If the speed of light is constant no matter the speed of the source, shouldn't we be able to know the direction and exact speed we are going relative to the absolute 0 in speed? If a photon is sent in the direction we are going and one at the opposite, one of them should appear to go slower then the other because of our current speed, we would travel at some part of his speed, so from our perspective it wouldn't go at $c$/light speed and that difference would be our current speed no? All this assuming $c$ is constant and I still don't know why we believe in that.