L. Tenuta
Quasi-static Limits in Nonrelativistic Quantum Electrodynamics
(116K, LaTex2e)

ABSTRACT.   We consider a system of N nonrelativistic particles of spin 1/2 interacting 
with the quantized Maxwell field (mass zero and spin one) in the limit when the 
particles have a small velocity, imposing to the interaction an ultraviolet 
cutoff, but no infrared cutoff. 
 Two ways to implement the limit are considered: c going to infinity with the 
velocity v of the particles fixed, the case for which rigorous results have 
already been discussed in the literature, and v going to 0 with c fixed. The 
second case can be rephrased as the limit of heavy particles, m_{j} --> 
epsilon^{-2}m_{j}, observed over a long time, t --> epsilon^{-1}t, epsilon --> 
0^{+}, with kinetic energy E_{kin} = Or(1). 
 Focusing on the second approach we construct subspaces which are invariant 
for the dynamics up to terms of order (v/c)sqrt{\log(v/c)^{-1}} and describe 
effective dynamics, for the particles only, inside them. At the lowest order, 
(v/c)^{0}, the particles interact through Coulomb potentials. At the second 
one, (v/c)^{2}, the mass gets a correction of electromagnetic origin and a 
velocity dependent interaction, the Darwin term, appears. Contrary to the case 
c going to infinity, the effective dynamics are independent of the spin of the 
particles. 
 Moreover, we calculate the radiated piece of the wave function, i. e., the 
piece which leaks out of the almost invariant subspaces and calculate the 
corresponding radiated energy. As in classical electrodynamics the leading part 
of the radiation vanishes when the particles have the same ratio of charge to 
mass.