Research Notes II, 1949

In the first approx. no pair production can be realized in the presence of Coulomb field + a plane wave, because the conditions of momentum energy conservation cannot be fulfilled. We must go to the second order approx.

Vacuum Polarization (Virtual production of pairs giving rise certain charge distribution)

Dirac density matrix

[equation] converges [?] ≠ 0 The subtraction device is made in such a way that it [gives?] a [?] result.

Dirac 1934

Heisenberg 2s.f Phys. 90, 209 ('34)

Wentzel 183 - 6 196 - 201

J. Schwinger P.R. 75, 651 ('49)