1. The problem statement, all variables and given/known data An atom of mass M decays from an excited state to the ground state with a change in mass of ΔM<<M. In the decay process, the atom releases a photon. Use the laws of energy and momentum to determine the energy of the photon, assuming the atom decays from rest. 2. Relevant equations The 4-momenta of the atom before the emission: P=(Mc2,0,0,0) After the emission: P=(M-ΔM)c2, -pxc,0,0) And the photon: E=(M-ΔM)c2 p = hf My main concern here is that this seems too straightforward. The book labels this problem as a challenging one. I think I'm missing a subtlety in the problem.