Sci-fi staple, the parallel universe, examined by Sci. A lot to take in here even with Maciej’s excellent bullet-point summary:
“The first kind of parallel universe is an inevitable consequence of the fact that we seem to live in an open universe with a uniform distribution of matter. Because all energy is quantized, a given volume of space can only contain a certain (unbelievably huge) number of configurations of matter (unless you do something crazy like turn the temperature to infinity). In an infinite Universe like ours, that means any finite volume of space is bound to repeat itself somewhere. An infinite number of somewheres. You just have to be willing to travel. According to Tegmark, the nearest copy of yourself is about 10^(10^29) meters away (he declines to say in which direction). Finding the nearest copy of our entire visible Universe is more of a slog – it’s about 10^(10^181) meters away. There’s also every imaginable near-variant to be found – a mirror-Earth where you’re wearing a different colored shirt, a mirror-Earth with a monkey typing Shakespeare, an infinite number of mirror-Earths where Ann Coulter gets sacrificed to a volcano god tomorrow.”
Petafloptimism 