Can anyone explain why a photon seems to travel in a perfectly straight 3D line over unimaginable distances? What exactly defines a straight-line? The concept appeals to Cartesian coordinate space. Is there any way to quantify the straightness of a photon path? Or, does the path taken by a photon thru empty space define straightness? It seems like the optical straight line is one of the fundamental phenomena of physics and mathematics, yet I seldom see it discussed.