of the test masses and subtracting motion from them, primarily ground motion at low frequencies.
Only when all these systems are active are the test masses "free" to the degree that the noise in these systems allows.
You can think of it as the noise cancelling system in a pair of headphones: a microphone peaks up background noise and an actuator feeds a control signal to the speakers so that you get a silent playback. The LIGO mirrors are free or "quiet" in a similar way when all the control systems are online.
The reaction mass itself is another, rather big and sophisticated, actuator. It controls the longitudinal degree of freedom of the test mass via electrostatic force.
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