Explanation
According to quantum field theory, the ground state of a system with interacting particles is not simply empty space. Rather, it contains short-lived "virtual" particle–antiparticle pairs which are created out of the vacuum and then annihilate each other.
Some of these particle–antiparticle pairs are charged; e.g., virtual electron–positron pairs. Such charged pairs act as an electric dipole. In the presence of an electric field, e.g., the electromagnetic field around an electron, these particle–antiparticle pairs reposition themselves, thus partially counteracting the field (a partial screening effect, a dielectric effect). The field therefore will be weaker than would be expected if the vacuum were completely empty. This reorientation of the short-lived particle-antiparticle pairs is referred to as vacuum polarization.
The one-loop contribution of a fermion–antifermion pair to the vacuum polarization is represented by the following diagram:
Read more about this topic: Vacuum Polarization
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