We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time.
So, how do we assume that light speed is constant? Does the distance moved change relative to the proximity to mass, and the time unit vary in value as well? If not, do we just assume that the lack of proximity to mass in between galaxies doesn't change the behavior of light when we say that the proximity to black holes does?
We assume that light speed is a constant, but we also know that speed is measured as distance/time. We know that in the presence of high gravity (or proximity to very high mass), or very high speed, we experience time dilation, and that perspective and location matters WRT actual time.
So, how do we assume that light speed is constant? Does the distance moved change relative to the proximity to mass, and the time unit vary in value as well? If not, do we just assume that the lack of proximity to mass in between galaxies doesn't change the behavior of light when we say that the proximity to black holes does?