Actually that’s a good point about interferometers, the only detectable change whould be in the difference between each arm’s length.
Gravitational waves do behave like EM waves, we’ve seen a neutron star merger simultaneously in gravity and light. If there was a difference, one observation would lag behind.
How exactly would we measure an absolute value of distance? The whole thing about general relativity is than everything is relative. If everything was scaled up such that the fine structure constant stayed the same, we wouldn’t be able to measure a difference.
Which brings us back to the question I have with your model: How can a changing distance be measured by light to be the same (metre bar) but also different (redshift)? If light is scaling with the rest of the universe, it shouldn’t get shifted. This in the crux of my confusion.
The answer is there’s no such thing as absolute distance. Because there’s no such thing as absolute position. Quantum garuntees inaccuracies in position.
And your right. We can’t actually measure the expansion of the universe directly. It’s actually because of the red shift we do.
The reason we can see the red shift is because the universe holds the speed of light in a vacuum constant.
So if the universe is expanding, and the speed of light is expanding with it, in-order for the speed of light to stay the same, it has to travel more distance in a time. Meaning it’s stretching it’s wavelength as it moves. Just like something moving away from us does. IIRC it’s because of observations that everything is constantly moving further from us, the further out you go, the faster it’s moving away.
But everything is moving from everything, including itself.
I do apologize if I’m a little muddy, I did my physics degree about a decade ago.
Edit as for why gravitational waves travel at the same as E&M waves is because “information” is what travels at the speed of light. For an electro magnetic wave that’s disturbances in E&M. For gravity that’s ripples in the fabric of space-time. For quantum there’s experiments showing that entangled particles will collapse together, if sperated by distance, the lag time is also the speed of light.
EDIT 2:
The only thing faster than the speed of light, is actually the expansion of the universe beyond a certain distance. Don’t remember what it is. But because distance istself is expanding, that’s proportional to distance. So the expansion rate is actually faster than the speed of light far enough out. But no SINGLE point is expanding faster than the speed of light.
If there’s no such thing as absolute distance, then how can you say that a metre bar (and the metre) is larger than it used to be?
If distance is relative, and matter isn’t expanding relative to anything else, then matter isn’t expanding.
We ultimately define distance in terms of c, and the fundamental forces agree with this. We do not observe atoms expanding, but we do observe the space between galaxies expanding. Presumably the space we occupy is also expanding, but it’s such a small effect as to be irrelevant.
Back to my original question, is the boundary between irrelevantly small and detectable above or below the galactic scale?
I think the trouble is also partly based around thinking of then universe as a volume, which implies a centre. And that’s where this analogy falls apart.
Because everything is expanding from everything, there is no centre. YOU are always centre. So you are “expanding” but you don’t change volume.
This is why I keep saying space isn’t getting bigger, distance is.
It’s not that a sheet of paper becoming bigger so the grid paper becomes larger,. It’s changing it’s distance of something, not it’s size and shape.
We don’t observe galaxies getting bigger. We observe them constantly moving away from us. Even. When they’re moving to us, but it’s done at a slower pace than expected. The further away you are, the faster you move away. And it’s a universal constant of 73km/s/Mpsc.
Notice that is a speed per distance. It’s not saying space is getting bigger, it’s saying things are moving faster away from you the further you go away.
The universe isn’t expanding like a loaf of bread because it has a volume. It’s expanding from one volume to another. Where the universe doesn’t.
