John Archibald Wheeler and Kip Thorne wrote the definitive book on the subject of gravity many years ago, Gravitation, 1973, 1336 pages. Everybody and his brother now knows that gravity is actually the warping or distorting of spacetime as a consequence of mass. That’s the reason the LIGO Project detected gravity waves a couple years ago. It detected gravity waves created by a merger of two monster size black holes. The LIGO sensors detect ripples in spacetime. The head of the LIGO Project for most of its life was Kip Thorne, former student of Wheeler and co-author of Gravitation.
Addendum for the advanced student: Considering the notion that positrons were electrons that were traveling backwards in time, Wheeler came up in 1940 with his one-electron universe postulate: that there was in fact only one electron, bouncing back and forth in time. His graduate student while was a professor at Princeton, Richard Feynman, found this hard to believe, but the idea that positrons were electrons traveling backwards in time intrigued him and Feynman incorporated the notion of the reversibility of time into his Feynman diagrams.[24]
With Neils Bohr Wheeler helped explain nuclear fission.