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.

  • $\begingroup$ Could you add a link to the place you read about this? $\endgroup$
    – Gremlin
    Oct 9, 2017 at 13:39
  • $\begingroup$ @Gremlin I added a link in the question. Thanks for the suggestion. $\endgroup$
    – user23823
    Oct 9, 2017 at 13:43
  • $\begingroup$ In case you cannot find a good answer here, you may send your question directly to LIGO at questions@ligo.org $\endgroup$ Oct 9, 2017 at 19:44
  • $\begingroup$ On a side note: "Motors" might give a bit of a wrong picture. The so-called electrostatic drives work by electrostatic repulsion between the test mass and the reaction mass without any physical contact. $\endgroup$
    – Emil
    Oct 17, 2017 at 7:10

1 Answer 1


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.

  • 1
    $\begingroup$ On the face of the reaction mass there are 4 quadrants of traces. The electrostatic forces from voltages put on these traces can move the mirror longitudinally and also rotate it so that the laser beam correctly hits the splitter mirror 4 km away. The voltages also (unfortunately) cause the mirror to charge up causing undesired noise, and making LIGO a sensitive radio receiver for any EM waves that get inside their vacuum chamber. LIGO works to minimize this charge by periodically reversing the pusher plate voltage polarities. $\endgroup$ Oct 9, 2017 at 19:03
  • $\begingroup$ Better wording: LIGO works to minimize this charge by periodically reversing the voltage polarities on the reaction mass traces. $\endgroup$ Oct 9, 2017 at 21:25
  • $\begingroup$ Also, the mirror mass is electrically insulated from the world since it hangs on fused silica fibers. The mirror gets charged up by ions in the vacuum following electric field lines. $\endgroup$ Oct 9, 2017 at 21:34

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