Alternate universes or alternate realities are often used in time travel stories. I have usually seen it done in two different ways. One is that a change in the timeline creates an alternate reality that has off-shot from the actual timeline. The other is that all possibilities exist simultaneously in an infinite number of alternate realities and universes. The first option is usually used as an explanation for the complication of time travel, although sometimes not explained in a satisfactory matter because the very act of traveling back in time should technically change the timeline. Unless it already happened that way to make the world what it is – you trying to change time is actually what causes the timeline – but that is an entirely separate discussion.
The other option just opens up so many possibilities of how things can be done, especially if you allow the characters the ability to jump between the various realities. This leads to interesting questions of how many realities the characters themselves actually exist in. Then would you traveling to a different time give you the memories of the alternate reality or would the time traveller / reality jumper exist outside of that reality? So many directions you could take this sort of story and so many ways for alternate realities to just break your brain!
Created Alternate Reality
So the most obvious example of this can be seen in Back to the Future, when, after going to the future, Marty comes back and everything has changed because Biff gave his younger self a copy of a Sports Almanac. In the Back to the Future example Marty does not have the new memories, and we are never aware of whether an alternate Marty exists or when he comes back he becomes the Marty of that reality, but with none of the memories. It is interesting to think of Marty coming to the alternate reality and the time traveler version taking over the version of that reality. In this instance it is used to say that there was such an extensive change to the timeline to create an alternate timeline.
At the same time my big question is didn’t they already create an alternate timeline when they when back in time originally, which is an idea they never address. Things had already happened differently once, but now it is just a more extreme change so now apparently it is an alternate timeline? I love the movies, but they do not spend too much time talking about the fact that anytime you time travel you have the possibility of creating changes. Apparently only the big changes are the problems. The created alternate reality is a situation that can just be changed. It comes from the idea that time is linear and that the order that things have happened can be affected by simple events in your past to bring you to your current reality. Personally, it is a little too fixed in how things happen and does not take in to affect the other things that could happen. Another example of this is the Butterfly Effect, which is kind of a terrible movie, but the idea of it is very much along these same ideas.
The big thing about the created timeline is that the new timeline overwrites the old and you have to fix whatever changed to make the timeline go back to the original. All time follows along a single fixed path and if something changes then you cannot get back to the original reality of the time traveler.
Multiple Alternate Realities
Personally, I am a big fan of the idea of an infinite number of universes / realities existing that play out the probabilities that exist in the way we live. This is where you can explore the ideas of “what if,” from the small to the very large. The game Chrononauts is entirely based on this concept, by taking what is considered lynchpin historically events and changing the results, which then affects the events after them. So all these different combinations of events exist and the point of the game is to get back to your reality. Chrononauts deals with this idea on a meta level, but the more interesting point comes from the more personal items.
This idea was the main point of Sliders, has been touched on in Doctor Who, and is adjacent to End of Eternity (check out my discussion of that book here). When you think about how many decisions people make every single day and think of what that means for the outcomes that happen… How many times do we wonder what would have happened if we had chosen differently? Being able to travel to or even just see the various probabilities answers these questions. It would be amazing if you could actually explore all the different possibilities. It was not explained too much in End of Eternity, but one of the big things is mapping the changes that might happen and what then happens. Now they still exist on a single timeline and they are trying to make a reality exist, but they can map the probabilities of the other realities existing.
There are just so many ways you can play around with the ideas presented with multiple realities or universes. There is blindly traveling between realities and like in Sliders how do you find your way back home? The other interesting thing is to play with the memories of the person traveling between realities and what happens if the time travel persona and new persona join into one person? Whose memories do they have and what if they come in to a reality where they never existed or died at an earlier point? There are just so many questions and there is not really an answer, but that is what I love about it. At the same time it is hard to think about what my life would be like if things had not happened exactly the way they did because I only know what happened – not what could have been.