A physical distinction between a covariant and a non-covariant reduction process in relativistic quantum theories

Abstract

Causality imposes strong restrictions on the type of operators that may be observables in relativistic quantum theories. In fact, causal violations arise when computing conditional probabilities for certain partial causally connected measurements using the standard non-covariant procedure. Here we introduce another way of computing conditional probabilities, based on an intrinsic covariant relational order of the events, which differs from the standard one when this type of measurement is included. This alternative procedure is compatible with a wider and very natural class of operators without breaking causality. If some of these measurements can be implemented in practice, as predicted by our formalism, the non-covariant, conventional approach should be abandoned. Furthermore, the description we promote here would imply a new physical effect where interference terms are suppressed as a consequence of the covariant order in the measurement process.