I wasn't sure whether this question should go in advanced or introductory physics but I decided to post here since it doesn't involve any complex maths. 1. The problem statement, all variables and given/known data Part 1 Consider a neutral pion at rest. On the basis of conservation of energy and momentum alone, show that it is possible for the pion to decay into two real photons (ie: two photons that obey the classical energy momentum relation for photons). Part 2 Show that if a neutral pion decays into a single photon then the photon is virtual. Hint: Consider the decay in the rest frame of the pion. 2. Relevant equations Photon energy = |p|c E = mc^2 Conservation laws 3. The attempt at a solution I'm not sure if I've been going about this the right way but I've been trying all day. Since the pion is at rest its energy is E(pion) = mc^2. Then by the conservation of energy, the sum of the energies of the two photons is equal to E(pion). Also, the pion has zero momentum so the momentum of the two photons is also zero (thus they are moving at the same speed in opposite directions). From here I've tried to show that the energy of each photon is given by the first equation I listed. I know that the pion's energy should be evenly distributed between the two photons but I'm not sure how to show it. Thanks in advance.