1. The problem statement, all variables and given/known data An electron and a positron is on a collision course. Both have a speed of 1,80*10^8 m/s. What are the frequencies of the two photons created after the electron and the positron has annihilated? Electron mass=positron mass= 9,11*10^-31 kg 2. Relevant equations E=γmc^2 P=E/C E=hf 3. The attempt at a solution When I first tried solving this problem using the fact that momentum is conserved before and after the collision, I got the wrong answer: Pbefore=γmv=4,10*10^-22 kgm/s Pafter=Pbefore Pafter=2*(hf/c) => f=(c*Pbefore)/(2h)=9,27*10^19 Hz. This is wrong according to my textbook. But when I do the same calculation, using that energy is conserved, I get that the frequency of each photon is 1,55*10^20, which is correct according to the textbook. This is weird because my textbook says that momentum and the total energy of an isolated system is always conserved. When I calculate the momentum of the system after the collision, using that each photon has a frequency of 1,55*10^20 Hz, I get: Pafter= 2(hf)/c = 6,85*10^-22 kgm/s. Doesn't that mean that the momentum of the system has INCREASED? What am I doing wrong?