1. The problem statement, all variables and given/known data two photons travel along the x-axis of S , WITH A CONSTANT DISTANCE L betweenthem. Prove that in S's the distance between these photons is L(c+v)^1/2/(c-v)^1/2. 2. Relevant equations x'=gamma*(x-vt), x=gamma*(x'+vt), t=gamma*(t'+vx'/c^2), t=gamma*(t'-vx'/c^2) 3. The attempt at a solution L(c+v)^1/2/(c-v)^1/2=L((c+v)/(c-v))^.5=L((+v/c)/(1-v/c))^.5. So will the two photons reach a midpoint along the x-axis. I think I should either find the difference between x_2 and x_`1 or the difference between x'_1 and x'_2. I thinkl in one reference frame , the time would be dilated with the moving frame for both photons . Is my line of thinking correct?