1. The problem statement, all variables and given/known data Mass M travelling at v=c/3 decays into 2 photons in x and -x directions. Mass M orginally formed by mass m travelling at v=3c/5 hitting a stationary mass m. This gives M v=c/3 and M=3m/√2. 2. Relevant equations E=K + mc^2 p = gamma m v E = gamma m c^2 K = (gamma -1) mc^2 E^2 - p^2 c^2 = m^2 c^4 3. The attempt at a solution I am confused how the photons form from a moving mass. How would I find out the ratio of the energies of the photons?