In the Dirac sea a particle-antiparticle couple can be created,
and then annihilate in such a short time that the energy conservation
principle is not violated.
Say I have two photosensitive parallel plates placed in vacuum, and
distance between them < < their size .
If I wait for an infinite time,
a couple of photon-photon will be created (traveling to opposite directions) between them
and each of them will react with one of the plates.
How exactly is the energy conserved then?