Why light can travel at c?

[PeterDonis]

The fact of the matter is we observe light speed to be constant/invariant via experimentation.

But we also observe the homogeneity and isotropy of space. Those are experimental facts as well: that the same experiments done in labs at different places and with different orientations give the same results. And they affect everything, not just light, so they at least seem more general on the face of it.

 

[Herman Trivilino]

I think, from a logical point of view, the invariance of the speed of light is equivalent to both original postulates.

Therefore, the invariance of $c$ implies that all inertial reference frames are equivalent?

Or, the existence of a invariant speed implies that all inertial reference frames are equivalent?

 

[PeroK]

Therefore, the invariance of $c$ implies that all inertial reference frames are equivalent?

Or, the existence of a invariant speed implies that all inertial reference frames are equivalent?

The former. The intention was that the speed of light was $c$, as suggested by Maxwell's equations, in all inertial reference frames

Einstein chose to say this through two postulates. You could argue that the second postulate alone infers invariance.

The invariance of the speed of light implies that Maxwell's equation hold in all inertial reference frames. You could argue that thus doesn't cover all laws of physics. So, perhaps you'd want to add the principle of relativity!

 

[Matterwave]

But we also observe the homogeneity and isotropy of space. Those are experimental facts as well: that the same experiments done in labs at different places and with different orientations give the same results. And they affect everything, not just light, so they at least seem more general on the face of it.

Fair enough.

Perhaps I was not as charitable with my language in my prior post (#23) as I should have been. Indeed, I quite like the argument from homogeneity and isotropy in general. Landau and Lifshitz also go down a similar path and I quite like their pedagogy.

Still, for me, the constancy / invariance of the speed of light is a "just as good" postulate. But I think at this point I'm just talking about personal taste.

 

[javisot]

The sun?

We can study the propagation of light in a vacuum without considering the source that originated it.