  Experimentally, certain degrees of freedom may appear classical because their
quantum fluctuations are smaller than the experimental error associated with
measuring them. An approximation to a fully quantum theory is described in
which the self-interference of such ``quasiclassical'' variables is neglected
so that they behave classically when not coupled to other quantum variables.
Coupling to quantum variables can lead to evolution in which quasiclassical
variables do not have definite values, but values which are correlated to the
state of the quantum variables. A mathematical description implementing this
backreaction of the quantum variables on the quasiclassical variables is
critically discussed.

Quantum Physics;General Relativity and Quantum Cosmology