  In this paper, we take a control-theoretic approach to answering some
standard questions in statistical mechanics. A central problem is the relation
between systems which appear macroscopically dissipative but are
microscopically lossless. We show that a linear macroscopic system is
dissipative if and only if it can be approximated by a linear lossless
microscopic system, over arbitrarily long time intervals. As a by-product, we
obtain mechanisms explaining Johnson-Nyquist noise as initial uncertainty in
the lossless state as well as measurement back action and a trade off between
process and measurement noise.
