As far as I understand, even if c is different in some circumstances or changes over time it would be hard to measure because everything else is expressed with c

It's like trying to measure if your fingers have grown longer, but doing it with only those fingers as a measure

[–] HubertManne@kbin.social 2 points 6 months ago (1 children)

so you believe c was never directly measured?

[–] bitfucker@programming.dev 1 points 6 months ago (1 children)

Well, how would you measure C directly? You can only always get 2C.

[–] HubertManne@kbin.social 2 points 6 months ago (1 children)

I wouldn't but it has been historically. Unsurprisingly with mirrors but always under the not insignificant influence of the suns gravity. Our most recent measurements I believe use cosmic bodies I believe which is what makes me wonder if our measurement is accurate. https://www.speed-of-light.com/historical_measurements.html

[–] bitfucker@programming.dev 1 points 6 months ago (1 children)

Oh sorry, I was talking about measuring C rather than 2C (since that is the only way we can get C, IIRC, you cannot measure C directly since SOME information must be conveyed when measurement begins AND ends, hence 2C). For C in a gravitational field, I have no idea but I suspect it will have something to do with relativity and time dilation if it has any effect at all.

[–] HubertManne@kbin.social 2 points 6 months ago

well we measure it assuming it has no effect and that is why going way back in this chain I said I would like a measurement outside the influence of a gravity well.

[–] victorz@lemmy.world 1 points 6 months ago (1 children)

Isn't Earth a gravity well, or nah?

[–] HubertManne@kbin.social 1 points 6 months ago (1 children)

when not under the effect and the big gravity well in teh area is the sun. maybe the voyager craft are far enough out for it to be weak enough.

[–] statist43@feddit.de 1 points 6 months ago (1 children)

Even then, we are still in the gravitational field of our galaxys black hole.

[–] HubertManne@kbin.social 2 points 6 months ago (1 children)

Good point and black holes are part of make me wonder about that given what it does to light and spacetime. All our measurements of the galaxy and universe is on a speed of light is in our gravity well and even though it drops off so quickly the suns is so huge we have to have quite a distance to get to where its inconsequential is way beyond earth. Knowing there it is the same 1000 au from the sun at high precision would be nice to know. It it showed any difference. Even slightly then it would be massive in our understanding of the universe.

[–] statist43@feddit.de 1 points 6 months ago (1 children)

Yeah, this is a teally intresting thought! The observation should be really somewhere outside of galaxies, or where there is almost no gravitation.

[–] HubertManne@kbin.social 2 points 6 months ago

not necessarily but at least far enough away that gravitational forces are way different. the sun contrls orbits for over 1000 au but neptune I think is the farthest circular orbit at 30 au. Our measurements at 1au have no practical gravitational variance at all but we assume light is uneffected by it.