@JSS
What is the purpose of focusing on what interferometers were FIRST built to measure?
You might have arrived late to the party. Stack has made an assumption that they are attempting to disingenuously/erroneously pass off as historical fact without any support (even anecdotal / personal "reasoning"). I am endeavoring to help keep them honest with themselves and others (only if they wish it, however).
What the interferometer does, and how it works, is actually much more relevant to the shape of the world and the observation of various phenomena (several erroneously attributed to the earth's presumed rotation) than it appears at first glance!
I agree that it is a minor point, but if it weren't important to understand I would have dropped it long before now. The other reason I continue to focus on it is because I have made mistakes like this before, and I wish to help stack to do better - if I can and they will allow me.
Doesn't matter, they still do both.
In the case of the interferometer, that is completely correct. However what is key is HOW it is doing it. We might need another thread where all this jibber jabber can go and stop cluttering this one.
There is no reason the LIGO interferometers would not work just as well as my home-brewed version.
Definitely true! In fact, there is every reason to suspect that they would work much better! However, ligo's arms are of fixed length, calibrated, and insulated from local noise and vibration. Great pains are taken to avoid any path length deviation. It is true that a vibration could cause a mirror (or the arm itself) to move and cause fringe patterns - it's just that that is not what causes the fringe patterns in ligo - nor is that the source of the fringe patterns in stationary and uniformly moving FOG's/RLG's.
When gravitational waves pass through the two arms, the interferometers detect the change in distance and record the results.
That is their earnest belief/conviction, yes. However, it is not what is happening. The interferometer is measuring perturbation in the medium of which light is comprised and travels within. There is no "gravitational wave" - that is largely a made up term to mislead laypeople into thinking "gravity" has been found/detected.
This is direct evidence of gravitational waves,
It is direct measurement of the light waves traveling in each arm. From it we can infer about the media through which light travels and is comprised. The arms of the interferometer did not change length. Noise and vibration did not move the mirrors. The path lengths remain fixed as best as can be achieved by ungodly amounts of money being thrown at the problem.
they are literally measuring space stretching and compressing in real time.
This is essentially correct in my view, assuming the data is real - which is highly suspect.