LIGO's vibration isolation system | Hacker News Reader

LIGO's vibration isolation system

ligo.caltech.edu

159 pointslisper20 comments

10 to the minus 19! It's mind boggling. Semiconductor process nodes constantly boggle my mind, and yet they sit at a cool 10 to the minus 9. These vibrations are smaller relative to a CPU transistor than a CPU transistor is to me.

Also, a single hydrogen atom is on the order of 10^7 larger than that... absolutely amazing!

LIGO scientist here. The way this is presented can be a little deceptive - the isolation is very frequency dependent. At high frequencies (>10Hz), the pendulums and blade springs in the suspension isolate the mirrors very well, so they are moving by only these small amplitudes (10^-19 meters). But at low frequencies (<1Hz) the isolation ratio is essentially 1, so the amplitude of the mirror motion is roughly the same as that of the ground (about 10^-6 meters).

Yes, which makes sense physically. If the earth's rotation slowed by a constant velocity, in order to damp that motion the mirrors would have to displace themselves in the opposite velocity. Of course, there's no room for them to do so, so damping such low frequencies is not possible.

> LIGO's mirrors must be so well shielded from vibration that the random motion of the atoms within the mirrors and their housings can be detected.

It's just simply amazing to me that this is possible.

That's impressive. They didn't have to put it in orbit to get sufficient vibration isolation. The eLISA team must be so frustrated about this.[2] Their gravitational wave detector is in orbit at the L1 point, and their test masses just float in space. But they're not quite ready to turn everything on; that happens in early March.

XKCD: [1]

[1] http://xkcd.com/1642/ [2] https://www.elisascience.org/

A picture is worth a thousand words : https://upload.wikimedia.org/wikipedia/commons/e/e2/LISA_orb...

Source : https://www.wikiwand.com/en/Evolved_Laser_Interferometer_Spa...

LISA and LIGO aren't in competition. They are very different projects trying to be sensitive to very different frequencies. (Likewise, Hubble isn't competing with ground-based arrays of radar antenna even though both are looking for electromagnetic waves.)

Furthermore, the only satellite currently in orbit is the LISA pathfinder mission, which is just a testbed for some of the requisite tech. It is incapable of detecting gravitational waves.

The multiple pendulum systems sound really cool. I wish they would put up more images, and high resolution ones, to show what is going on from different angles.

A Video. We need a video!

i think a video of a system that achieves 10^-19 vibration sensitivity would be stiller - far stiller - than a still photograph

Not a video of the equipment working. More of a 3D video explaining stuff would be awesome.

This is the sort of thing that makes me proud to be human.

what about molecular motions of the mirrors themselves?

That averages out over the timespan involved in gravitational wave detection. The waves detected by LIGO are on the order of 100 Hz while molecular vibrations are many orders of magnitude higher.

Ah! This is a key point! Thanks.

I understand that's why https://en.wikipedia.org/wiki/KAGRA is cryogenic. Kip Thorne mentioned it during the Q&A@ https://www.youtube.com/watch?v=aEPIwEJmZyE

Are their suspected GW sources in the thermal ~(7um/42MHz) range?

Found more info on thermal mirror effects, see around 12m @ https://www.youtube.com/watch?v=RGqZZthc_l4

hey you can set timestamps on youtube links like so:

https://www.youtube.com/watch?v=RGqZZthc_l4&t=12m

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