  We have recently introduced a realistic, covariant, interpretation for the
reduction process in relativistic quantum mechanics. The basic problem for a
covariant description is the dependence of the states on the frame within which
collapse takes place. A suitable use of the causal structure of the devices
involved in the measurement process allowed us to introduce a covariant notion
for the collapse of quantum states. However, a fully consistent description in
the relativistic domain requires the extension of the interpretation to quantum
fields. The extension is far from straightforward. Besides the obvious
difficulty of dealing with the infinite degrees of freedom of the field theory,
one has to analyze the restrictions imposed by causality concerning the allowed
operations in a measurement process. In this paper we address these issues. We
shall show that, in the case of partial causally connected measurements, our
description allows us to include a wider class of causal operations than the
one resulting from the standard way for computing conditional probabilities.
This alternative description could be experimentally tested. A verification of
this proposal would give a stronger support to the realistic interpretations of
the states in quantum mechanics.
