After the success of quantum electrodynamics, Feynman turned to quantum gravity. By analogy with the photon, which has spin 1, he investigated the consequences of a free massless spin 2 field, and derived the Einstein field equation of general relativity, but little more. The computational device that Feynman discovered then for gravity, "ghosts", which are "particles" in the interior of his diagrams that have the "wrong" connection between spin and statistics, have proved invaluable in explaining the quantum particle behavior of the Yang–Mills theories, for example, QCD and the electro-weak theory.
క్వాంటం గురుత్వాకర్షణపై పనిచేస్తున్నప్పుడు ఫేన్మాన్ ఏమి పరిశోధించాడు?
consequences of a free massless spin 2 field