Schematic cartoon of gravitational interactions between positive (in yellow) and negative (in purple) mass particles. Black vectors indicate the direction of the gravitational force, F_g = −GM₁M₂/r², that is experienced by a given particle. Red vectors indicate the direction of the acceleration, a = F_g/M, that is experienced by a given particle when the weak equivalence principle holds. There are three possible cases: (i) top row: the familiar positive–positive mass interaction, in which both particles accelerate towards one another via gravitational attraction, (ii) middle row: the positive–negative mass interaction, in which both particles accelerate in the same direction – pointing from the negative mass towards the positive mass. This case is sometimes referred to as runaway motion, and (iii) bottom row: the negative–negative mass interaction, in which both particles accelerate away from one another via gravitational repulsion.