Time Travel, Part 2
Can the past be changed?
If you can travel through time, can you make changes? There are many scientific theories that deal with this very question from the standpoint of paradoxes not being possible. The Novikov self-consistency principle for example states that if an event has the chance to create a paradox, the probability of that event occurring will be 0. Essentially this theory proposes that if you try to change the past something will happen to prevent you from making the change.
This can be interpreted two ways. One would be that the universe will not allow the paradox to happen. I personally discount this entirely, because it would require the universe to have desires and intervene. The other concept is that the past has already occurred, so if someone goes back to try to change it, they obviously have failed because here in the present that change did not occurred. This is a far more acceptable theory, but leads to a boring story. If your time traveler cannot make changes, then what is the point of the trip. It could be a trip of discovery, but following this theory puts many limitations on your story.
The only theories scientists seem to take seriously in regard to making changes revolve around alternate realities. If a person makes a change they are then creating an alternate reality. Their original reality still exists, but they are now cut off from it and there is the potential for a temporal clone to exist.
Alternate realities provide many options, but they also present one major issue. Any time traveler from the future — by the nature of just being there — has cut themselves off from their own time. How big of a change would need to be made before a temporal clone situation arises? Because decisions have not yet been made and time travelers can change things, then any trip to the future would have to be only a potential future. By seeing that future you are then ensuring that at least minor changes will occur because of your knowledge. You also then have to deal with many potential futures for every universe. If time travel occurs after the split why wouldn’t multiple versions of you from different alternate realities run into each other when they go back to a time before the split occurred? This methodology can be interesting, but tends to just be too convoluted for my tastes. A paradox cannot occur because there are an infinite number of realities.
Another method that allows for changes to be made is similar to the previous method. In this case an alternate reality is created by the change, but the original one now no longer exists. There is only one timeline and any change causes it to readjust itself. This is my preferred method of dealing with time travel. It allows for changes, paradoxes, and can provide the largest sense of drama. If your actions can risk your own existence, it makes everything you do that much more important. If you subscribe to the butterfly effect this method also has the most potential for suspenseful stories.
The down side is that this method also doesn’t attempt to answer paradoxes at all. Your characters typically have to find a way to avoid changes themselves and are thus combating paradoxes. The real question is that if they change their world enough for them to have never time traveled to begin with, if they were to return to their own time would it have to be a temporal clone? Also since the characters are typically trying to fix what they have changed, you generally don’t see what would happen to the world if the character that made the change had suddenly never been born and thus could not have traveled back in time. The way comic books typically deal with this concept is to say that while the person may not exist anymore in the future, they did exist at a point before the change was made since they were the one making the change. Thus a character may fade, but their actions did occur because they were there before the timeline diverged.
However you want to handle time travel in your story or game, you have to decide in the beginning if the past can be changed, and if so what method you will be using to show these changes. While I’m sure it is possible to make it work, I just don’t personally see a way to make a game work if you are traveling through time but cannot change it. That would likely just lead to frustration from your players.