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Comment by nyssos | Hacker News Reader

It has two arms at right angles to each other, and measures the difference between the two length changes via laser interferometry. This gets you the component of the wave polarized along the corresponding axes.

No clue why GP thinks it "shouldn't work": it's an extremely difficult engineering problem, but the physics of it is relatively straightforward.

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"but the physics of it is relatively straightforward."

Amusingly Einstein both identified gravitational waves in his then new general relativity and then changed his mind about whether they existed. Indeed he had found three different types of these waves, and two of them were simply coordinate artifacts (they could be made to travel at any speed, Eddington famously quiped that they could be made to move at the "speed of thought"). The third type however didn't have this problem (though again here Einstein would write a paper where he claimed these gravitation waves required singularities, but these turned out these were all coordinates singularities .... sort of like how late and long coordinates misbehave at the poles)

In some ways the physics of it is straightforward, in other ways....not so much.

I think you have an intuitive mental model of how it works, but don't actually understand it, or aren't able to explain it well.

This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300... which may strengthen your understanding.

> This question is important enough that there have been papers written on it. I suggest you read https://pubs.aip.org/aapt/ajp/article-abstract/65/6/501/5300...

This is a physics education journal: its papers are supposed to be interesting and accessible to undergrads, not on the frontier of the field.

What the linked paper amounts to is noticing that the sensitivity of a detector depends on the relative length scales of the arms and the waves you're trying to detect. This is true, and also one of the first things you would consider when figuring out what size your detector should be. It's a good homework problem, but not news to anyone actually working on LIGO.

I legitimately find your cognitive dissonance interesting. I think you're on the other end - you may understand the fundamentals so well that your brain isn't processing the question / conundrum in the way that others see it. Honing in on the the scales of the arms as your reaction the paper is why I think this, as that's not related to the problem. And, well, you said it yourself, you don't understand the question:

> No clue why GP thinks it "shouldn't work"

It's also possible you think that this is all about laser interferometry, and aren't properly considering how it could work in the context of compressing space, since a laser interferometry system in compressed space wouldn't produce interference.

Yeah, the physics is straightforward. Even "laser interferometry" is super simple to explain. It is getting enough resolution and noise immunity from laser interferometry where all the interesting stuff is.

I think GP's point is that you can't measure changes in the length of an arm if the ruler you're using is changing in the same way.

I think this misrepresents the situation, but I can't say how, so I can't dismiss it quite so quickly.

The "ruler" you're using is the other arm, which doesn't change in the same way: one axis stretches while the other compresses.

True, but with only two arms, I think LIGO is somewhat directional, is it not?

Stronger signals from some black holes depending on which way they're oriented relative to the arms. And if there were many different simultaneous signals, some could mask the others.

Clearly, we need a 3-armed, space-based LIGO. Or better, a dozen of them.

There are two LIGO interferometers in the US [0], Virgo in Italy [1], GEO600 in Germany [2], something in Japan [3] and something being planned out being built in India. There is LISA as space-based project in the works too [4].

[0] https://en.m.wikipedia.org/wiki/LIGO

[1] https://en.m.wikipedia.org/wiki/Virgo_interferometer

[2] https://en.m.wikipedia.org/wiki/GEO600

[3] https://en.m.wikipedia.org/wiki/KAGRA

[4] https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

Thanks!

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