I know the fact that one photon can not decay into electron-positron pair.
at the CM frame, the momentum of electron + positron is zero. However, the momentum of a photon can not be zero.
In these cases, the virtual photon is emitted, and decay into a lepton pair. How can it work? I just thought that a virtual photon is almost the same as a real photon, except a virtual photon is created and annihilated inside a diagram.
Is there any other rule for the virtual photon?