As far as I'm aware, pair production always involves the initial energetic photon interacting with another body (e.g. nucleus) to conserve momentum when creating the matter/antimatter products (e.g. electron and positron). Although "secondary" high-energy photons (e.g. ## e^+ + e^- \rightarrow 2\gamma##) produced by annihilation of the matter/antimatter products does not require any additional body. I was thus simply wondering about the likelihood of the reverse process of two photons (possibly of different energies) interacting to produce matter/antimatter products without a third body. Does this readily occur and/or has it been observed? To my knowledge, two photons only interact beyond the Schwinger limit, and the above annihilation process is essentially irreversible below this threshold. Any further thoughts or resources you may have on the above and considerations I may be overlooking would be greatly appreciated.