  Experiments are beginning to probe the interaction of quantum particles with
gravitational fields beyond the uniform-field regime. In non-relativistic
quantum mechanics, the gravitational field in such experiments can be written
as a superposition state. We empirically demonstrate that alternative theories
of gravity can avoid gravitational superposition states only by decoupling the
gravitational field energy from the quantum particle's time evolution.
Furthermore, such theories must specify a preferred quantum reference frame in
which the equations of motion are valid. To the extent that these properties
are theoretically implausible, recent experiments provide indirect evidence
that gravity has quantum features. Proposed experiments with superposed
gravitational sources would provide even stronger evidence that gravity is
nonclassical.
