posted by walter .
A particle called neutral pi meson (or pion) decays into two photons (and nothing else).If the pion (whose rest mass is 135Mev) is initially at rest,
What are the energies of the photons?
How are they moving with respect to each other ?
It's easier to start with the second part of the question. The momentum of the poin is zero, so the sum of the momenta of the two photons (p1 and p2) is zero:
p1 + p2 = 0 ------->
p1 = - p2
Then because the energy of a photon with momentum p = |p| c, you see that the energies of the two photons are equal. The sum of the energies of the two photons has to be 135 MeV, because of energy conservation, so each photon has half this energy.