Richard Schroeppel <rcs@cs.arizona.edu> writes:

The finite speed of gravitational attraction should be an issue for galactic simulations, even if other relativistic effects do not arise. E.g., suppose that some configuration several light years away suddenly flies apart. The observer (viewer _or_ body effected by the gravitational change) wouldn't notice it for quite a while.

That always made me wonder. After all, if the configuration is far enough away to notice the delay, it ought to gravitate the observer much like a point mass. If the configuration flies apart, conservation of momentum keeps the center of mass fixed (or on the same inertial path), and conservation of mass keeps the magnitude the same. Now some of the mass needs to become energy to make the body fly apart, but I think the energy has the same gravitational effect, no? This is all very confusing to me, but I suppose we could turn all the mass into photons, half toward the observer and half away. The attraction will approach infinity when the photons arrive. But with lightspeed propagation of gravity, the gravity does not change until the photons arrive, so it essentially jumps to infinity rather than increasing gradually. Is that a sensible scenario for testing the difference, or is there something I've missed? Dan Hoey@AIC.NRL.Navy.Mil