  Ultimately, any explanation of quantum measurement must be extendable to
relativistic quantum mechanics (RQM), since many precisely confirmed
experimental results follow from quantum field theory (QFT), which is based on
RQM. Certainly, the traditional "collapse" postulate for quantum measurement is
problematic in a relativistic context, at the very least because, as usually
formulated, it violates the relativity of simultaneity. Among alternatives to
the traditional collapse interpretation, the Everettian approach of an
unmodified, unitary quantum formalism is the only one that has been clearly
extended to RQM and QFT. However, the usual "many worlds" interpretation of
such an approach leads to to difficulty in how to even define probabilities
over different possible "worlds". The present paper addresses this difficulty
by providing a relativistic model of measurement, in which the state of the
universe is decomposed into decoherent histories of measurements recorded
within it. Zurek's concept of envariance can be generalized to this context of
relativistic spacetime, giving an objective definition of the probability of
any one of these quantum histories, consistent with Born's rule. This then
leads to the statistics of any repeated experiment also tending to follow the
Born rule as the number of repetitions increases. The wave functions that we
actually use for such experiments are local reductions of very coarse-grained
superpositions of universal eigenstates, and their "collapse" can be
re-interpreted as simply an update based on additional incremental knowledge
gained from a measurement about the "real" eigenstate of our universe.
