Primary Source: "The Paradoxes of Time Travel," by David Lewis
I started this summary ten years from now... It took a while, ok? The good news is that I reach 1032 posts. The bad news is that I only have 941 views :\ also, emoticons are still used in the future :)
There are two types of timelines; External Time, which is the objective time in which we are all caught up in and which binds us together, and Personal Time, which is how we experience external time. External Time is absolute and affects everyone that is in its state of affairs. A cannot come after B, nor can C come before B; all things happen in causality. Personal Time, on the other hand, is able to loop behind, over, and around itself. Personal Time is bound to External Time but only in the sense that the former must happen in the course of the latter. Both focus on states of affairs (how things are arranged at any given moment) and the sets of facts about what is possible during each state of affairs... That was a little heavy philosophically.
Imagine a past relationship you had, and also that you are a guy since I am writing this from my point-of-view. External Time is the account of all the people involved in that relationship from the time you are introduced to the time she cheats on her boyfriend with you to the time she leaves you for that same douche. This is a completely causal relationship wherein your combined actions of texting each other leads to your relationship and her ex being a douchebag leads to her dumping him for the second time. Personal Time is your own experience within that External account, from the time she tells you she loves you to the point where you want to kill yourself since you don't know how to live without her.
In the earlier state of affairs, you are best friends and it is entirely possible for you to be in a relationship given all logical possibilities at the time, but, since it is in the past Externally, you know that is not a fact since she was dating that over-controlling douche. It is also true in the External account that she was cheating on you for a week before she broke up with you, since that is also in the External past.
That should be more understandable.
Now, the Personal account is your own experiences within this External account. They are not capable of changing the External past since that is the one that is affected by everyone and affects everyone. Personal Time really only comes into play when someone travels through time... WITH SCIENCE! I will write a later post about general problems associated with time travel, but for now we shall assume that zombie-Jesus has ordained it as being possible.
In the External account things have already happened, but when someone travels through time, forwards or backwards, they sort of "skip the record." When they go forward in time, it can be assumed that the External account simply goes on the same as though they had disappeared, and will continue to do so until the External point where they return. This isn't too hard to conceptualize and isn't very fun to write about since no real paradoxes happen, so I'm done with this part.
TRAVELING TO THE PAST, HOWEVER, yields quite a few perceived problems, but this is one time where Lewis and I will agree; that the 2009 Star Trek movie got it right. When a person travels backwards in the External account it seems like things should work out in such a way that it is logical for them to appear in the past (i.e., they go back in time to kill someone, end up betraying their original master, and are killed by their "past" self while saving the person they were meant to kill. Thus, the "past" self goes back in External time to repeat the circle.) The Personal account is bound to the External account in terms of possible states of affairs but not bound BY those affairs.
What this means is that whenever you go back in time, you create a new Personal Timeline within the External Timeline. You Alpha, the one that traveled backwards in time, and You Beta, the one that exists in the current External Timeline, are two completely different people with different Personal Timelines. They diverge at the time when You Alpha entered the External Timeline which You Beta resides in since You Alpha has experienced all of the things You Beta was supposed to experience in the future. Theoretically, as long as You Alpha does nothing to disrupt You Beta's experiences, You Beta will at some point in the External Timeline resurface at the same point as You Alpha... Again, this is very intense philosophically. Let's bash my ex some more to help you guys understand.
Steve Alpha catches his ex with some douche in a beret. For several months Steve Alpha is depressed and punishes himself. Several years later, Steve Alpha discovers a way to travel backwards in External Time. Steve Alpha decides to go to the day he became best friends with his ex. So Steve Alpha reverses the External Timeline to that point, but this does not make him the same Steve in External Time; Steve Alpha exists in the sense that he traveled to the past, but Steve Beta, the one that existed in the External Timeline at this moment STILL EXISTS. Steve Beta does not just disappear due to Steve Alpha traveling to this point. Essentially, Steve Alpha has jumped to a new branch of External Time, creating a new state of affairs simply by being there. Steve Alpha decides to beat the crap out of the douche with the beret to the point where he is in a coma, completely differing Steve Beta's Personal Timeline. Steve Alpha still has his own Personal timeline, but Steve Beta will not have the same Personal Timeline experiences as Steve Alpha did up to the point where Steve Alpha intervened. Steve Alpha could even make it so that Steve Beta does not even meet his"later to be" ex, preventing a lifetime of sadness and self-worth issues.
Steve Alpha and Steve Beta will not only have different Personal Timelines, but the External Timeline of Steve Alpha, and all of the possible states of affairs, are rewound to the point in External Time where Steve Alpha and Steve Beta are in the same External Time for the first time. Imagine listening to a song all the way though, then listening to the same song but it is completely different than it was before. That is the same way it works with time; the External Account is how the computer plays the song regardless of what is one it, whereas your Personal Account is how you hear the song every time you play it. It is this principle that makes the multiverse theory somewhat plausible, but I will argue that some other time (pun intended.)
10 Second Version: There are no paradoxes in time travel. You cannot change the past External Account in the present but you retain your Personal Account when you travel backwards in the External Account, making you capable of changing certain facts in the "new" External Account. My ex is a jerk. Time is hard.
Time is one of the hardest things to understand. I don't expect anyone to be able to understand it without extreme concentration and studying the subject for several months. Not even I, The Amazing Steve, was able to pick it up easily. I will continue to try to explain it as best I can and I encourage anyone genuinely interested in the topic to not give up. It really is just a matter of conceptualizing all of these crazy concepts. In retrospect, though, being able to understand it has been so fulfilling.
Steve
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