1. The problem statement, all variables and given/known data A neutral pion traveling along the x axis decays into two photons, one being ejected exactly forward and the other exactly backward. The first photon has three times the energy of the second. Prove that the original pion had speed 0.5c. 2. Relevant equations for m=0, E=p*c conservation of Energy E^2=(c*p)^2+(m*c^2)^2 gamma=1/sqrt(1-Beta^2) Beta = v/c p=gamma*m*v E=gamma*m*c^2 3. The attempt at a solution I know momentum and energy are conserved giving me the following equations. Epion = 3*Ephoton1 + Ephoton2 and Ppion = Photon1 - Pphoton2. I know the mass of a pion at 140MeV/C^2. For a massless particles E = pc and for the pion E = gamma*m*c^2. I want to find expressions for Ephoton1 + Ephoton2 that do not have P or E in it to solve for the velocity. Though I cannot seem to make any progress with this approach. I need a hint.