Continuing on yesterday's theme of light being bent by gravity, what happens when gravity is really strong, such as around a block hole? At 1.5 times the Schwarzchild radius (the "point of no return") of a black hole, light is bent so much that it actually orbits around and around the black hole! If you were at that distance from a black hole and looked out at a tangential angle, the surface below you would look perfectly flat and you would see the back of your own head all along the horizon!
But if light going around in circles is weird, time going around in circles is even weirder. According to General Relativity, deep within the bowels of certain rotating black holes it may be possible to move around in a circle and end up not only where you started but also when you started. Gravity is so strong that it bends time into loops and events can occur which are their own cause and their own effect. A time machine. All this is hidden behind the event horizon of the black hole so we could never actually observe this time travel going on but it's pretty weird nonetheless.
This is one of the most mind-boggling things I learnt about while studying physics at university. That and one of the last lectures I ever attended, which was given by Sir Martin Rees and was on the subject of the fate of the universe in the extremely long term. I'll write about that tomorrow.