  For any choice of initial state and weak assumptions about the Hamiltonian,
large isolated quantum systems undergoing Schrodinger evolution spend most of
their time in macroscopic superposition states. The result follows from von
Neumann's 1929 Quantum Ergodic Theorem. As a specific example, we consider a
box containing a solid ball and some gas molecules. Regardless of the initial
state, the system will evolve into a quantum superposition of states with the
ball in macroscopically different positions. Thus, despite their seeming
fragility, macroscopic superposition states are ubiquitous consequences of
quantum evolution. We discuss the connection to many worlds quantum mechanics.
