Thought Experiments on Gravitational Forces
  Large contributions to the near closure of the Universe and to the
acceleration of its expansion are due to the gravitation of components of the
stress-energy tensor other than its mass density. To familiarise astronomers
with the gravitation of these components we conduct thought experiments on
gravity, analogous to the real experiments that our forebears conducted on
electricity. By analogy to the forces due to electric currents we investigate
the gravitational forces due to the flows of momentum, angular momentum, and
energy along a cylinder. Under tension the gravity of the cylinder decreases
but the 'closure' of the 3-space around it increases. When the cylinder carries
a torque the flow of angular momentum along it leads to a novel helical
interpretation of Levi-Civita's external metric and a novel relativistic
effect. Energy currents give gravomagnetic effects in which parallel currents
repel and antiparallel currents attract, though such effects must be added to
those of static gravity. The gravity of beams of light give striking
illustrations of these effects and a re-derivation of light bending via the
gravity of the light itself. Faraday's experiments lead us to discuss lines of
force of both gravomagnetic and gravity fields. A serious conundrum arrises if
Landau and Lifshitz's definition of Gravitational force is adopted.
