zlacker

>>qqqqqu+(OP)
The LIGO detector. I've never heard a logical explanation for it. If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works. If your explanation doesn't involve higher spatial dimensions (not time) then you don't understand it. If you haven't even considered higher spatial dimensions when explaining LIGO then you shouldn't even try for the incorrect explanation above because you don't have any of the pieces of the puzzle.

>>steveb+2m
> If your explanation is that gravitational waves stretch and squash spacetime so light takes different amount of time to bounce from an emitter back to a measurer, then you don't have even a slight understanding of how it works.

Well, I'll bite. I'm a physicist and I understand LIGO. What's your alternative explanation?

>>knzhou+cm
I have no idea how it works, I don't have an explanation, I just know the one spouted by people doesn't make sense

>>steveb+Bm
Well, if you're uncomfortable with LIGO, you're in good company. Once a whole decade went by in physics where people couldn't agree if it would work in principle or not. And it is true that a lot of common explanations are bad (e.g. "it's just a ruler" is not complete by itself because "why doesn't the light get stretched too?"). Nonetheless, today we have a variety of independent explanations.

Maybe you'll find this paper helpful: https://aapt.scitation.org/doi/10.1119/1.18578

>>knzhou+Tm
Thank you for this link. I read it once and think I've gotten more of the picture and I'm going to re-read it to see if I can get more. What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either.

If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension?

>>steveb+Gx
> If light is emitted at a constant wavelength independent of the stretching of the universe, doesn't that imply light is traveling through a higher spatial dimension, otherwise the emitter itself would be stretched with the universe and we'd never be able to observe differences in the speed of light? If I understand this paper, once light is emitted, it's "stuck" to space and will stretch along with it. But if the emitter wavelength stays constant doesn't that imply it's waving through a higher dimension?

I'm not totally sure what you mean by a higher dimension. The properties of the emitter (which is, e.g. a laser cavity) aren't affected by the gravitational wave because the emitter is a rigid body, which doesn't get stretched. (It's the same thing as described here: https://news.ycombinator.com/item?id=22990753 ) So it puts out light of a given frequency.

By contrast, LIGO is not a rigid body, because the mirrors at the ends of the arms hang freely, hence allowing gravitational waves to change the distance between them.

> What's baffling to me is everyone who has tried to explain the LIGO detector doesn't even realize this question exists. I've independently thought this question and when people start explaining LIGO to me, and I take the time to spell out the question, they realize they don't understand LIGO either.

Yup, it generally is the case in physics that over 95% of people who claim they can explain any given thing don't actually understand it! But the professionals are aware. I even know a LIGO guy who goes to popular talks armed with a pile of copies of the paper I linked.

>>knzhou+pA
I've read this paper twice now and it's very good. I still don't have a fundamental understanding, I'm having trouble visualizing what happens over time with the stretching and squashing of space vs the light in the tunnel. I've come away with this paper with more questions than answers, but I think all of it is there if I study it hard enough.

- It sounds like (based on the answer you linked) the "expansion" of the universe is a lie in the sense that the fabric of space is not actually expanding, things are just moving farther away from each other via motion. So it's not points on a balloon being blown up (in which case the points themselves are also growing in size), but a force pushing things apart

- If that's true, then I don't understand why the paper talks about light waves expanding with the cosmological expansion, implying that the fabric of space itself is indeed expanding, and talks about how that makes the doppler effect make no sense since the light wavelength expands with the universe. It sounds like there's a fundamental incompatibility with your explanation ("this doesn't expand small objects") and the paper ("the wave itself expands with the expanding space in which it travels, so that its wavelength grows with the cosmic scale factor"), which implies the fabric of space, eg all particles, is expanding

- It sounds like light "sticks" to space as space expands, but new light emitted after an expansion will still have some constant wavelength. So in this way in an expanding universe, light which has a constant wavelength will have further to go between particles, so light will appear to slow down as the universe expands

- If light does stick to space, and the fabric of space is expanding, then I never realized that the doppler effect makes no sense for measuring cosmological expansion, because we wouldn't be able to see it (hinted at in the paper)

- Maybe I don't fully understand why LIGO needs two arms. If you had a clock that could accurately measure light wave crests, could you do it only with one arm? i'll take a leap of faith in believing that a gravitaional wave compresses in one dimension and expands another (maybe not if the wave hits it exactly at 45 degrees?). Maybe the two arms are just for convenience to get phase difference for free?

- I think what I'm missing still is what is actually being measured and how it happens. Space expands, the wave gets longer in one direction, so it has further to go (only for a fraction of time), and it will take longer for the next crest to get to the detector, (I guess the crest itself is still moving at C? but through a farther distance?) so for a tiny blip of time, there will be a phase difference, not for all the light in the arm but just for the one or few crests that make it back along the further length until the wave resets the overall distance?

- Does space compressing and expanding prove that its compressing and expanding through a higher dimension? Especially if new light emitted is at some constant wavelength independent of the stretching of the spacetime it enters? Does that also imply this constant wavelength is happening independent of our (3d) space stretch, so it's a constant through some higher dimension?