1. The problem statement, all variables and given/known data Hi, My question probably has a simple answer, but I've been scratching my head over it a little too long so I thought I would ask it here. I have three initial photons involved in a collision with 4-momenta k1, k2 and k3. I have two reference frames: frame 1: the centre of mass frame of photons k1 and k2 (so that 3-momenta k1+k2=0) frame 2: the centre of mass frame of all photons k1, k2 and k3 (so that 3-momenta k1+k2+k3=0) I want to find out what the relativistic beta v/c and gamma 1/Sqrt(1-(v/c)^2 are. 2. Relevant equations beta=v/c gamma=1/Sqrt[1-(v/c)^2] (observed frequency/source frequency)^2=(1+beta)/(1-beta) 3. The attempt at a solution So there seems no way to directly find the relative velocity of the two frames is as only photons are involved (relative velocity=c ?!?). I thought to use the expression for the relativistic doppler shift, but that doesn't seem appropriate.