  Macroscopic systems are described most completely by local densities
(particle number, momentum and energy) yet the superposition states of such
physical variables, indicated by the Everett interpretation, are not observed.
In order to explain this, it is argued that histories of local number, momentum
and energy density are approximately decoherent when coarse-grained over
sufficiently large volumes. Decoherence arises directly from the proximity of
these variables to exactly conserved quantities (which are exactly decoherent),
and not from environmentally-induced decoherence. We discuss the approach to
local equilibrium and the subsequent emergence of hydrodynamic equations for
the local densities. The results are general but we focus on a chain of
oscillators as a specific example in which explicit calculations may be carried
out. We discuss the relationships between environmentally-induced and
conservation-induced decoherence and present a unified view of these two
mechanisms.
