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Homework Statement
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.
Homework 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.