A particle with mass M a rest decays into two particles a and b. I know that Ea + Eb = Mc2, from conservation of energy. But I'm pretty confused about signs in the conservation of momentum equation, and I've actually seen two versions! pa + pb = 0, so pa = - pb. But I've also seen pa = pb! I know one is magnitudes and the other takes account of directions. Both are right, but which applies for the situation described above? As in, don't they conflict?