Watsisname wrote:midtskogen wrote:Source of the post What I don't quite catch here is that the black hole could make me see the redshifted things behind me in two different directions, once directly (behind) and once bent around the black hole (in front). Therefore I would expect bands.
Aye, I think I see your confusion, and it's my fault for not explaining the special relativistic aberration effect very well. Yes, the black hole generates many images of the same object, because light rays may circulate around many times near the photon orbit before spiraling back to reach you. But the special relativistic red or blueshift of the image due to your velocity does not depend on whether the source of that light was behind or in front of you, but rather only on the direction from which the light ray ultimately intersects you, and aberrated (appearing to come more from the direction you are flying to). Normally in special relativity this distinction wouldn't matter, but with gravity bending the paths of light rays, it is surely confusing.
So an image that was lensed by the black hole to appear in front of you -- of an object that is actually behind you -- will be seen to be blueshifted, not redshifted (if you're hurtling toward the black hole with a great initial velocity). This is because the change in the photon's energy did not depend on the path it took around the black hole (that's what I mean earlier by the path independence due to gravity being a conservative force), but you are "driving into it" (invoking the driving in rain analogy for aberration), and because you can't measure a faster speed of light you instead see it have a higher energy (bluer color). You'll get multiple images, but they'll progressively go from redder appearing behind you, to bluer appearing more directly ahead.
hmmm this sounds a lot like how the sound of a train whistle changes pitch to an observer on a platform (but in reverse, since the observer in that case is the stationary one.)