My question refers to the following from this source:
Within this context, what does it mean for a positron and electron to be in thermal equilibrium and why would that mean the process is reversible? Does this mean that their collision would not produce the gamma photons if they were not in thermal equilibrium? I thought the particles just needed enough energy so to not "break" any laws.
Additionally, what does it mean for the temperature to drop below the electron mass? What I understand is that the photons don't have enough energy to create an electron and positron pair following E=mc^2, right?
