  The Bose-Marletto-Vedral experiment tests a non-relativistic quantum effect
due to a gravitational interaction. It has received attention because it may
soon be within observational reach in the lab. We observe here that: (i) in
relativistic language the experiment tests an interference effect between
proper-time intervals; (ii) the relevant difference of proper times is of the
order of the Planck time if the masses of the particles in the experiment are
of the order of the Planck mass (micrograms); (iii) the experiment might open a
window on the structure of time at the Planck scale: if time differences are
discrete at this scale ---as quantum gravity research may suggest--- the
Planckian discreteness of time could show up as quantum levels of a measurable
entanglement entropy.
