  Interplays between quantum physics and gravity have long inspired exciting
studies, which reveal subtle connections between quantum laws and the general
notion of curved spacetime. One important example is the uniqueness of
free-falling motions in both quantum and gravitational physics. In this work,
we re-investigate the free-falling motions of quantum test wave packets that
are distributed over weakly curved spacetime backgrounds. Except for the de
Broglie relations, no assumption of priori given Hamiltonians or least actions
satisfied by the quantum system is made. We find that the mean motions of
quantum test wave packets can be deduced naturally from the de Broglie
relations with a generalized treatment of gravitational time dilations in
quantum waves. Such mean motions of quantum test masses in gravitational field
are independent of their masses and compositions, and restore exactly the
free-falling or geodesic motions of classical test masses in curved spacetime.
This suggests a novel perspective that weak equivalence principle, which states
the universality of free-fall, may be deeply rooted in quantum physics and be a
phenomenon that has emerged from the quantum world.
