Why is photon self-energy called vacuum polarization?

[cello]

Can electron self energy diagram contribute to the vacuum polarization? Or is the question meaningful? What's the exact meaning of vacuum polarization? Does it necessarily involve some charged particle? For example, for the ordinary phi^4 theory, can I identify any diagram that can be called vacuum polarization?

 

[Lapidus]

Why is photon self-energy called vacuum polarization?

Because it can be interpreted that while a photon propagates, the vacuum affects the photon to fluctuate into pairs of (virtual) electrons and positrons. Further, the electron and positron interact by exchanging a photon, which also can turn into a pair of an electron and a positron, and so forth.

Can electron self energy diagram contribute to the vacuum polarization?

Well, now we have an electron constantly emiting (virtual) photons. They again turn into lots of electron-positron pairs, which surround the electron. So the vacuum around the electron is polarized.

Does vacuum polarization necessarily involve some charged particle?

The term vacuum polarization, as I understand, is restricted to processes where virtual electron-positron pairs are created. In the phi^4 theory, virtual phi particles are created.

 

[Bill_K]

Vacuum polarization is a term applied to the special case of the photon self-energy in which the photon is not a free photon but bound to a classical external field like a charge. Although today you're more likely to see the term charge screening. The standard electric charge or coupling constant is the one observed a large distance from the source. The central charge is supposed to be screened by a cloud of virtual positron-electron pairs. At shorter distances as one gets inside these pairs the effective charge increases.

 

[A. Neumaier]

The name is purely historical and means not really anything.

Can electron self energy diagram contribute to the vacuum polarization? Or is the question meaningful? What's the exact meaning of vacuum polarization? Does it necessarily involve some charged particle? For example, for the ordinary phi^4 theory, can I identify any diagram that can be called vacuum polarization?

In QED, any connected diagram with just two external photon lines contributes to vacuum polarization. Thus in QED, it always involves at least one fermion loop. If within the fermion loop there are subdiagrams with only two external electron lines, you have contributions of the electron selfenergy that also contribute 9at higher order) to the vacuum polarization.

in phi^4 theory, you'd have to think of the particles as scalar photons to apply the term.