1. The problem statement, all variables and given/known data An atom initially at rest emits a photon. Explain why the mass of the atom decreases, taking into account energy and momentum conservation. 3. The attempt at a solution I get the energy conservation part. E=delta(m)c^2=hf. But how would momentum conservation cause an atom at rest to decrease in mass? Unless it's talking about the zeroth component of a 4-vector - in which case that's just energy, surely?