Quote by chrispb
In a free field theory, the true ground state of the theory, Omega>, corresponds with 0>, the eigenstate of the number operator. [...]However, in an interacting field theory, like QED, 0> is no longer equal to Omega>.

That's correct, but I'm still a little hazy on how this shows that virtual particles are a real, nonperturbative phenomenon, rather than an artifact of a perturbative calculation.