  Investigation into the applicability of the equivalence principle in quantum
mechanics has taken many forms, with varying conclusions. Here, a dynamical
semi-classical description of a wave packet in terms of its center of mass and
higher quantum fluctuations is applied to the case of a quantum particle in
gravitational free fall. The analysis provides an intuitive account of the
origin of mass-dependence in quantum-gravitational dynamics through an
effective potential that enforces the uncertainty principle. This potential has
two implications: (i) The lowest order quantum fluctuations encoding the width
and spreading of the wave packet obey an uncertainty relation whose observance
is mass-dependent. (ii) In an inhomogeneous gravitational field tidal effects
couple the center of mass motion to the quantum fluctuations. The combined
effect results in a clear demonstration of how some conceptions of the weak
equivalence principle, based on mass dependence, are violated. The size of this
violation is within sensitivities of current Eotvos and clock-based return time
experiments.
