  According to Feynman, the universe consists of two parts - the system in
which we are interested and the rest of the universe which our measurement
process does not reach. Feynman then formulates the density matrix in terms of
the observable world and the rest of the universe. It is shown that coupled
harmonic oscillators can serve as an illustrative example for Feynman's ``rest
of the universe.'' It is pointed out that this simple example has far-reaching
consequences in many branches of physics, including statistical mechanics,
measurement theory, information theory, thermo-field dynamics, quantum optics,
and relativistic quantum mechanics. It is shown that our ignorance of the rest
of the universe increases the uncertainty and entropy in the system in which we
are interested.
