  Although the standard viewpoint in theoretical physics is that the
unification of quantum theory and general relativity requires the quantization
of gravity and spacetime, there is not consensus about whether spacetime must
fundamentally have any quantum features. Here we show a theorem stating that
spacetime degrees of freedom and a quantum system violate a Bell inequality in
a background Minkowski spacetime if a few properties of general relativity and
quantum theory have a broad range of validity, and if the quantum state
reduction upon measurement is a real physical process that is completed
superluminally when acting on distant quantum particles in a quantum entangled
state. We argue that this implies that spacetime cannot be sensibly called
classical if the assumptions in our theorem hold. In contrast to the
Eppley-Hannah argument for the necessity of quantizing the gravitational field,
we discuss the validity of our assumptions, our thought experiment does not
require to manipulate or detect gravitational waves, and our theorem does not
rely on the conservation of momentum or on the uncertainty principle.
