The Bound States Problem in Quantum Electrodynamics and the Lamb Shift in Muonic Atoms

The bound state problem in quantum electrodynamics is discussed. Adiabatic $S$-matrix formalism, with which radiative corrections to energy levels of atoms are usually evaluated, is stated to provide only an approximate solution. It is shown that the radiative corrections are significant to the energy levels of muonic atoms that can not be predicted by the adiabatic $S$-matrix formalism.