I have just started special theory of relativity. The limiting speed known as speed of light fascinated me most. I asked my teacher:
Consider two massless objects moving in the same direction at the speed of light. What would be their relative velocity w.r.t each other?
My teacher tells me that their relative velocity would be zero. But then the speed of light is universal constant regardless of the motion of its frame of reference, so shouldn't their relative velocity be $c$? What is their relative velocity and how? Do objects moving at the speed of light obey law of addition of velocities?