  A pedagogical introduction is given to the quantum mechanics of closed
systems, most generally the universe as a whole. Quantum mechanics aims at
predicting the probabilities of alternative coarse-grained time histories of a
closed system. Not every set of alternative coarse-grained histories that can
be described may be consistently assigned probabilities because of quantum
mechanical interference between individual histories of the set. In the quantum
mechanics of closed systems, containing both observer and observed,
probabilities are assigned to those sets of alternative histories for which
there is negligible interference between individual histories as a consequence
of the system's initial condition and dynamics. Such sets of histories are said
to decohere. Typical mechanisms of decoherence that are widespread in our
universe are illustrated. Copenhagen quantum mechanics is an approximation to
the more general quantum framework of closed subsystems. It is appropriate when
there is an approximately isolated subsystem that is a participant in a
measurement situation in which (among other things) the decoherence of
alternative registrations of the apparatus can be idealized as exact. Since the
quantum mechanics of closed systems does not posit the existence of the
quasiclassical domain of everyday experience, the domain of the approximate
aplicability of classical physics must be explained. We describe how a
quasiclassical domain described by averages of densities of approximately
conserved quantities could be an emergent feature of an initial condition of
the universe that implies the approximate classical behavior of spacetime on
accessible scales.
