  Feynman's multifaceted contributions to gravitation, as can be inferred from
several published and unpublished sources, are here reviewed. Feynman thought
about this subject at least from late 1954 to the late '60s, giving several
contributions to it. Even if he only published three papers on gravity, much
more material is available, especially the records of his many interventions at
the Chapel Hill conference in 1957, which are here analyzed in detail. There,
Feynman showed that he had already developed much of his picture of gravity,
and he expressed deep thoughts about fundamental issues in quantum mechanics,
such as superpositions of the wave functions of macroscopic objects and the
role of the observer, which were suggested by the problem of quantum gravity.
Moreover, Feynman lectured on gravity several times. Besides the famous
lectures given at Caltech in 1962-63, he extensively discussed this subject in
a series of lectures given at the Hughes Aircraft Company in 1966-67, whose
focus was astronomy and astrophysics. Here Feynman gave a somewhat simplified
exposition with respect to the Caltech lectures, but with many original points.
All this material allows to reconstruct a detailed picture of Feynman's ideas
on gravity and of its evolution until the late sixties. The main points are
that gravity, like electromagnetism, has a quantum foundation, so that
classical general relativity has to be regarded as emerging from an underlying
quantum theory, and that this quantum theory has to be investigated by
computing physical processes, as if they were measurable. The same attitude is
shown with respect to gravitational waves, as evident both from the Chapel Hill
records and a letter written to V. Weisskopf. As a bonus, an original approach
to gravity, which closely mimics the derivation of the Maxwell equations he
gave in that period, is hinted at in the unpublished lectures.
