A road map for Feynman's adventures in the land of gravitation

TL;DR

This paper reviews Feynman's extensive work and ideas on gravitation from the 1950s to the 1960s, highlighting his contributions to quantum gravity, gravitational waves, and his evolving perspective on the quantum nature of gravity.

Contribution

It reconstructs Feynman's detailed ideas on gravity and their evolution, emphasizing his view of gravity as a quantum phenomenon and his original approach inspired by electromagnetism.

Findings

Feynman viewed gravity as having quantum foundations.

He proposed that general relativity is the classical limit of a quantum theory.

Feynman's lectures and interventions reveal a consistent development of his ideas on quantum gravity.

Abstract

Richard P. Feynman's work on gravitation, as can be inferred from several published and unpublished sources, is reviewed. Feynman was involved with this subject at least from late 1954 to the late 1960s, giving several pivotal contributions to it. Even though he published only three papers, much more material is available, beginning with the records of his many interventions at the Chapel Hill conference in 1957, which are here analyzed in detail, and show that he had already considerably developed his ideas on gravity. In addition he expressed deep thoughts about fundamental issues in quantum mechanics which were suggested by the problem of quantum gravity, such as superpositions of the wave functions of macroscopic objects and the role of the observer. Feynman also 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 delivered at the Hughes Aircraft Company in 1966-67, whose focus was on astronomy and astrophysics. All this material allows to reconstruct a detailed picture of Feynman's ideas on gravity and of their evolution until the late sixties. According to him, gravity, like electromagnetism, has quantum foundations, therefore general relativity has to be regarded as the classical limit of an underlying quantum theory; this quantum theory should be investigated by computing physical processes, as if they were experimentally accessible. The same attitude is shown with respect to gravitational waves, as is evident also from an unpublished letter addressed to Victor F. Weisskopf. In addition, an original approach to gravity, which closely mimics (and probably was inspired by) the derivation of the Maxwell equations given by Feynman in that period, is sketched in the unpublished Hughes lectures.