Answer:
By conservation of momentum
Explanation:
By conservation of energy, the photon needs at least the same amount of energy as the energy rest mass of the electron and positron together, which is around 1,02 MeV. But, by conservation of momentum, the electron-positron do not have momentum, while the photon had some. This means that, in order to conserve momentum, we need another neighboring particle, normally an atomic nucleus, to receive the photon's momentum.
