When he was first thinking about relativity (and before he developed his theory), Einstein
considered how the universe would look to a person who traveled on a light beam. Suppose you
are traveling on a light beam when you encounter a friend who is traveling on a second light
beam in the opposite direction. What is your friend's speed relative to your reference frame?
(Express your answer in terms of c and use the postulates of special relativity to find the correct
answer.)
Solution
According to special relativity, the speed of light is same in all reference frames. So even though
i am watching my friend(moving with c) from a frame which is itself moving with c(me), my
friend's speed relative to my reference frame would be c(speed of light).