1. The problem statement, all variables and given/known data Two particle P and Q each of restmass m0 and moving in collision course at 2/3c in the laboratory frame of reference. In the same collision but in particle P's frame of reference, P is at rest. 2. Relevant equations As the total energy of the particles depends on the frame of reference, do the observers in each frame of reference agree on the number of particles and photons formed in the collision? 3. The attempt at a solution From E=[tex]\gamma[/tex]m0c2 p=[tex]\gamma[/tex]m0v where v=v' from relativistic velocity addition. and E2=p2c2+m02c4 I can conclude that the total energy and momentum of the the collision differs. Furthermore, as I do not se how the available energy for particle formation can be the same when the total energy is different, I would conclude that the number of particles would differ depending on the reference frame. However, this does not feel right. As the collision takes place at a "single point" in space, wouldn't it be measured the same from all inertial frames of reference? Thanks in beforehand.