In classical physics the self-energy of a system is the contribution to the energy of the system resulting from the interaction between different parts of the system. In quantum field theory the self-energy of a particle is the contribution to the energy of the particle due to virtual emission and absorption of particles. In the many-body problem in quantum mechanics the self-energy E_s of a particle is the difference E_Q−E_B, where E_Q is the energy of the quasiparticle associated with the particle and E_B is the energy of the ‘bare’ particle. The particle interacts with its surrounding medium, which in turn acts back on the original particle.