Has anyone measured the speed of light in one direction?

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• OscarCP
In summary, the video discusses the idea that the speed of light cannot be measured one-way, only it is possible to obtain a value for it in two-way measurements. It has implications for the whole of physics, and if the speed of light were different in different directions, many things in reality would be stipulated.
Care to explain? Reading argued comments about it was the whole point of putting the URL here.

OscarCP said:
Becase if the light sources' positions change with their coordinates and their distances to us and between themselves change, all of it so the redshifts stay the same if light is stipulated to be different in different directions, those things, distances and age of the universe, it seems to me, won't stay the same.
That is correct. In a universe with anisotropic speed of light the age of the universe is also anisotropic due to anisotropic time dilation. From the “fast direction” we receive immediate light from a very young part of the universe as it is now. From the “slow direction” we receive delayed light from a currently old part of the universe back when it was much younger. Thus we currently receive light from the early universe in all directions.

Dale said:
That is correct. In a universe with anisotropic speed of light the age of the universe is also anisotropic due to anisotropic time dilation. From the “fast direction” we receive immediate light from a very young part of the universe as it is now. From the “slow direction” we receive delayed light from a currently old part of the universe back when it was much younger. Thus we currently receive light from the early universe in all directions.
Thanks for this answer. A very interesting concept that helps me understand this fact never encountered before that the one-way speed of light is impossible to measure.
But I wonder now, following this idea of what it would be like with an anisotropic speed of light, how would one know that what is seeing are "now" and "long ago" parts of the same earlier universe and not parts of the universe of different ages, without knowing first that the speed of light is not the same in all directions and what is it in any given direction? Could it be by knowing the two-way speed already measured in different directions?

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OscarCP said:
Thanks for this answer. A very interesting concept that helps me understand this fact never encountered before that the one-way speed of light is impossible to measure.
But I wonder now, how would one know that what is seeing are "now" and "long ago" parts of the same earlier universe and not parts of the universe of different ages, without knowing first that the speed of light is not the same in all directions and what is it in any given direction?
Why would it be “without knowing first … what it is in any given direction”. Since the one way speed of light is a convention, it cannot be an unknown. We know it as soon as we choose it. We don’t have to wait to know.

I guess it could be undecided. But then your “how” is easy to answer: you decide.

Dale said:
Why would it be “without knowing first … what it is in any given direction”. Since the one way speed of light is a convention, it cannot be an unknown. We know it as soon as we choose it. We don’t have to wait to know.
Well, confused again, I'm afraid. You seem to be saying, keeping your most recent statement in mind, that by changing our choice of one-way speed of light convention we change how the universe looks like to us: some old parts as they are now, some as they were then. I really doubt you meant to say that. Do you mean: "We know as we choose it, based on the two-way measured speed of light?" (And on the assumption that this speed, in a given direction, is the same anywhere and anywhen in this direction?)

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OscarCP said:
some old parts as they are now, some as they were then
No. We see the young side of the universe as it is now (because the light is fast on that side) and the old side of the universe as it was long ago (because the light is slow on that side).

OscarCP said:
Do you mean: "We know as we choose it, based on the two-way measured speed of light?"
We don’t choose the two way speed. That is measured and not a convention. We only choose the convention for the one way speed

malawi_glenn
Dale said:
No. We see the young side of the universe as it is now (because the light is fast on that side) and the old side of the universe as it was long ago (because the light is slow on that side).
Hmmm ... And why couldn't it be the other way around? Or some other way that is neither?

OscarCP said:
Hmmm ... And why couldn't be the other way around?
Because the other way around wouldn’t fit observations.

Dale said:
Because the other way around wouldn’t fit observations.
For example ... Or, because it would result also in some kind of causality violation?

OscarCP said:
You don't mean to say that the distance to stars and galaxies and the age of the universe are irrelevant, do you?
They're both coordinate dependent, not invariants. (It is actually possible to define an invariant "age of the universe", but it takes some care.)

OscarCP said:
Becase if the light sources' positions change with their coordinates and their distances to us and between themselves change, all of it so the redshifts stay the same if light is stipulated to be different in different directions, those things, distances and age of the universe, it seems to me, won't stay the same.
That's correct, all of these things are coordinate dependent and will change if you change coordinates.

OscarCP said:
rather than a repeated blanket statement about "invariants", I would prefer an explanation or a reference that actually explains things beyond technicisms.
You can start here:

https://en.wikipedia.org/wiki/Lambda-CDM_model

That is a reasonable overview of our best current model, and there are plenty of references for further reading.

You could also try the cosmology chapter of Sean Carroll's online lecture notes on General Relativity. They are easy to find.

Another good reference is Davis & Lineweaver 2003:

https://arxiv.org/abs/astro-ph/0310808

A few posts have been deleted and we will leave this thread closed.

In addition to the resources mentioned by @PeterDonis regarding cosmology, I would also like to post the Anderson paper which is the best one I know regarding the conventionality of the one way speed of light:

R. Anderson, I. Vetharaniam, G.E. Stedman, Conventionality of synchronisation, gauge dependence and test theories of relativity, Physics Reports, Volume 295, Issues 3–4, 1998, Pages 93-180

https://doi.org/10.1016/S0370-1573(97)00051-3

If you search online you can find a free link but I am not certain that it is legitimate so I won’t link to it directly.

I recommend that you read that paper and come back with specific questions about the paper rather than about the video.

vanhees71, PeterDonis and berkeman