I had a few questions about black holes and photons that get stuck in an orbit around them. With some Googling, I was able to get the answers I so badly needed, but, as it is with these sorts of things, I only got curioser and curioser. So, let's review what I wanted to find out in the first place. The question that kept me awake last night (fortunately my girlfriend wasn't around, else I'd have been distracted from nookie -- and yes, unfortunately that has happened before with such issues) was a rather simple one. Black holes can bend light, but can a photon get stuck in an orbit around a black hole? The answer is yes. Thanks to this page: http://www.physics.nus.edu.sg/~phyteoe/kerr/ Coming from a computer science and maths background, it occured to me that a photon has no mass, so it's not actually "falling" into a black hole while furiously travelling in another direction, therefore getting locked into orbit. Rather, the photon drives straight, it's the road that's developed a bend. But is it possible for a region of space to exist that turns back on itself? Topologically it doesn't make much sense. But that's not why I'm posting. The way I understand it, light gets blueshifted when it approaches a black hole and redshifted when it moves away from a black hole. But what exactly happens to a photon that's locked into orbit around a black hole? There can be two explanations. First, the shift is due to the direction in which the photon is moving. Towards a massive object means blueshift and away from a massive object means redshift. In that case once a photon is in orbit, nothing much happens to it. Just keeps on squiggling around the black hole. In the second case, moving through a gravitational field causes redshifting. Period. The "blueshift" is just how an observer falling towards a black hole would see stuff behind him. But that makes me curioser once again and I ask you: how much redshifting can a poor photon take and what happens to it when it can't take no more? It flatlines? So which is the case? And what actually happens to the poor photon that's been circling a black hole for ages?