What about M-Theory (which used to be called String Theory) where there are an infinite number of universes? This was a key point in my ginormous novel. If there are multiple universes and each one is created when a different choice was made that altered the course of history, then there would be millions or billions of versions of us in each, some where we have already died, some where we are older, some younger, some where we never existed at all because of a break in the family tree. Every time I make it safe after a car or plane trip home, I wonder if in one of the universes I didn't survive and my family is grieving. It's a weird thought to wrap your head around. I posited in the novel that if such universes exist, we would always be alive in at least one universe and so never die. Because perhaps on that one they have figured out how to cheat death with technologies we are only beginning to explore on this particular planet in this universe. Fun stuff.