  The equilibrium and fluctuations of an ideal gas in a rigid container are
studied by every student of statistical mechanics. Here we study the less
well-known case when the box is floating freely; in particular we determine the
fluctuations of the box in velocity and position due to interactions with the
gas it contains. This system is a toy model for the fluctuations in velocity
and position of a black hole surrounded by stars at the center of a galaxy.
These fluctuations may be observable in nearby galaxies.
