Speed of light travel 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? 
 A: 
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?

EDIT Upon further review (special thanks to Alfred's comment), I think my original answer is incorrect. It turns out that the question of relative velocities of photons moving in the same direction is a meaningless question. The reason is as follows.
For two objects A and B moving as such,

                       v


               u      -------> A
            -------> B
            ------------------ (ground)

The velocity of B in A's frame is then
$$
u'=\frac{u-v}{1-\frac{uv}{c^2}}
$$
Notice the denominator? For $u=v=c$, this is zero and we get 0/0 which is an undefined operation, hence the meaningless question.

Do objects moving at the speed of light obey law of addition of velocities?

Not exactly. The Galilean velocity addition, $s=u+v$ does not hold for large-velocity objects. We use the "composition law",
$$
s=\frac{u\pm v}{1\pm\frac{uv}{c^2}}
$$
where $\pm$ depends on directions/frames. If $uv\ll c^2$, then this does reduce to the Galilean transformation.
A: The answers can be :

*

*You said that they travel in parallel then you know that the relative speed between them is 0.

*But suppose that one, or both, intends to measure the relative speed. They send a light beam and wait forever for the returning reflected beam. Obviously they must conclude that the other is at an infinite distance apart. This is the way we measure distances: with light.

IMO both answers are correct.
note: although the speed of light is constant and measured $c$ by any observer it does not invalidate (it is easy to explain) that the speed of light is constant irt to the background, the medium.
A: You state "two massless objects moving in the same direction at the speed of light".
This implies the two objects have the same velocity, so the relative velocity is zero.
If two massless objects travel in opposite directions, both at the speed of light relative to a stationary observer, then the relative velocity of the two objects is still the speed of light.
A: The second postulate of special relativity is:

As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.(Wikipedia)

Your question is not regarding the second postulate because no inertial observer can observe the constellation of your thought experiment. Thus there is no contradiction.
