Undergrad LIGO: Detecting Differences Less Than a Proton Length - How is It Possible?

[thegroundhog]

TL;DR

How is the LIGO detector able to be so accurate?

I read that the LIGO detector in the US was able to detect a difference of less that the length of a proton, or maybe even less than this. How is this possible? The perpendicular arms won't be the same length down to the nearest proton length. Also, at such small lengths the microclimate on each arm might be enough to shift the apparatus 1000x more than a proton length. What about minor tremors and other meteorological phenomena? I would love to know the exact detail of how it is able to be so accurate.

 

[anorlunda]

Try this video. Veritasium answers your question with a very good explanation.

 

[f95toli]

Well, it took them many years to get to that level of sensitivity so it is presumably not easy...
The LIGO collaboration has published a large number of technical papers describing their setups (I have read some of them since I've used some related signal processing techniques). There are also a large number of popular articles. Have a look at the LIGO website.

Anyway, one of the key points here is they are detecting a difference between two signals/path. This is much, much easier than e.g. measuring the absolute position of two objects. That is, you don';t need to know WHERE the protons are in order to detect a relative change in position.

 

[Ibix]

Very precise mirror mounts that are extremely well isolated from their surroundings, mounted in extremely high vacuum, illuminated with extremely stable lasers, and surrounded by lots of sensors to detect uncontrolled vibration, and with extensive post processing of the signals to correct for that, is my understanding.

 

[mfb]

LIGO and Virgo look for gravitational waves in the range of ~50-1000 Hz, optimal for merging neutron stars and stellar mass black holes. Motion that has a much lower frequency is not disturbing the measurement unless it's excessive. A multi-step pendulum suspension dampens motion in the sensitive range. They keep the interference near the dark fringe because that leads to larger relative changes in brightness from small changes in length difference. Sometimes noise is so large that they lose that alignment, during that time that individual detector cannot take data.

 

[Demystifier]

So basically, the change of interferometer arm length by a tiny fraction of laser wavelength transforms into a tiny fraction of laser light power compared to laser source power. By taking the source power big enough, even this tiny fraction of power becomes detectable.