Pair Production: Why Not in Empty Space?

[Ratzinger]

Is there a short answer why pair production can not happen in empty space, but that photons must interact with an external field?

thanks and a nice weekend

oh, just saw someone asked the same question in the nuclei section today and answers already there

 

[Kruger]

Listen, Go and look for "pair creation" in "Nuclei and particles". The answer is given there.