I Has anyone measured the speed of light in one direction?

[OscarCP]

You can't stipulate that either. You can only stipulate the one-way speed of light in one particular direction. Then the known fact that the two-way speed of light is $c$ in all directions is sufficient to dictate the one-way speed of light in all other directions besides the one you stipulated.You keep using the word "interpretation" Red shifts aren't an interpretation. They are a direct observable. You don't "interpret" them. You just measure them.

If you mean, what kind of spacetime model would you infer from the redshifts and other observable data, of course it would be the same one we infer with the standard choice of coordinates that cosmologists use, because all of the invariants are the same, and, as I have already said several times now, the physical content of the model is in its invariants. Same invariants, same model. How you choose to put coordinates on it is irrelevant.

You don't mean to say that the distance to stars and galaxies and the age of the universe are irrelevant, do you? 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.

In other words: rather than a repeated blanket statement about "invariants", I would prefer an explanation or a reference that actually explains things beyond technicisms.
And to hear what people here make of the video.

 

[malawi_glenn]

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.

Very hard to read that sentence.

We do use SR (and GR) in our GPS satellites, and they seem to work fine... there is no problem in "just being able to measure the two-way speed of light".

 

[Ibix]

In other words: rather than a repeated blanket statement about "invariants", I would prefer an explanation or a reference that actually explains things beyond technicisms.

Spacetime is what it is. The paths of objects through spacetime are what they are. The interaction between a photon and your detector is what it is, so the wavelength you measure is what it is. Why the photon has the frequency you measure has an invariant explanation in terms of those invariant things. They do not always match up nicely to familiar terms like "position" or "age".

If you want to split spacetime up into space and time so that you can talk in 3 dimensional terms about where the photon was emitted and where it was received, and separately talk about when it was emitted and when it was received you need to make a decision about how you want to do that splitting. How you do it defines what you mean by space and times, so it defines what you mean by when something was emitted and where, and it defines what you mean by what happens to the light as it travels. None of this makes a blind bit of difference to the universe, it just changes how you are describing it.

 

[OscarCP]

Very hard to read that sentence.

We do use SR (and GR) in our GPS satellites, and they seem to work fine... there is no problem in "just being able to measure the two-way speed of light".

Yes, both Special and General relativity work very well when corrections based on them are applied to the signals of the GPS satellites. (NB: the measurements made using their signals are one way.) I happen to have worked for almost thirty years, among several other things, developing techniques to position objects to better than a couple of inches, some of those fixed to the Earth, as in geodetic stations, and others while moving (from Antarctic glaciers, cars, ships, jet planes, to artificial satellites) using GPS and its more recent comparable system GALILEO from the European Union.
And the best answer so far on this topic, I think, has been this one about the ability to measure the two-way speed of light in all directions. I'll go with that. Thanks.
And about that video ...

 

[Ibix]

And about that video

I think most of us have seen it already, it is not new. My opinion of the video is that it is a decidedly low point for Veritasium.

 

[OscarCP]

Care to explain? Reading argued comments about it was the whole point of putting the URL here.

 

[Dale]

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.

 

[OscarCP]

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?

 

[Dale]

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.

 

[OscarCP]

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?)

 

[Dale]

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).

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

 

[OscarCP]

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?

 

[Dale]

Hmmm ... And why couldn't be the other way around?

Because the other way around wouldn’t fit observations.

 

[OscarCP]

Because the other way around wouldn’t fit observations.

For example ... Or, because it would result also in some kind of causality violation?

 

[PeterDonis]

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.)

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.

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

 

[Dale]

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.