  Local gravitational theories with more than four derivatives have remarkable
quantum properties, e.g., they are super-renormalizable and may be unitary in
the Lee-Wick sense. Therefore, it is important to explore also the IR limit of
these theories and identify observable signatures of the higher derivatives. In
the present work we study the scattering of a photon by a classical external
gravitational field in the sixth-derivative model whose propagator contains
only real, simple poles. Also, we discuss the possibility of a gravitational
seesaw-like mechanism, which could allow the make up of a relatively small
physical mass from the huge massive parameters of the action. If possible, this
mechanism would be a way out of the Planck suppression, affecting the
gravitational deflection of low energy photons. It turns out that the mechanism
which actually occurs works only to shift heavier masses to the further UV
region. This fact may be favourable for protecting the theory from
instabilities, but makes experimental detection of higher derivatives more
difficult.
