Hello. Firstly, I hope this is the right sub-forum for this question. Here's a quick question that I have. After an electron-positron pair production, do the angles that the paths of the electron and the positron make with the original path of the photon have to be equal? Another way to ask this is, do the velocities of the created electron and positron have to be equal? If they do have to be equal, I'd really like to know how this can be proved. Thanks.