Gene: The basis of this caution is the fact from electromagnetic theory that the electric field of a charge moving at constant velocity is directed towards (or away from) the instantaneous position of the charge. It does not point to where the charge was at an earlier time as if the field propagated at the speed of light. Warren: --supermassive BHs in galaxies are only, like, 0.5% of the galaxy mass, or less. So they don't really matter for this, but they do matter in the sense that they seem to stabilize galaxies, and may even be crucial for that. ------------ Points taken. Re Gene: I was going to suggest using the GR ray-tracing program developed for the movie "Interstellar", but if they've taken Gene's comments to heart, then perhaps the Interstellar SW might not do such a good job calculating the gravitational lensing for focussing *gravitational forces*. It would be interesting to ask the Interstellar folks in any case. In any case, Gene's comments might change exactly where a focal point might be and what the point spread function would look like, but it shouldn't change the fact that there *is* a focal point. Re Warren: Of course, the galaxy lensing effect isn't just due to the black hole itself, but to the entire amount of mass influencing the curvature of the space that the gravitational force travels through; the black hole is one certain place where the space is guaranteed warped enough to cause a lensing effect. Furthermore, if there really is "dark matter", then that should cause even greater lensing effects. I continue to believe that any black hole in the galactic center would be the "same" black hole for both normal matter and for dark matter. Perhaps that assumption is wrong?