Self-energy
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In quantum field theory and statistical mechanics one encounters the 
concept of the self-enrgy of a particle.

But self-energy is not a fraction of the energy associated with a 
single particle but a function of momentum describing the off-shell 
dispersion of the associated wave functions in interactions.
 
The self-energy governs the form of the kinetic energy term in a 
single-particle equation, and describes kinetic corrections to the 
Schroedinger or Pauli equation for nonrelativistic particles, and to 
the Klein-Gordon equation, the Dirac equation, or the  Maxwell 
equations. for relativistic scalar, spinor, and vector particles. 
(The correction to the interaction term is goverend by form factors;
see the FAQ entry ''Are electrons pointlike/structureless?''.)

The ''self'' is a misnomer, due only to historical reasons. 
Instead, the self-energy is created by the cooperative effects 
of the environment of a particle.

In statistical mechanics, another term for the self-energy is 
''memory function''. These terms are used synonymously, and describe 
(together with form factor corrections, if they exist) the memory a 
particle has of its multiparticle origin, when a multiparticle system 
is (approximately) separated into a single particle and an effective 
field representing the remainder of the universe.