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@3ammo the problem with that logic is that we already can experementally prove that relativity is correct so any conventional measurement is moot going in.
Moreover the idea that speed of light is a constant is not what is the issue. The speed of light **is** a constant when measured round-trip, that is experimentally provable. What is not a postulate and the part we cant prove is that the speed of light is a constant one-way regardless of orientation. We take that to be the case in relativity as a matter of **convention** not as a postulate. The reason we do so is because even if it is not symmetric it wont change any of the results, therefore any arbitrary convention works, so we pick the easiest one.
Regardless you still cant actually measure it one way, if you do so conventionally you wont account for time dilation at all and get an incorrect answer where using the relativity approach you will get a more accurate answer but it will be interently a two-way measure.
Yea though keep in mind that is also a very different effect than what I am describing. The sagnac effect is about the relative motion of the medium through which light travels and compares a two-way or round-trip path clockwise vs counterclockwise. So its still comparing two different two-way trips and at no point is it measuring a one-way trip.
