  A topic of present interest is the application of experimentally observed
quantum mechanical levels of ultra-cold neutrons in the earth's gravitational
field for searching short-range modifications to gravity. A constraint on new
forces in the nanometer-range published by Nesvizhevsky and Protasov follows
from inadequate modelling of the interaction potential of a neutron with a
mirror wall. Limits by many orders of magnitude better were already derived
long ago from the consistency of experiments on the neutron-electron
interaction.
