This question already has an answer here:
What would the result of a vertical variation of the Michelson-Morley experiment be? I.e., if one were to compare light traveling along a horizontal arm with light traveling along a vertical arm (perpendicular to the surface of the Earth), what would the result be?
My understanding is that light traveling along each arm will both travel at c and consequently the result of such a variation of the experiment will be exactly as all of the previous (fully horizontal) versions of the experiment. Namely, that there is no directional dependence in the speed of light.
Is this understanding correct?