This was posted before, way down in another thread and went unanswered. So I thought I will start a new thread of the interesting question. Quote: Originally Posted by Farsight I release a test particle of mass mp a given distance away from a black hole of mass M and measure their initial closing acceleration as a. If I now substitute the test particle with a second black whole of mass M, is the initial closing acceleration of the two black holes a or 2a or something else?. I think the locally Lorentz acceleration of a test particle starting from rest outside a black hole with mass M at circumferential distance 2 Pi r is given by: (with c=G=1) a = -(1-2M/r)^(-0.5) M/r^2 What is the equivalent for two black holes, momentarily at rest relative to each other, like Farsight asked above? I am pretty sure we can't just add the calculated accelerations like in the Newton case. Isn't there a "commandment" in relativity that says: "Thou shalt not add thine own acceleration directly to that of thine fellow traveler"?