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 particles 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.
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