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.cello said: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?
The term "vacuum polarization" refers to the phenomenon in quantum field theory where the presence of a charged particle in a vacuum causes the vacuum itself to become polarized. This polarization leads to the appearance of virtual particles, including virtual electron-positron pairs, which can influence the properties of the vacuum. In the case of a photon, the presence of these virtual particles causes the photon to have a self-energy, which is responsible for the observed phenomenon of vacuum polarization.
Vacuum polarization affects photons in several ways. First, it causes a shift in the energy levels of the photon, known as the Lamb shift. It also leads to a small deviation from the expected strength of the electromagnetic force, known as the anomalous magnetic moment. Additionally, vacuum polarization plays a role in the scattering of photons off of charged particles, known as the Schwinger effect.
One of the earliest pieces of evidence for vacuum polarization was the Lamb shift, which was observed in atomic spectra. This was later confirmed by precision measurements of the anomalous magnetic moment of the electron. Another important observation is the Casimir effect, which demonstrates the existence of virtual particles and their effects on the vacuum. Additional evidence comes from high-energy particle physics experiments, which have observed phenomena such as the Schwinger effect and the production of electron-positron pairs from vacuum fluctuations.
In quantum field theory, the vacuum is seen as a state filled with virtual particles that constantly appear and disappear. These particles can influence the behavior of particles such as photons through their interactions. Vacuum polarization is the manifestation of these interactions, where the presence of virtual particles causes a change in the properties of the vacuum. This concept of the vacuum as a sea of virtual particles is crucial in understanding the behavior of particles and fields at the quantum level.
Direct observation of vacuum polarization is not currently possible, as it occurs at extremely small scales and is a property of the vacuum itself. However, its effects can be observed through various experimental techniques, as mentioned in the previous questions. The existence of vacuum polarization has been confirmed through a multitude of experiments and is an essential concept in modern physics.