thought: Two atoms close together will also change thelocal vacume aroundThem. If onefthem emits a virtual photon, which is almost instantaly ab-sorbed by the other, then on any timescale larger than the brief mom ent
of the photons existence the total enery is conserved. But the fact that
virtual particles can be exchanged modifies the vacume around the atoms
modifies the vacume around the atoms, and thisleads to aforce.