  We analyze the recent proposal of measuring a quantum gravity phenomenon in
the lab by entangling two particles gravitationally. We give a generally
covariant description of this phenomenon, where the relevant effect turns out
to be a quantum superposition of proper times. We point out that measurement of
this effect would count as evidence for quantum superposition of spacetime
geometries. This interpretation addresses objections appeared in the
literature. We observe that the effect sheds light on the Planck mass, and
argue that it is very plausibly a real effect.
