In quantum electrodynamics (QED), which is the quantum field theory describing the electromagnetic force, photons are the quanta or particles associated with electromagnetic fields. Feynman diagrams are a graphical representation used to calculate and visualize the interactions between particles in quantum field theory.
In the context of QED, the interaction of photons with electromagnetic fields can be explained through the process of photon absorption and emission by charged particles. Let's consider a simple example: the scattering of an electron off a photon.
According to QED, the electron and the photon are described by their respective quantum fields. The interaction between them is mediated by the exchange of virtual photons, represented by a Feynman diagram. Here's a step-by-step explanation:
Incoming electron: We start with an initial electron represented by a solid line pointing to the right.
Incoming photon: We also have an incoming photon represented by a wavy line pointing to the right.
Vertex interaction: The electron interacts with the photon at a vertex. This interaction is represented by a point where the electron line and the photon line meet. At this vertex, the electron absorbs or emits a virtual photon.
Outgoing electron: After the interaction, the electron continues its trajectory, which is represented by a solid line pointing to the right.
Outgoing photon: Similarly, after the interaction, the photon continues its trajectory, represented by a wavy line pointing to the right.
In this Feynman diagram, the interaction between the electron and the photon is shown as the exchange of a virtual photon at the vertex. The virtual photon represents the exchange of energy and momentum between the electron and the photon. It is important to note that virtual particles, such as the virtual photon in this case, are not directly observable.
The probability of this interaction occurring is calculated by evaluating the Feynman diagram and summing up all possible contributions from different Feynman diagrams that describe the same physical process. This calculation involves complex mathematical expressions and perturbative expansions, but it provides a powerful framework for understanding and predicting the behavior of particles in quantum electrodynamics.
Overall, Feynman diagrams provide a visual representation of the interactions between particles, and in the case of QED, they help explain how photons, as particles, interact with electromagnetic fields through the exchange of virtual photons.