Sorry for that, I'm new to PF:bow:
And in fact, what I need is not the answer but your thinking which can help me to think throughly about the question.:kiss:
In my opinion, I think it depends. It could be correct in some situations, like the two photons travel in the opposite direction, it is possible to find a point where the total momentum equals zero, and I think if they travek in other different directions, it also makes sense.
While if they...