  Local gravitational theories with more than four derivatives are
superrenormalizable, and also may be unitary in the Lee-Wick sense. Thus, it is
relevant to study the low-energy properties of these theories, especially to
identify observables which might be useful for experimental detection of higher
derivatives. Using an analogy with the neutrino Physics, we explore the
possibility of a gravitational seesaw mechanism, in which several dimensional
parameters of the same order of magnitude produce a hierarchy in the masses of
propagating particles. Such a mechanism could make a relatively light degree of
freedom detectable in low-energy laboratory and astrophysical observations,
such as torsion balance experiments and the bending of light. We demonstrate
that such a seesaw mechanism in the six- and more-derivative theories is unable
to reduce the lightest mass more than in the simplest four-derivative model.
Adding more derivatives to the four-derivative action of gravity makes heavier
masses even greater, while the lightest massive ghost is not strongly affected.
This fact is favorable for protecting the theory from instabilities, but makes
the experimental detection of higher derivatives more difficult.
