Why Is the Photon One-Point Function Zero in QED?

In summary, Peskin explains in "An Introduction to Quantum Field Theory" that in order to analyze the photon one-point function, the external photon must be attached to a QED vertex. By neglecting the external photon propagator, the resulting amplitude is equivalent to the diagram with two fermion lines and a vertex. If the photon propagator is included, the expression gives the same diagram as if the current were replaced by the photon field, representing the photon one-point correlation function.
  • #1
taishizhiqiu
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In 'an introduction to quantum field theory' by peskin, he writes: To analyze the photon one-point function, note that the external photon must be attached to a QED vertex. Neglecting the external photon propagator, this amplitude is therefore:

upload_2015-10-1_16-44-37.png


I really cannot justify this equation. Can anyone explain it to me?
 
  • #3
Intuitively, the expression is essentially the diagram with the two fermion lines and vertex (by definition of [itex]j_{\mu}[/itex]). Note the, "neglecting the external photon propagator." If you include the photon propagator as a factor, the expression gives rise to the same diagram as if the current were to be replaced by the photon field, i.e. the photon one-point correlation function.
 

Related to Why Is the Photon One-Point Function Zero in QED?

What is a photon one point function?

A photon one point function is a mathematical expression that describes the behavior of a single photon in a given system. It takes into account the properties of the photon, such as its energy and momentum, as well as the properties of the system it is interacting with.

How is a photon one point function calculated?

A photon one point function is calculated using the principles of quantum mechanics, specifically the Schrödinger equation. This equation takes into account the wave-like nature of particles, such as photons, and describes how they evolve over time in a given system.

What is the significance of the photon one point function?

The photon one point function is significant because it allows scientists to make predictions about the behavior of photons in a given system. This is important for understanding and manipulating light in various applications, such as in lasers and telecommunications.

Can the photon one point function be applied to other particles?

Yes, the principles behind the photon one point function can be applied to other types of particles, such as electrons and protons. However, the specific equations and calculations may differ depending on the properties of the particle.

Are there any practical applications of the photon one point function?

Yes, the photon one point function has practical applications in various fields, including quantum computing, optical communications, and medical imaging. It allows scientists to accurately predict the behavior of photons in these systems, leading to new technologies and advancements in these areas.

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