  We propose an approach that allows to systematically take into account
gravity in quantum particle physics. It is based on quantum field theory and
the general principle of relativity. These are used to build a model for
quantum particles in curved spacetime. We compute by its means a deviation from
a classical geodesic in the Earth's gravitational field. This shows that free
fall depends on quantum-matter properties. Specifically, we find that the
free-fall universality and the wave-packet spreading are mutually exclusive
phenomena. We then estimate the E\"{o}tv\"{o}s parameter for a pair of atoms
freely falling near the Earth's surface, provided that the wave-packet
spreading is more fundamental than the weak equivalence principle.
