In quantum field theory, the energy that a particle has as a result of changes that it causes in its environment defines self-energy
In a condensed matter context, self-energy is used to describe interaction induced renormalization of quasiparticle mass (dispersions) and lifetime.
Another example of self-energy is found in the context of phonon softening due to electron-phonon coupling.
Mathematically, this energy is equal to the so-called on mass shell value of the proper self-energy operator (or proper mass operator) in the momentum-energy representation (more precisely, to
Using a small number of simple rules, each Feynman diagram can be readily expressed in its corresponding algebraic form.
In general, the on-the-mass-shell value of the self-energy operator in the momentum-energy representation is complex.
Neutral particles with internal quantum numbers can mix with each other through virtual pair production.
Under appropriate simplifying assumptions this can be described without quantum field theory.
[citation needed] In solid state and condensed-matter physics self-energies and a myriad of related quasiparticle properties are calculated by Green's function methods and Green's function (many-body theory) of interacting low-energy excitations on the basis of electronic band structure calculations.