Self-energy

In quantum field theory, self-energy${\displaystyle \Sigma }$ is the energy of a particle due to the interaction with its own field. In electrostatics, an electron is never really free. It is always interacting with its own electric field, independent of the environment it is in. In condensed matter physics, self-energy creates quasiparticles and gives them a lifetime. Self-energy is especially used to describe electron-electron interactions in Fermi liquids. Another example of self-energy is found in the context of phonon softening due to electron-phonon coupling.