[time-nuts] LIGO detects gravitational waves

[Tom Van Baak]

> How much of a shift did they actually see in their 2.5 mile long laser paths?
>
> The news article I saw talked about a distance change of “1/10,0000 the size 
> of a proton”.  That didn’t seem to make much sense.
> 
> Bob

Hi Bob,

The unit of measurement that gravity wave folks use is "strain" which is unit-less meters per meter. It's analogous to how we T&F people use unit-less Hz per Hz for oscillator frequency offset and stability measurements. Plus they have their strain spectral plots like we have our phase noise plots.

>From what I understand, the GW signals they're looking for create a distortion on the order of 1e-21 so they want a detector that's in the 1e-22 or 1e-23 range; in a 20 to 500 Hz bandwidth. This level of precision is just mind-blowing. But as you read the wealth of PDF's out there about LIGO, and drool at photos of the optics, and understand the plots showing strain sensitivity as a function of frequency, you start to believe that this is actually possible. Ok, given a thousand PhD's, a billion dollars, and a couple of decades.

Yes, the interferometer is 4 km in length but they bounce the beam back and forth 400 times so the effective length is more like 1600 km. They keep the mirrors stationary to "picometers". They use hundreds of clever tricks to pull this off.

Since the press is averse to using scientific notation they tend to make up units. So it's common to read units like Rhode Island, football field, human hair, and now, proton. A proton diameter is about a femtometer so 1/10,000th of that is about 1e-19 meter.

LIGO publishes the raw and processed data -- and this is time nuts -- so attached is a TimeLab plot for you showing the chirp of the century. The LIGO/Hanford and LIGO/Livingston data is from:

https://losc.ligo.org/s/events/GW150914/P150914/fig1-waveform-H.txt
https://losc.ligo.org/s/events/GW150914/P150914/fig1-waveform-L.txt

For TimeLab, set scale to 1e-21 and tau to 6.1035e-5 s (1/16384 s). The time axis is relative to 2015-09-14 09:50:45 UTC plus about 0.25 s. 

/tvb
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