  We argue that the light particles in string theory obey an effective quantum
mechanics modified by the inclusion of a quantum-gravitational friction term,
induced by unavoidable couplings to unobserved massive string states in the
space-time foam. This term is related to the $W$-symmetries that couple light
particles to massive solitonic string states in black hole backgrounds, and has
a formal similarity to simple models of environmental quantum friction. It
increases apparent entropy, and may induce the wave functions of macroscopic
systems to collapse.
