I'm having trouble understanding conservation of 4-momentum. My problem is about dertermining the threshold for triplet production from a photon and an electron using 4-momentum conservation. The answer is 4m0c^2. So far, I say the initial energy is: E1^2=(pc)^2 + (m0c^2)^2=(hv)^2+(m0c^2)^2 At threshold energy after collision, three particles with no momentum are produced, so E2^2=(3m0c^2) E1=E2, but substituting and solving for hv does not work out correctly, and I have not used 4-momentum at all anyhow. What am I doing wrong/forgetting? Help!