  We propose a quantum optical version of Schr\"{o}dinger's famous gedanken
experiment in which the state of a microscopic system (a cavity field) becomes
entangled with and disentangled from the state of a massive object (a movable
mirror). Despite the fact that a mixture of Schr\"{o}dinger cat states is
produced during the evolution (due to the fact that the macroscopic mirror
starts off in a thermal state), this setup allows us to systematically probe
the rules by which a superposition of spatially separated states of a
macroscopic object decoheres. The parameter regime required to test
environment-induced decoherence models is found to be close to those currently
realizable, while that required to detect gravitationally induced collapse is
well beyond current technology.
