  We show how to apply post selection in the context of weak measurement of
Aharonov and collaborators to construct the quantum back reaction on a
classical field. The particular case which we study in this paper is pair
creation in an external electric field and the back reaction is the counter
field produced by the pair \underline {as} it is made. The construction leads
to a complex electric field obtained from non diagonal matrix elements of the
current operator, the interpretation of which is clear in terms of weak
measurement. The analogous construction applied to black hole physics (thereby
leading to a complex metric) is relegated to a future paper.
