Multi-local Relational Description of the Measurement Process in Quantum Field Theory

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. This 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 and solve the objections raised by Sorkin concerning the impossibility of ideal measurements in quantum field theory. Here we show that in order to retain a causal theory it is necessary to modify the standard Wigner expression for the cases of partial causally connected local measurements. Our description could be experimentally tested. A verification of the new expression would give a stronger support to the realistic interpretations of the states in quantum mechanics.