Malcolm A. H. MacCallum (non-tilted spatially homogeneous cosmologies, exact solutions book), M. Mathisson (Mathisson–Dixon–Papapetrou equations, Mathisson–Pirnai condition in the Kerr metric), Richard A. Matzner (popularized Penrose picture of gravitational wave, rotating cosmologies), Marc Mars (Mars vacuum), David Maxwell (Yamabe number criterion for existence of asymptotically flat vacuum solutions), R. G. McLenaghan (CM invariants), Reinhard Meinel (Neugebauer/Meinel dust disk solution), M. A. Melvin (Melvin electrovacuum), A.W.K. Metzner (Gravitational waves, Bondi–Metzner–Sachs Group) Hermann Minkowski (spacetime), Charles W. Misner (mixmaster model, ADM initial value formulation, ADM mass, textbook) John Moffat (various classical gravitation theories) Vincent Moncrief (global properties of spatially compact dynamical vacuum spacetimes), C. Møller (energy-momentum complex), Moustafa Mosharafa (Relation of radiation, mass and energy),