Radiation Damping and Velocity of Light

Abstract

IN a recent communication, Ferretti and Peierls1 have applied the theory of radiation damping suggested by Peng and me2,3 to the problem of the propagation of light. By considering the exchange of excitation energy between two like atoms through emission and absorption of light, they came to the conclusion that, according to this theory, light does not travel with the velocity c. The result must seem surprising for the following reasons : (i) The propagation of light is a property of the vacuum and has nothing to do with the approximation in which the interaction with electrons is treated. (ii) The problem has already been treated by Kikuchi4 on the grounds of the earlier theory of Weisskopf and Wigner5, which is a special case of the more general theory quoted above. Kikuchi found the correct velocity of light. (iii) The theory of damping consists of nothing but a systematic elimination of the infinite parts of quantum-electrodynamics, a procedure which for special problems has been, and had to be, adopted before in nearly all applications of quantum-electrodynamics, including the theory of Weisskopf-Wigner (which does not mean that this crude procedure yields always exact and correct results. It is particularly to be noted that the avoidance of the infra-red catastrophe rests on the low-energy end of the neglected diverging integrals6.)