The opening montage of Six Feet Under's first episode of the third season got me thinking. It explores the "Many Worlds" quantum theory with Nate walking through his house seeing alternate lives of his own. All lie in the future and all are vastly different. There have been many, many other examples of this in Hollywood. However, Back to the Future, dealt with only one continuous life, but allowed the future to exist in the past, constantly interacting with each other and shaping outcomes by altering important events.
Does everything happen in alternate realities? If so, how small are the branches? If I decide to cross the street in front of a car (and subsequently get hit), the resulting "life" will obviously be different than if I choose to let the car pass first. That's a big branch. But, if I decide to put my left pant leg on before my right like normal, how much different will my life be? Although it's cool to think about, I am not really buying it. As complex as the human body is, can you imagine an infinite amount more, all living out an infinite amount of your life?