Scott and Micheal are editing a dramatic piece right now about a series of alternate earths. They all have nexus points where history went on a different path (some in the distant past, such as a world where lizard like creatures developed with opposable thumbs). All conform to the laws of physics and most saw history change at a point where it very well might have (the Cuban Missile Crisis developed into a nuclear exchange, a fascist dictator became President of the United States in the late 1920s, etc.). To do such fiction or films correctly, you have to be a student of both science fiction and history.