1 gamma ray photon produces 1 electron & 1 positron then why 1 positron & 1 electron produce 2 gamma photons?

Question

If,
1 Y (gamma photon) = e- + e+ (pair production)

then why not e- + e- = 1 gamma photon instead of 2 ? (annihilation)

Would you plz answer another question of mine about 'INDUCTION' ?
No one is answering it,i have a confussion about that.

Answer 1

Conservation of momentum.
In the centre of mass frame of the e- and e+, momentum is zero. If they made one photon, it would have energy equal to the masses of the particles but it would have non-zero momentum.
So instead, two photons travelling in opposite directions with equal energy (net momentum = 0) are created.

In very high energy situations, three or more photons can be created.

Answer 2

I think it is to do with the momentum conservation. Good question.