“If you are in a spaceship that is traveling at the speed of light, and you turn on the headlights, does anything happen?” - Stephen Wright
Ahh, yes. The paradox concerning the speed of light. If you are traveling in a bus at 50 MPH and you throw a baseball from the back of the bus to the front of the bus at 50 MPH, how fast was the baseball traveling? It depends on where you made the observation from.
Why doesn't this apply to the speed of light?
We are on a planet that is in motion revolving around a star that is also in motion in a galaxy that is in motion. We observe light that is emanating from stars that are in motion likely in directions not synonymous with ours. What have we really used to measure the speed of light and how can we be sure that is constant. Consider that we are measuring distances using the constant that we have established here in our solar system from one point to another where all variables are known. Then we have applied that which we have called a constant to everything from the size of the universe and our galaxy to how close the nearest star outside our solar system is.
I believe the best constant we may presume is how fast photons move from the source. I speculate the speed of a photon through a vacuum might change relative to the motion of its source depending on where the observation is made. Theoretically, if a star were moving at the speed of light in one direction, light would never be seen by any object traveling in a direction opposite the source. Light would not "travel" at all.
How much do we really know? Have we really found the edge of our Universe, or just the end of observable light?