  The physics of low-energy quantum systems is usually studied without explicit
consideration of the background spacetime. Phenomena inherent to quantum theory
on curved space-time, such as Hawking radiation, are typically assumed to be
only relevant at extreme physical conditions: at high energies and in strong
gravitational fields. Here we consider low-energy quantum mechanics in the
presence of gravitational time dilation and show that the latter leads to
decoherence of quantum superpositions. Time dilation induces a universal
coupling between internal degrees of freedom and the centre-of-mass of a
composite particle. The resulting correlations cause decoherence of the
particle's position, even without any external environment. We also show that
the weak time dilation on Earth is already sufficient to decohere micron scale
objects. Gravity therefore can account for the emergence of classicality and
the effect can in principle be tested in future matter wave experiments.
