Thought experiments on relativistic forces

Abstract: For stationary space-times an object is at rest if its world line lies along the time-like Killing vector that becomes (1,0,0,0) at infinity. The gravitational force on an object at rest is minus the force required to keep it at rest. We shall be interested in the forces due to energy, currents of energy, currents of momentum and currents of angular momentum on both static and moving bodies. We also discuss Bonnor’s beautiful 1969 work on the gravity of a beam of light. Finally (if there is time) we show that Landau and Lifshitzs’ gravitational forces omit one cause of changes in momentum of free bodies.