As I explain here, one reason why I enjoy science fiction is its ability to wrestle with ideas. Sometimes those ideas are deep questions Sometimes they are fun. Sometimes they are just weird or confusing. Stories involving time travel could fit into any or all of these categories. This is not intended to be a serious philosophical analysis. I also do not believe in time travel, but I still think it makes fascinating stories. This post is merely fun speculation, nothing more.
Think about all of the time travel stories you have watched or read. Is there any time travel story that does not create paradoxes or contradictions when followed to its logical conclusions? Temporal paradoxes can be divided into two categories: Causal loops and grandfather paradoxes. In this post I will only discuss grandfather paradoxes. Maybe I will consider causal loops in a future post.
A grandfather paradox, also called a consistency paradox, occurs when changing the past creates inconsistencies in the timeline. The classic example involves a time traveler who goes back in time and kills his grandfather before his grandfather had children, thereby rendering his own birth and future murder of his grandfather impossible. In a similar example, a time traveler goes back in time to kill Adolph Hitler and thereby prevent the Holocaust. If he succeeds, then he will alter history in such a way that he will have no reason to go back in time and therefore will never have a reason to kill Hitler.
For a more familiar example, consider the Back to the Future trilogy. Even people who normally do not watch science fiction have likely seen these movies. In these stories, Doctor Brown and Marty sometimes use a photo or a newspaper to document an event, such as the arrest of Marty McFly, Jr. or the murder of Brown. When history is changed, then the images and text change. This is a neat idea, and it makes the movies entertaining, but it creates at least two problems. First, if a change in history creates a ripple effect that changes a newspaper even after it has been taken to a different time, then why does the ripple effect not also affect Brown’s and Marty’s memories. The second problem is similar to the Hitler scenario. If Brown and Marty change history to prevent the arrest of Marty Jr., then Brown never would have collected the newspaper, and they never would have observed the change. If Marty never was fired, then Jennifer never would have kept the “YOU’RE FIRED” fax (Marty’s boss never would have sent it), and she never would have watched the ink disappear. Even if she could still watch the ink disappear, then one must wonder why the paper did not disappear as well.
Another example, although more obscure, is one of my favorite stories from my middle school years, The Green Futures of Tycho by William Sleator. Many paradoxes could be found in this book, but the best example involves the beginning and end. Tycho finds a handheld time machine while digging a vegetable garden in his back yard. That time machine (left on Earth by aliens in a pre-historic age) alters his future in ways he finds very disturbing, and so he goes back in time to prevent it from being left on Earth so that he will never find it. However, if he never finds the time machine then he can never go back in time to prevent himself from ever finding it. He nonetheless does prevent himself from finding it and eliminates the disturbing future. Strangely, this change also affects his present relationships with his siblings for unexplained reasons, and when digging his vegetable garden he finds an empty lipstick tube instead of the time machine.
Some people will argue that the logical contradiction indicates that time travel, or at least time travel which would cause a contradiction, is impossible. That certainly would solve the problem. On the other hand, if such time travel is possible, then how do we resolve stories in which changing the past does create contradictions?
Some people might propose that changing history would create a parallel timeline in which the changes to the timeline are effective, while the original timeline remains unchanged. However, that creates problems. First, the alternate timeline might as well be another universe. Where did the matter and energy come from? Second, would the newly created timeline have the same past history as the parent timeline? Such a duplicate history would not be the real history of that timeline because it only just now came into existence. Is it a fake or illusory history? Then one could ask which history is the real one and which is the illusory duplicate and how one would tell the difference. Who would have thought time travel could lead to skepticism about the reality of one’s history? What is next? A time traveler without a past? Imagine that.
Thinking about time travel easily creates confusing logical problems. Maybe that is one reason why the idea is so fun. In any case, regardless of how one settles these problems, Sheldon makes one thing clear. Thinking about time travel can make your head hurt.