  We analyse the wave function collapse as seem by two distinct observers (with
identical detectors) in relative motion. Imposing that the measurement process
demands information transfer from the system to the detectors, we note that
although different observers will acquire different amount of information from
their measurements due to correlations between spin and momentum variables, all
of them will agree about the orthogonality of the outcomes, as defined by their
own reference frame. So, in this sense, such a quantum mechanical postulate is
observer invariant, however the effective efficiency of the measurement process
differs for each observer.
