According to this Wikipedia article, the Electron–positron annihilation forms two gamma photons from an electron and a positron collision. However, according to this other article from Wikipedia, pair production can yield the positron and electron from a single gamma photon. How come this be the case? Why would two particles collide and form 2 photons and then each one of these photons would make 4 particles?
This behavior is reversible, so we would generate more and more electrons and positrons implying an increase in both energy and mass as time progresses. In other words:
This is wrong because of conservation laws.
Now, this diagram shows a somewhat more understandable process, however, I do not understand what exactly does the line in between both circles represents. And what do the red arrow heads mean? Wikipedia article for the image.