In the thought experiment I mentioned, we have two objects with masses m,-m in an otherwise empty universe. Newton's gravitational law says they repell each other. But Newton's law of motion says that when you push rightwards on an object with negative mass, it moves leftwards. For this reason, the object with mass m moves away from the object with mass -m (since it is being repelled), but the object with mass -m moves in the same direction with the same acceleration. Thus, the pair maintain constant distance and accelerate as a system wrt any inertial observer, with constant acceleration, indefinitely. Thus after finite time they achieve any given desired velocity wrt any given inertial observer!

That is a Newtonian thought experiment, so something similar must happen in any gravitation theory which has a Newtonian limit.

__________________
Chris Hillman

Read these PF posts. Avoid Wikipedia--- except for these versions. Read this and this suggested sticky. When asked for advice, I always say: never take advice!