  Motivated by the advances of quantum Darwinism and recognizing the role
played by redundancy in identifying the small subset of quantum states with
resilience characteristic of objective classical reality, we explore the
implications of redundant records for consistent histories. The consistent
histories formalism is a tool for describing sequences of events taking place
in an evolving closed quantum system. A set of histories is consistent when one
can reason about them using Boolean logic, i.e., when probabilities of
sequences of events that define histories are additive. However, the vast
majority of the sets of histories that are merely consistent are flagrantly
non-classical in other respects. This embarras de richesses (known as the set
selection problem) suggests that one must go beyond consistency to identify how
the classical past arises in our quantum Universe. The key intuition we follow
is that the records of events that define the familiar objective past are
inscribed in many distinct systems, e.g., subsystems of the environment, and
are accessible locally in space and time to observers. We identify histories
that are not just consistent but redundantly consistent using the partial-trace
condition introduced by Finkelstein as a bridge between histories and
decoherence. The existence of redundant records is a sufficient condition for
redundant consistency. It selects, from the multitude of the alternative sets
of consistent histories, a small subset endowed with redundant records
characteristic of the objective classical past. The information about an
objective history of the past is then simultaneously within reach of many, who
can independently reconstruct it and arrive at compatible conclusions in the
present.

Quantum Physics