# Measuring the speed of light in one direction? [closed]

I have this thought that probably is dumb. But regarding the problem to measure the speed of light in one direction, we know the refractive index is $$c/v$$. Now, by measuring the refraction we can arrive at $$c$$. Also since this experiment is done in one direction, is this experiment not measuring the speed of light in one direction and arriving at $$c$$?

"by measuring the refraction we can arrive at c" - No, that's not true. We also need to know v i.e. the speed of light in the medium in order to determine c from n. If you want to determine the speed of light in one direction in the medium you then encounter the same problems as when you try doing this in the vacuum.

For more on the problem I recommended this interesting stack exchange entry:

Can we measure the speed of light in one direction?

(It is stylistically interesting that both you and the person who asked the previous question started with a version of "my question is probably dumb" , which you really do not have to do!)

• I guess what I meant was, the speed of light in another medium is precalculated (using the two direction measurement), but even then if we know n, and the speed of light in another medium (v), are we not arriving at c in an experiment where the light travels in only one direction. And in multiple experiments if speed of light is not c, we'd get different values of n. But we don't see that! Sep 18, 2021 at 11:06
• Well, I think IF the speed of light is less than c in the direction we are doing the experiment, v will also adjust so that n is constant. That basically means we have no way of knowing if c is constant in all the directions. Sep 18, 2021 at 11:30
• One could imagine that in some strange world in the vacuum c changes with the direction while the speed of light in some hypothetical material changes differently, so that you can measure preferred directions of the universe by rotating your special material in the vacuum. For example, something like this this would apply for certain materials in a univererse where the vacuum contains some uniform magnetic field. But the fact that we don't observe something like this in our universe has nothing to do with measureing the speed of light in one direction exactly by your last argument. Sep 18, 2021 at 11:49

Okay, that experiment is not actually measuring the speed of light in one direction as other have pointed that out. We can't get to v without actually measuring it in both the directions.

And, IF the speed of light is less or greater than c in the direction we are doing the experiment ( which is spooky to me), v will also adjust so that n is constant. That basically means we have no way of knowing if c is constant in all the directions.