The Grandfather Paradox illustrates why time travel into the past is impossible.
The paradox goes like this: Sally is an angry young woman who blames her grandfather, and his dedication to the family business, for most of her problems in life. She is also an expert sniper and engineer. One day, Sally constructs a machine that allows her to travel back in time and, using the machine, she journeys 80 years into the past, when her grandfather was a child. Fueled by hatred of her grandfather and armed with her trusty rifle, which she brought with her, Sally finds her grandfather and kills him.
But the murder of Sally’s grandfather as a young boy, before he fathered any children, means that one of her biological parents will never be born. This, in turn, means that Sally will never be born and, thus, cannot travel back in time and kill her grandfather.
The Grandfather Paradox thus seems to lead to the conclusion that backwards time travel is impossible.
Is there a way out of this paradox?
One response to the Grandfather Paradox appeals to the concept of alternate timelines. We might say that when Sally travels back 80 years, she emerges on a parallel or alternate timeline. In other words, Sally goes into the past, but lands on a different plane of time (or possibly spacetime) than the one she left, and so it is possible for her to kill her grandfather, who exists on the alternate timeline, without undoing her existence, on her own timeline, in the process.
But what this scenario describes is not time travel, as the concept is generally understood. If a traveler jumps 80 years into the past and onto a separate plane of existence, the traveler has not really traveled through time, because she has not landed in a time that was prior to the one she left. Rather, she has traveled to a whole new existence—maybe a whole new universe—that is apparently 80 years behind her own but identical in all other respects. In contrast, time travel as we imagine it, and as the Grandfather Paradox conceives of it, requires a unity of the time sequence being traveled. (Imagine a bus terminal with several ticket windows. You are in the front of the line at Window A. Traveling back in time would be like journeying to the beginning of the Window A line, while traveling onto an alternate timeline at a past time would be like journeying to the end of the line at Window B.)
Another answer to the Grandfather Paradox is to say that, while it might preclude some types of backwards time travel, it need not render all travel to the past impossible. In other words, the most the Grandfather Paradox shows is that backwards time travel is impossible only if it is used to upset the conditions that allowed the backwards time travel to happen. Using this approach, we can concede that Sally cannot travel back in time to kill her grandfather (because once she does, there will be no Sally to travel back in time to kill her grandfather), while still imagining all sorts of travel to the past that might be possible: Jim travels back 100 years and drinks some water from an isolated stream; Carol travels back 40 years and pets a cat; Roger travels back two weeks and moves a book from one end of a bookshelf to another. This type of explanation is referred to as the “self-consistent” approach. It says that backwards time travel is possible, so long as anything the time traveler does is “self-consistent” with the set of circumstances that allowed the time travel in the first place.
The problem with this approach is that it is very difficult—if not impossible—to know what actions in the past will affect the future. The Grandfather Paradox is gripping because it offers a stark and obvious example of an action in the past that prevents the time travel itself. But how would it ever be possible to determine whether any action—a sip of water from a stream, a stroke underneath a cat’s chin, or moving a book from one place to another—affects the future in some way that is inconsistent with the time travel that made the action possible? Unless and until we can say for certain that a particular event will not have such an effect, the “self-consistent” approach will be an unsatisfying response to the Grandfather Paradox.
It’s also possible to explain the Grandfather Paradox by various “plot tricks” that would be at home in any Hollywood movie. For example, we might imagine that Sally kills her grandfather and yet is mysteriously not erased from existence. Sally then travels back to her own time, utterly confused by her continued presence on Earth, only to discover years later that the man she murdered was actually not her grandfather. We might also imagine that although Sally succeeds in killing her actual grandfather, after the murder he is somehow brought back to life and goes onto father one of Sally’s biological parents. These explanations are fun and can make for good cinema, but they are ultimately unsatisfying, since they deny the Grandfather Paradox the very thing that makes it paradoxical: the idea that one can travel backwards in time and take actions that affect the future.